Epidemiology of Human ListeriosisANNE SCHUCHAT,* BALA SWAMINATHAN, AND CLAIRE V. BROOME
Meningitis and Special Pathogens Branch, Division of Bacterial Diseases, Center for Infectious Diseases,Centers for Disease Control, Atlanta, Georgia 30333
INTRODUCTION .............................................. 169MICROBIOLOGY OF L. MONOCYTOGENES ............................................. 169
Identification and Biochemical Characteristics ............................................. 169Isolation of L. monocytogenes............................................. 170Systems for Typing and Subtyping L. monocytogenes ............................................. 171Rapid Confirmation of L. monocytogenes............................................. 172Rapid Detection of Listeria Species and L. monocytogenes .............................................172
L. MONOCYTOGENES IN THE ENVIRONMENT ............................................. 173L. MONOCYTOGENES IN ANIMALS ............................................. 173L. MONOCYTOGENES IN HUMANS............................................. 173
Pathogenesis............................................. 174Invasive disease in nonpregnant adults............................................. 174Listeriosis during pregnancy ............................................. 175Neonatal disease: early onset ............................................. 175Neonatal disease: late onset ............................................. 175
Noninvasive Disease: Mild Syndrome due to Listeriosis ............................................. 175EPIDEMIOLOGIC PATTERNS OF DISEASE............................................. 176Epidemic Disease ............................................. 176Community outbreaks: foodborne disease............................................. 176Nosocomial outbreaks ............................................. 177
Sporadic Listeriosis ............................................. 177Surveillance .............................................. 177Dietary risks for sporadic disease ............................................. 178Possible other sources of sporadic disease ............................................. 178
DIAGNOSIS, TREATMENT, AND PREVENTION ............................................. 178ISSUES FOR THE FOOD INDUSTRY ............................................. 178SUMMARY ............................................. 179REFERENCES ............................................. 179
INTRODUCTION
Listeria monocytogenes is a gram-positive bacillus foundfrequently in the environment that causes stillbirths andmeningoencephalitis in a wide range of animals (53). Humandisease due to L. monocytogenes usually occurs in thesetting of pregnancy, immunosuppression, or extremes ofage. Epidemiologic investigation of several outbreaks oflisteriosis during the 1980s (12, 41, 93, 145) demonstratedthat epidemic listeriosis is a foodborne disease. Recent studysuggests that a substantial portion of sporadic listeriosiscases may also be caused by foodborne organisms (23, 147).Laboratory advances in detection and subtyping of theorganism have recently improved efforts to study humanlisteriosis.The successful use of immunosuppressive medications in
the treatment of malignancy and the management of organtransplantation, as well as the more recent epidemic ofacquired immunodeficiency syndrome, have led to an expan-sion of the immunosuppressed population at increased riskof listeriosis. Attention has therefore been directed to liste-
* Corresponding author.
riosis, both as a clinical entity of increasing importance andas a substantial problem for the food industry.
In this article, we discuss recent developments in micro-biologic detection and subtyping of L. monocytogenes andreview current information on epidemic and sporadic dis-ease, specifically detailing each of the major outbreakscaused by foodborne organisms in the past decade. We alsodiscuss recent efforts to determine the magnitude of diseasedue to L. monocytogenes in the United States and to identifyrisk factors for sporadic listeriosis. The proceedings of tworecent symposia on listeriosis provide detailed reviews ofvirulence factors and immunologic aspects of infection (1, 2).
MICROBIOLOGY OF L. MONOCYTOGENES
Identification and Biochemical CharacteristicsListeria spp. are gram-positive rods 0.4 to 0.5 ,um in
diameter and 0.5 to 2 ,um in length (151). Members of thegenus Listeria are aerobic and facultatively anaerobic, donot produce spores, and demonstrate characteristic motilitywhen cultured at 20 to 25°C. Colonies are bluish gray bynormal illumination, but a blue-green sheen is visible byoblique light. Listeria spp. can grow between pH 6 and pH 9and in temperatures ranging from 1 to 45°C. Optimum
169
170 SCHUCHAT ET AL.
TABLE 1. Characteristics of Listeria Spp.a
Acidproductionfrom: Result of CAMP test
Species Hemolysis Acid production from: Nitrate with:reduction
D-Glucose D-Xylose D-Mannitol L-Rhamnose S. aureus R. equi
a Adapted from reference 168. +, .90% of strains were positive; -, 290% of strains were negative; V, 11 to 89% of strains were positive.
growth occurs between 30 and 37°C. Listeria spp. arecatalase positive, oxidase negative, methyl red positive, andVoges-Proskauer positive.
L. monocytogenes is beta-hemolytic on blood agar andforms a narrow zone of hemolysis around colonies. Incontrast, L. ivanovii forms double or triple hemolytic zoneswhen grown on sheep or horse blood agar, and other Listeriaspecies are nonhemolytic. The CAMP test is useful indifferentiating hemolytic species of Listeria and in separatingL. innocua from L. monocytogenes, since these two speciesgive similar reactions to biochemical tests. The CAMP testdetects synergistic reactions of hemolysins of Listeria spp.with the beta toxin of Staphylococcus aureus and with anexofactor of Rhodococcus equi. L. monocytogenes gives apositive CAMP reaction on sheep blood agar with Staphy-lococcus aureus but not Rhodococcus equi, while L. ivanoviiproduces a positive CAMP reaction with R. equi but not S.aureus. L. innocua gives a negative CAMP reaction with S.aureus and R. equi. All L. monocytogenes strains produceacid from L-rhamnose and ox-methyl-D-mannoside but notfrom D-xylose or D-mannitol. Table 1 summarizes featuresdistinguishing L. monocytogenes from other Listeria spe-cies.
Isolation of L. monocytogenesL. monocytogenes is readily isolated from clinical speci-
mens obtained from normally sterile sites such as cerebro-spinal fluid, blood, and amniotic fluid. L. monocytogenesgrows on a wide variety of nonselective plating media (e.g.,blood agar, chocolate agar, tryptic soy agar, and brain heartinfusion agar). When L. monocytogenes is present in highnumbers (104 CFU/ml), direct plating on these media yieldsvirtually pure cultures (159).The isolation of L. monocytogenes from foods and clinical
specimens that are not usually sterile requires selectiveenrichment before specimens are plated on selective isola-tion agars. Ever since Gray et al. (54) discovered thepsychrotrophic characteristics of L. monocytogenes, coldenrichment (4°C) in nonselective broth medium has beenused to enhance its isolation. However, cold enrichment for1 to several weeks may be required for isolation of theorganism from foods and clinical specimens (59, 73). Fur-ther, other psychrotrophic bacteria present in foods mayovergrow Listeria spp. during cold enrichment (71). Becauseof these disadvantages of cold enrichment, other selectiveenrichment procedures have been developed for the isola-tion of Listeria spp. from foods.Chemicals used to render liquid enrichment media selec-
tive for L. monocytogenes include acriflavine, glycine anhy-dride, lithium chloride, nalidixic acid, nitrofurazone, potas-
sium tellurite, and potassium thiocyanate (83). Among themost frequently used selective chemical enrichment mediaare (i) Food and Drug Administration (FDA) broth, anenrichment broth prepared from tryptic soy broth, acrifla-vine, and nalidixic acid (95, 97); (ii) U.S. Department ofAgriculture-Food Safety and Inspection Service (USDA-FSIS) broth, containing esculin, acriflavin, and nalidixicacid; developed by Dominguez Rodriguez et al. (31), modi-fied by Donnelly and Baigent (32), and used by the USDA-FSIS for the isolation of L. monocytogenes from meats andpoultry products (109); (iii) nutrient broth containing 3.75%potassium thiocyanate and nalidixic acid, developed byWatkins and Sleath (167) and used by Hayes et al. (59) toisolate L. monocytogenes from epidemic-associated rawmilk; and (iv) L-PALCAM broth, a medium developed inThe Netherlands (165) and used widely in Europe. Foods areenriched in these media for 24 to 48 h and plated onListeria-selective agars. Hayes et al. (59) used the potassiumthiocyanate-nalidixic acid-containing medium for the sec-ondary enrichment of cold-enriched cultures.McBride and Girard (108) developed the first selective
plating agar for L. monocytogenes. Their formulation wasblood-containing medium composed of phenylethanol agarbase, lithium chloride, and glycine. Lovett (95) used amodification of the McBride agar (without blood) to isolateL. monocytogenes from foods at the FDA laboratories.Researchers at USDA-FSIS improved the selectivity of theMcBride medium by increasing the concentration of lithiumchloride 10-fold, substituting glycine anhydride for glycine,and incorporating the antibiotic moxalactam (91). This for-mulation, commonly known as LPM agar, is highly selectiveand is useful for the isolation of L. monocytogenes fromhighly contaminated specimens. The FDA amended its List-eria isolation procedure (164) by including LPM agar forplating enrichment cultures. Neither the modified McBride'sagar nor LPM agar contain any chemicals that would allowfor the differential presumptive identification of Listeriacolonies by colony morphology and color. Growth on thesemedia is examined under oblique 450 illumination to detectthe typical light blue colonies formed by Listeria species.The PALCAM medium developed by van-Netten et al. (165)and the Oxford agar developed by Curtis et al. (27) attemptto overcome this disadvantage by incorporating differentialagents in the agar medium. On PALCAM agar, Listeriacolonies appear grey-green, are approximately 2 mm indiameter, and have black, sunken centers. They also have ablack halo against a cherry red background (165). On Oxfordagar, L. monocytogenes colonies appear black, are 1 mm (24h) to 3 mm (48 h) in diameter, and are surrounded by blackhalos (27). PALCAM and Oxford agars are preferred by
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 171
those who have difficulty observing the blue color of Listeriaspp. on LPM and McBride agars under oblique illumination.Oxford agar was useful for the isolation of L. monocyto-genes from milk and dairy products (124).
Several investigators have compared the various selectiveenrichment and plating methods. Many comparative studiessuffer from poor design and very low positivity rates, limit-ing the statistical evaluation of the results. Pini and Gilbert(132) compared the cold enrichment and FDA procedures forthe isolation of L. monocytogenes from soft cheeses and rawchickens. They found that the FDA procedure was moreproductive for the isolation of L. monocytogenes fromchickens, while the methods were essentially equivalent forisolation of L. monocytogenes from cheeses. In contrast,Doyle and Schoeni (34) found cold enrichment to be superiorto the FDA procedure for the isolation of L. monocytogenesfrom soft cheeses. Parish and Higgins (128) found the FDAmethod to be more productive than cold enrichment (59) forthe isolation of L. monocytogenes from artificially inocu-lated single-strength orange juice. Lammerding and Doyle(87) compared the FDA, USDA, and cold enrichment pro-cedures for the isolation of L. monocytogenes from dairyproducts and concluded that the USDA-FSIS procedure wassuperior to the FDA and cold enrichment procedures. Hayeset al. (60) compared cold enrichment and USDA-FSISprocedures for the isolation of L. monocytogenes from 402foods obtained from the refrigerators of patients with liste-riosis. This study was conducted as part of an activesurveillance project to determine the role of foods in spo-radic human listeriosis. They found the USDA procedure tobe significantly better (P < 0.001) than cold enrichment;isolation efficiencies of the two procedures were 96 and 59%,respectively.
All the comparative studies agree on one point. No singleisolation procedure gives consistent isolations from all List-eria-positive samples (34, 36, 86). Further, several food-processing methods (heating, freezing, chemical treatment,etc.) may cause sublethal injury to the microbial flora.Sublethally injured listeriae may show greater sensitivity tothe selective agents used in the selective chemical-enrich-ment broths (87, 132). New approaches leading to theefficient recovery of sublethally injured listeriae from foodsare currently being investigated in several laboratories (26,52, 154, 155).
Systems for Typing and Subtyping L. monocytogenes
L. monocytogenes is subdivided into serotypes on thebasis of somatic (0) and flagellar (H) antigens (150). Elevenserotypes (1/2a, 1/2b, 1/2c, 3a, 3b, 4a, 4ab, 4b, 4c, 4d, and 4e)of L. monocytogenes have been identified. Because the vastmajority of cases of human disease are caused by only threeserotypes (1/2a, 1/2b, and 4b), serotyping is only minimallyhelpful in epidemiologic investigations. Various subtypingmethods have been applied to collections of L. monocyto-genes isolates. One goal of subtyping Listeria isolates is topermit a more precise comparison of clinical and environ-mental specimens than is afforded by serotyping alone.A phage typing system for Listeria spp. has been devel-
oped (5) and evaluated for use in epidemiologic studies (112).With a set of 28 phages tested on clinical strains collected inthe United Kingdom between 1967 and 1983, 64% of L.monocytogenes strains were typeable. Typeability varied byserogroup: 82% of serogroup 4 strains could be typed, whileonly 37% of serogroup 1/2 strains were phage typeable.Somewhat different results were obtained when 20 phages
were used to type strains isolated in France (4): 78% of allstrains were typeable (88% of serogroup 4 and 57% ofserogroup 1/2). Geographic variation in the prevalence ofListeria strains as well as methodologic differences mayaccount for the different study results. Although phagetyping has been successfully applied in epidemiologic inves-tigations (41, 93), the portion of strains that cannot be typedby this method suggested the need for additional methods.
Selander et al. (152) described a method that uses multi-locus enzyme electrophoresis to distinguish among isolatesof a specific bacterial genus. The method differentiatesorganisms by variations in the electrophoretic mobilities of apanel of constitutive enzymes. All L. monocytogenes strainsare typeable by this method. In addition, the method pro-vides information on the degree of genetic relatedness be-tween strains. In determining whether a relationship be-tween a clinical and an environmental isolate exists, it isimportant to consider the background distribution of sub-types in the environment. Such background information ondistribution of enzyme types was recently obtained for awell-characterized sample of L. monocytogenes isolatesacquired during population-based surveillance in the UnitedStates (11, 49). Similar information has been collected for alarge but less well defined sample of clinical and environ-mental isolates from several countries (131); however, 41 ofthe 114 human isolates characterized in the report wererecovered from patients associated with a common-sourcelisteriosis outbreak in Switzerland. Both studies suggest thatelectrophoretic enzyme typing permits L. monocytogenes tobe divided into a large number of distinct subtypes. Becausemultilocus enzyme electrophoresis permits typing of allstrains and because substantial diversity exists among List-eria strains, the method is extremely useful in epidemiologicinvestigations that attempt to compare clinical and environ-mental strains.
Perez-Diaz et al. (129) examined 32 Listeria strains fromspecies L. monocytogenes, L. murrayi, and L. grayi for thepresence of plasmids and found a 38.5-MDa cryptic plasmidin 4 strains of L. monocytogenes, 2 strains of L. grayi, and 1strain of L. murrayi. Restriction analysis of the plasmidsindicated that all were a single molecular species. Analysisof plasmids may not provide adequate discrimination ofListeria species and therefore may not be suitable forsubtyping L. monocytogenes.
Restriction fragment length polymorphism (RFLP) analy-sis was applied by Nocera et al. (123) to 28 strains of L.monocytogenes serotype 4b. They obtained 10 differentRFLP profiles with EcoRI restriction enzyme. However,four different RFLP profiles were observed for 19 strainsinvolved in a single outbreak that were a single cloneaccording to multilocus electrophoretic enzyme typing.Wesley et al. (175) used HhaI restriction enzyme to examineL. monocytogenes serotype 4b strains from the four NorthAmerican outbreaks for RFLP of genomic DNA. Theyobtained distinctly different patterns for strains from dif-ferent outbreaks but identical profiles for strains from thesame outbreak. While RFLP analysis appears to be usefulfor the characterization of L. monocytogenes, the highdegree of discrimination offered by this procedure may notalways be epidemiologically relevant.One of the problems of RFLP analysis is that a large
number of bands (100 to 1,000) are obtained by restriction ofgenomic DNA with restriction enzymes with four- or six-base recognition sequences. This makes it difficult to applyRFLP analysis to a large set of strains to determine thedegree of interrelatedness. Attempts have been made to
VOL. 4, 1991
172 SCHUCHAT ET AL.
simplify RFLP analysis by performing hybridization exper-iments on the restriction fragments of chromosomal DNAwith probes that recognize relatively stable chromosomalregions. Saunders et al. (141) used two cloned probesselected from a bacteriophage lambda library of L. monocy-
togenes to probe NciI digests of L. monocytogenes DNAfrom 64 strains. They found 19 different patterns and ob-served similar profiles in epidemiologically related strains.Ribosomal DNA fingerprinting (ribotyping) has also been
used to subtype L. monocytogenes. Ribotyping involvesprobing restriction fragments of chromosomal DNA with16S and 23S rRNA of Escherichia coli. This method has twodistinct advantages. Because ribosomal genes are highlyconserved across phyla, one universal probe can be used forthe characterization of several organisms (56). Also, becausebacteria usually contain multiple rRNA operons (seven in E.coli), an adequate number of bands (5 to 15) of differentmolecular sizes are obtained by this procedure. Swami-nathan et al. (158) characterized 89 strains of L. monocyto-genes by ribotyping. Sixteen different ribotypes were ob-served. In general, strains with a strong epidemiologicassociation were found to be the same ribotype. Further,ribotyping divided the strains into two distinct subgroups(separating serotypes 1/2a, 1/2c, and 3a from serotypes 1/2band 4b) in the same manner observed with multilocusenzyme electrophoresis. Ribotyping in conjunction withenzyme typing provided laboratory support for a suspectedassociation between an isolate of L. monocytogenes fromraw milk and a patient with listeriosis who drank the milk(166).Although several methods of subtyping L. monocytogenes
have been investigated, most subtyping methods are laborintensive and do not easily permit rapid screening of a largenumber of isolates. Questions concerning the reproducibilityand sensitivity of various typing methods continue to beinvestigated.
Rapid Confirmation of L. monocytogenes
Colonies that give a Listeria-like appearance on primaryplating media are subjected to biochemical tests for identifi-cation of L. monocytogenes. Confirmation of presumptivelypositive colonies as Listeria species or as L. monocytogenesby conventional methods requires 2 to 7 days. This isproblematic when a rapid answer is required, such as in thequality control of semiperishable food products which havea limited shelf life.
Several procedures have been proposed for the rapidconfirmation of Listeria species or of L. monocytogenes.Fraser and Sperber (43) modified the USDA secondaryenrichment broth by supplementing it with lithium chlorideand ferric ammonium citrate. Listeria species hydrolyzeesculin in the USDA secondary enrichment broth, producing6,7-dihydroxycoumarin. The ferric ions react with this com-
pound to produce a black precipitate. Hydrolysis of theesculin by enterococci in the sample is prevented by theaddition of lithium chloride to the medium. Kempton (77)proposed the use of a sodium chloride-esculin hydrolysistest, motility test, and hemolysis test to rapidly identifyListeria species. The use of a petri plate with four quadrants(quad plate) has been proposed as an aid for rapid identifi-cation of Listeria species (140). The quad plate allows theperformance of five tests: oxidase, catalase, CAMP test,
rhamnose, and xylose (159). Another procedure for rapidconfirmation, the API 20 STREP system, was successfully
used to differentiate L. monocytogenes and L. seeligeri fromL. ivanovii and nonhemolytic Listeria species in 4 h (101).DNA probes derived from the listeriolysin 0 gene se-
quence (24, 115), the gene coding for a putative invasionfactor of L. monocytogenes (28, 29, 40, 80), and the gene fora delayed-type hypersensitivity factor (125) of L. monocyto-genes have been used for the rapid confirmation of L.monocytogenes colonies isolated from food. Mengaud et al.(115) and Chenevert et al. (24) found a 651-bp internalfragment of the listeriolysin gene to be specific for L.monocytogenes under stringent hybridization conditions.Datta et al. (28) reported that pRF106, a 321-bp internalfragment of a gene coding for a 60-kDa secreted polypeptidein L. monocytogenes, was specific for L. monocytogenesunder moderately stringent hybridization conditions; how-ever, Kim et al. (80) reported that the same probe hybridizedto two of five strains of L. seeligeri even at stringenthybridization conditions. An oligonucleotide probe derivedfrom the nucleotide sequence of pRF106 reacted with L.monocytogenes and one strain of L. seeligeri (29). Interest-ingly, an oligonucleotide probe that differed in only onenucleotide from the probe used by Datta et al. (29) did notreact with the same L. seeligeri at stringent hybridizationconditions (79). A DNA probe derived from the nucleotidesequence of a putative delayed-type hypersensitivity factorof L. monocytogenes reacted with 175 of 186 strains of L.monocytogenes tested but also with L. ivanovii under strin-gent hybridization conditions. An L. monocytogenes-specific acridinium ester-labeled DNA probe was used in ahomogeneous protection assay for the confirmation of L.monocytogenes colonies in less than 2 h (126). This methodmay be very useful for the rapid confirmation of foodisolates.
Rapid Detection of Listeria Species and L. monocytogenes
Monoclonal antibody-based immunoassays and nucleicacid hybridization assays have been developed for the rapiddetection of Listeria spp. in foods and in clinical specimens.Foods generally contain low concentrations of Listeria spp.in relation to the total microbial load. Therefore, the analysisof foods by these methods is typically done after the samplehas been cultured in one or more selective enrichmentbroths.
Several monoclonal antibody-based tests are available forthe rapid detection of Listeria species. Monoclonal antibod-ies to cell surface antigens are only genus specific; thesehave been used to develop a dot-enzyme immunoassay, amicroplate enzyme immunoassay, and a direct immunofluo-rescence test for the rapid detection of Listeria species (19,37, 107, 113, 114). Although none of the monoclonal anti-bodies to cell surface antigens are specific exclusively to L.monocytogenes, monoclonal antibodies against the beta-hemolysin (listeriolysin 0) of L. monocytogenes are specificand could potentially be used in the rapid detection andconfirmation of L. monocytogenes (61).DNA probe assays have also been developed for the rapid
detection of Listeria species but are not yet available for therapid and specific detection of L. monocytogenes fromenrichment broth cultures of foods. A commercially avail-able DNA probe assay (Gene-Trak Systems, Framingham,Mass.) uses a fluorescein-labeled DNA probe targeted to aListeria species-specific rRNA sequence for rapid detectionof Listeria species in foods (82, 84).A two-step nested polymerase chain reaction has been
developed to detect L. monocytogenes DNA in paraffin-
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 173
embedded tissues (81). The diagnostic fragment is a 165-bpinternal fragment of the listeriolysin gene. The method, ifvalidated in further evaluations, may offer a means ofdetecting L. monocytogenes DNA retrospectively in pre-served clinical specimens and could potentially be modifiedfor food applications.
L. MONOCYTOGENES IN THE ENVIRONMENT
L. monocytogenes has been identified throughout theenvironment. The organism has been isolated from soil(171), water, and decaying vegetation (170, 172). Weis de-tected L. monocytogenes in 21% of 779 plant and soilsamples; serotypes 1/2b and 4b were found most frequently(170). The ability of L. monocytogenes to survive in theenvironment has been demonstrated quantitatively by Wat-kins and Sleath (167), who isolated L. monocytogenes fromsamples of sewage, river water, and sewage sludge by usingcold enrichment methods. They found that quantitativecounts of L. monocytogenes from sewage sludge sprayed onagricultural land remained unchanged for at least 8 weeks.The potential implications of using fecal material as agricul-tural fertilizer subsequently became evident when this prac-tice was suspected to have contributed to a large outbreak ofhuman listeriosis in Nova Scotia (145).
L. MONOCYTOGENES IN ANIMALS
Beginning with Murray and colleague's description in 1926of disease in rabbits caused by the bacterium now known asL. monocytogenes (120), listeriosis has generally beenthought of as a veterinary disease. In mammals, L. mono-cytogenes causes abortions and "circling disease" (menin-goencephalitis), and epizootics of listeriosis were observedin herds of cattle and sheep long before outbreaks oflisteriosis were recognized in humans. In addition, healthyanimals could be gastrointestinal carriers of L. monocyto-genes.
L. monocytogenes has been isolated from cattle, pigs,sheep, chickens, turkeys, ducks, and a variety of otherspecies (149). Systematic culture of stool from cows withListeria-related abortions, from healthy cows in herds withlisteriosis, and from cows in unaffected herds revealeddifferences in the carriage of L. monocytogenes (13). Thehighest rates were found in stool cultures of animals who hadaborted because of listeriosis (24% [53 of 219 samples]),while healthy cows from affected and unaffected herds hadlower rates of carriage (affected herds, 6.7% [41 of 622];unaffected herds, 1.7% [2 of 120]) (13). There may be arelationship between the feeding of poor-quality silage andthe onset of listeriosis in domesticated ruminants.Because clinical illness and asymptomatic carriage of L.
monocytogenes were well documented in animals, listeriosiswas considered a zoonosis; human disease was thought to bedue to illness occurring in animals, and the animal host wasconsidered the primary reservoir for the organism (13).Consistent with this view, conjunctivitis was reported inpoultry workers who handled infected chickens (38), andcutaneous lesions have been reported among veterinariansand cattle ranchers who delivered infected and abortedcalves (20, 127). In most reported cases of human illness,however, there is no history of direct contact with animals.Gray and Killinger stressed this discrepancy in their obser-vation that most human listeriosis occurred in urban resi-dents, while only rare cases occurred among residents of
rural areas where domestic animals were widely affected bylisteriosis (53).The idea that human listeriosis could be the result of
indirect contact with infected animals focused attention onthe possibility of transmission by foodborne organisms. Casereports had suggested that human listeriosis occurred afterconsumption of contaminated foods such as unpasteurizedmilk from cows known to be infected with the organism(133). However, Listeria species may be isolated frequentlyfrom the environment or from foods without a direct relationto human illness, and initial typing systems were not suffi-ciently precise to suggest that an environmental strain wasrelated to a clinical isolate. Investigation of large outbreaksof human listeriosis finally provided epidemiologic and lab-oratory support to confirm the suspicion that listeriosis wasa foodborne disease (12, 41, 93, 145).
L. MONOCYTOGENES IN HUMANS
Carriage
As early as 1926, Murray et al. (120) suggested that theintestinal tract might be the portal of entry for organismscausing Listeria infections. While most studies of nasopha-ryngeal cultures from healthy individuals failed to detectListeria spp. (105, 157), several investigators have found L.monocytogenes in fecal specimens from healthy people (13,55, 70, 72, 73, 88), a fact consistent with the suggestion thatthe gastrointestinal tract is the human reservoir of theorganism.
Bojsen-Moller used cold enrichment to study fecal car-riage in a number of population groups (13). He foundListeria in stool cultures from healthy slaughterhouse work-ers (4.8% [55 of 1,147 workers]), hospitalized adult patients(1.2% [12 of 1,034 patients]), patients with diarrhea (1% [6 of595 patients]), and household contacts of listeriosis patients(26% [9 of 34 household contacts]). Because up to eightspecimens were collected from individual household con-tacts of patients, the frequency of Listeria isolation in thisgroup is not directly comparable to the results from otherpopulations. Five of 14 households had at least one memberwith L. monocytogenes in stool. However, in only two of thehouseholds was the family member carrying the same sero-type as the patient. Cold enrichment was used in processingall the cultures, but transport of specimens to a centrallaboratory was delayed in the nonhospitalized patients andantibiotic use prior to culture was more frequent in thehospitalized patients, which complicated comparison be-tween groups.
Other studies from various pregnant and nonpregnantpopulations have identified wide-ranging estimates for stoolcarriage of L. monocytogenes among healthy adults. Kam-pelmacher and van Noorle Jansen (72) found that 11.9% ofoffice personnel sampled had fecal Listeria carriage, andslaughterhouse workers had a rate of 13.3%. Gregorio andEveland (55) found L. monocytogenes in 1.75% of stoolsamples from 400 patients hospitalized with nonlisteric con-ditions. Differences in culture methods as well as in dietaryand host factors may account for differences in the preva-lence of carriage. Higher rates of Listeria carriage have beenreported when serial specimens from individual subjectswere cultured, but comparing such estimates of cumulativeprevalence with estimates of point prevalence is proble-matic.
VOL. 4, 1991
174 SCHUCHAT ET AL.
Invasive Disease
Pathogenesis. L. monocytogenes can be found frequentlyin the environment, and carriage studies suggest that humanexposure to L. monocytogenes is not uncommon, yet inva-sive listeriosis occurs rarely. Factors that may influencewhether invasive disease will occur include the virulence ofthe infecting organism, the susceptibility of the host, and thesize of the inoculum.Transmission of L. monocytogenes by food first requires
penetration of the organism through the intestine (10). Intra-cellular multiplication can occur in various types of cells.Mackaness first showed that L. monocytogenes could growinside nonimmune phagocytes (102), and more-recent exper-imental work suggests that Listeria spp. can replicate inPeyer's patches (100). Gaillard et al. (44, 45) showed in an invitro model using human enterocytelike cells that L. mono-cytogenes multiplies intracellularly after internalization intovacuoles. This directed phagocytosis was observed for L.monocytogenes and L. ivanovii, the pathogenic species ofListeria, but not for nonpathogenic species of Listeria (45).Quantitative electron microscopic study suggests that L.monocytogenes moves from phagocytic vacuoles to thecytoplasm, where replication is improved by more-favorablegrowth conditions. The ability to disrupt vacuolar mem-branes, therefore, is considered important in the virulence ofL. monocytogenes.Transposon mutagenesis experiments suggest that a he-
molysin, listeriolysin 0, permits pathogenic Listeria speciesto escape the phagosome (46, 74). Nonhemolytic mutantswere avirulent in a mouse model, and virulence was restoredin a revertant strain which gained the ability to producehemolysin after spontaneous loss of the transposon (46, 74).Listeriolysin 0 was not involved in bacterial internalization,since nonhemolytic strains penetrated cells in vitro at thesame rate as hemolytic strains (45). Progress in elucidatingthe role of hemolysin in the virulence of L. monocytogeneshas included purification of listeriolysin 0 (50) and measure-ment of cytolytic activity under various experimental condi-tions. Cytolytic activity was greatest at pH 5.5 and unde-tectable at pH 7.0, suggesting that lytic activity would bemainly expressed in the phagolysosome rather than in extra-cellular fluids (50).Sword demonstrated that growth of L. monocytogenes in
mice and host susceptibility to experimental infection werecorrelated with the availability of iron to the organism (160).In contrast, iron deprivation stimulates hemolysin secretion.Iron deprivation within the phagosome favors production oflisteriolysin 0, leading to disruption of intracellular mem-branes. Iron availability and neutral pH in the cytoplasmtherefore favor bacterial replication after escape of thebacteria from the vacuole. Although listeriolysin 0 appearsto be important in virulence, levels of hemolysin do notdirectly correlate with virulence in experimental infection inmice (76), and discrepancies between experimental results inmurine and human cell lines still need to be clarified (75).Host susceptibility to L. monocytogenes depends primar-
ily on cell-mediated immunity, and most listeriosis occurs inpersons with impaired cell-mediated immunity due to dis-ease processes, medications, or pregnancy. L. monocyto-genes was used by Mackaness in early studies that delin-eated the nature of cell-mediated immunity (102). Protectionin mice against L. monocytogenes is mediated throughListeria-sensitized T cells, which activate macrophages.Mackaness showed that macrophage activation depended onspecific lymphocytes (103) and that antilymphocyte globulin
suppressed immunity to Listeria spp. (104). The main func-tion of Listeria-sensitized T cells appears to be attracting,focusing, and activating macrophages at infective foci (116).T cells activate macrophages by producing lymphokinesincluding gamma interferon, which has been shown to in-crease the listericidal activity of dexamethasone-treatedmonocytes (142). Prostaglandins may mediate suppressionof cellular immunity to L. monocytogenes (130, 163). Mac-rophage-derived thromboxane A2, via vasoconstriction, de-creases the numbers of bacteria that can leave the site ofinfection; treatment with the cyclooxygenase inhibitor indo-methacin was shown to increase the susceptibility of mice toListeria infection (163). Cytotoxic suppressor T cells appearto have a more important role in eradicating Listeria spp.than do helper cells (116).
Pregnancy-associated depression in cell-mediated immu-nity may be due to alterations in hormonal and serum factorsas well as to a decrease in the ratio of T helper to suppressorcells during pregnancy (169). In addition, the local placentalsuppression of cell-mediated immunity necessary to preventmaternal rejection of the placenta may contribute to suscep-tibility of the fetus to infection with listeriosis (138).
In addition to depressed cell-mediated immunity, deficien-cies in immunoglobulin M and complement activity associ-ated with the neonatal state may contribute to the propensityof infants to develop listeriosis (14). Local gastrointestinalfactors may play an additional role in disease in adults (62,148).Although the infectious dose of listeriosis in humans is not
known, host susceptibility probably influences the size of theinoculum that can cause infection. The lethal dose of L.monocytogenes is reduced substantially in steroid-treatedmice (117). Outbreaks of disease caused by foodborne liste-riae in which the majority of nonpregnant adults were notimmunodeficient (145) might conceivably involve a higherinfectious dose of Listeria spp. than outbreaks in whichillness occurred only in immunosuppressed persons (46, 62,148). Oral-feeding experiments with Sprague-Dawley ratssuggest that infection is dose dependent and that gastricacidity is protective; cimetidine-treated rats were suscepti-ble to lower inocula of L. monocytogenes than animals withnormal gastric acid (144).
Invasive disease in nonpregnant adults. In 1967, Louria etal. first described an association between listeriosis andmalignant disease (94), and more recent reviews suggest thatmost invasive listeriosis occurs in persons who are immuno-suppressed or elderly (111, 122, 147). Use of immunosup-pressive medications for the management of malignanciesand in organ transplantation has increased the immunosup-pressed population and brought increased attention to liste-riosis in the medical literature (122). With the epidemic ofacquired immunodeficiency syndrome (AIDS) and the re-sulting rapid expansion of the population at substantial riskfor listeriosis, this disease may be more frequently diag-nosed.
In their review of adult listeriosis cases reported in theliterature from 1968 to 1978, Nieman and Lorber (122) foundthat most cases of meningitis or bacteremia due to L.monocytogenes occurred in persons with malignancies (27%[40 of 148]) or receiving immunosuppressive treatments fornonmalignant conditions (31% [46 of 148]). Other conditionssuch as alcoholism, diabetes, and cirrhosis accounted forlower proportions of cases. However, in 30% of meningitispatients and 11% of bacteremic patients, no predisposingcondition was recognized.
In a more-recent study of sporadic listeriosis cases occur-
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 175
ring in 1986 to 1987 in a geographically diverse population inthe United States, 88% of nonperinatal cases occurred inpersons with one or more underlying diseases (147). Themost common conditions were cancer (23%), diabetes(20%), renal disease (18%), and heart disease (17%); severalpatients had more than one underlying condition.
Additional underlying illnesses that have been reported inassociation with listeriosis include sarcoidosis, chronic oti-tis, collagen-vascular disease, idiopathic thrombocytopenicpurpura, asthma, ulcerative colitis, and aplastic anemia(122).
Several investigators have reported listeriosis among per-sons with AIDS or human immunodeficiency virus infectionand have commented on the surprisingly "infrequent" oc-currence of listeriosis among this population with substantialdeficiency of cell-mediated immunity (58, 67, 69, 106, 136,137). Although it is not one of the more-common infectionsexperienced by human immunodeficiency virus-infected pa-tients, listeriosis does occur approximately 300 times morefrequently in persons with AIDS than in the general popu-lation (49, 173).Nonpregnant adults with listeriosis most often present
with meningitis, meningoencephalitis, or sepsis. Additionalsyndromes include abscesses of the brain (30) and spinalcord (119), endocarditis (7, 47), endophthalmitis (6), osteo-myelitis (63), and septic arthritis (17). Fever, ataxia, sei-zures, depressed consciousness, and altered mental statusare frequent presenting symptoms of central nervous systemlisteriosis. Spinal fluid examination may show pleocytosiswith predominantly polymorphonuclear leukocytes; Gramstain may show gram-positive bacilli but is more oftenunrevealing; the protein level is elevated; and the glucoselevel usually is within normal limits. However, many otherpatterns have been observed. Diagnosis is made by cultureof L. monocytogenes from spinal fluid, blood, or some otherusually sterile site.
Listeriosis during pregnancy. Listeriosis may develop atany time during pregnancy, although most infections aredetected in the third trimester. Infections occurring earlier inpregnancy may not be recognized if cultures are not ob-tained, and failure to culture the products of conceptionwhen spontaneous abortion or stillbirth occurs complicatesthe problem of estimating the proportion of fetal loss thatmay be attributable to listeriosis. Estimates of the proportionof stillbirths that may be attributed to L. monocytogenesinfection have varied considerably (3, 134, 135). Recentdevelopments in the polymerase chain reaction technique todetect L. monocytogenes in fixed pathologic tissue such asplacenta may eventually lead to improved understanding ofthe number of stillbirths attributable to this organism.
Pregnant women infected with L. monocytogenes mayexperience only a mild flulike illness, with fever, headache,myalgias, and occasionally gastrointestinal symptoms. Pro-dromal illness may affect two-thirds of women with listerio-sis complicating pregnancy (15, 110). The symptoms corre-spond to the bacteremic phase of infection and represent thetime when blood cultures should be taken to permit diagno-sis.
Intrauterine infection most likely results from transplacen-tal transmission following maternal bacteremia, althoughsome intrauterine infection may be the result of ascendingspread from vaginal colonization with Listeria spp. Intra-uterine infection can cause amnionitis, preterm labor, spon-taneous abortion, stillbirth, or early-onset infection of theneonate. Antibiotic treatment during pregnancy can preventneonatal illness, although case reports suggest that even
untreated maternal listeriosis may in rare instances be asso-ciated with the birth of an unaffected infant (65, 93). Micro-biologic screening or prophylactic antibiotics are not cur-rently recommended for pregnant women with a history ofpregnancy-associated listeriosis. However, pregnant womenshould receive appropriate dietary counseling. Early detec-tion and treatment of pregnancy-associated listeriosis will beimproved if all febrile episodes during pregnancy are evalu-ated by blood culture.
Neonatal disease: early onset. Neonatal listeriosis is di-vided into early- and late-onset disease syndromes, similarto the pattern of the more common neonatal infections due togroup B streptococci. Early-onset listeriosis results fromintrauterine infection, which can cause clinical illness in thenewborn at birth or shortly thereafter. The clinical presen-tation of early-onset disease is most often sepsis, althoughgranulomatosis infantisepticum, an overwhelming form ofinfection, occurs less frequently. This syndrome is charac-terized by widely disseminated granulomas in the liver andplacenta as well as other organs. Early-onset disease may beassociated with aspiration of infected amniotic fluid, whichcan lead to respiratory distress. Signs of meningitis are rarein early-onset infection.Neonatal disease: late onset. In contrast to early-onset
listeriosis, late-onset disease occurs several days to weeksafter birth. Infants are generally full term, healthy at birth,and delivered to mothers who have had uncomplicatedpregnancies. Late-onset listeriosis, like late-onset group BStreptococcus disease, is more likely than early-onset dis-ease to present as meningitis. Evidence of central nervoussystem infection was present in 93% (39 of 42) of late-onsetcases reported between 1967 and 1985 in Britain (110). Casefatality rates appear to be lower in late-onset disease than inearly-onset infection. In a retrospective series reported fromBritain, late-onset disease had a case fatality rate of 26% (6of 23) while early-onset disease had a case fatality rate of38% (40 of 104) (110).While vertical transmission from mother to fetus accounts
for early-onset listeriosis, modes of transmission for late-onset disease are poorly understood. Some infections maybe the result of acquisition of the organism during passagethrough the birth canal, although late-onset disease canoccur following cesarian delivery. Some postnatal infectionsmay be the result of nosocomial transmission.
Noninvasive Disease: Mild Syndrome due to Listeriosis
Because listeriosis presents with invasive syndromes suchas meningitis and stillbirth, the association of the illness witha foodborne organism has not always been apparent. Liste-riosis differs from other foodborne diseases in which nonin-vasive syndromes have been well characterized. One mayspeculate that a milder gastrointestinal illness might occurwhen persons without underlying immunosuppression eatfood contaminated by L. monocytogenes. If this were thecase, the numbers of cases of illness caused by Listeria spp.would be much greater than previously estimated. However,investigating mild illness associated with listeriosis has beendifficult. In Bojsen-Moller's study of carriage in the house-hold contacts of patients with listeriosis, establishing anassociation between symptoms and Listeria excretion wasnot possible (13). Three of the persons with stool carriage ofL. monocytogenes may have had diarrhea at the time thesample was obtained. However, information on gastrointes-tinal symptoms of controls was not obtained.The question of noninvasive illness was recently investi-
VOL. 4, 1991
176 SCHUCHAT ET AL.
gated among persons who had attended a catered partyfollowing which two guests delivered infants with listeriosiscaused by the same strain of L. monocytogenes (139). Manyof the partygoers experienced mild gastrointestinal symp-toms. However, stool cultures were not obtained untilseveral weeks after the party, and the outbreak strain of L.monocytogenes was isolated from only one additional guest.Listeria carriage and gastrointestinal symptoms could there-fore not be correlated.
EPIDEMIOLOGIC PATTERNS OF DISEASE
Epidemic Disease
Community outbreaks: foodborne disease. A large outbreakin the Maritime Provinces of Canada provided the firstevidence for transmission of listeriosis by foodborne organ-isms (145). The outbreak was recognized when perinatallisteriosis occurred in 1.3% of births at the maternity hospitalin Halifax, Nova Scotia. Between March and September1981, there were 7 adult and 34 perinatal cases of listeriosis,including 5 spontaneous abortions and 4 stillbirths. The casefatality rate among liveborn infants was 27%. In contrast tomost patients with sporadic disease, nonpregnant adultpatients with listeriosis in this outbreak had no evidence ofan underlying immunosuppressive condition.A case-control study conducted during the investigation
suggested that patients were more likely than controls tohave eaten coleslaw prior to their illness (P = 0.02). Cole-slaw from the refrigerator of one patient grew L. monocyto-genes serotype 4b, the same serotype as the epidemic strain.The coleslaw was prepared in the area, and two unopenedpackages of the coleslaw from the manufacturer also grewthe organism. The manufacturer had purchased cabbagefrom a farmer who also raised sheep. Two of the farmer'ssheep had died of listeriosis the year before the outbreak.The farmer used composted and raw sheep manure tofertilize his cabbage fields, which may have led to contami-nation of the crop. Cabbage was stored over the winter andspring in a large shed; growth of L. monocytogenes wasprobably enhanced by prolonged cold storage. The outbreakinvestigation documented that listeriosis is a foodbornedisease and suggested potential problems with consumptionof uncooked vegetables.Another outbreak of listeriosis may also have involved
raw vegetables. In September and October 1979, 20 patientswere hospitalized in Boston with listeriosis due to serotype4b; only 9 cases had been detected in the preceding 26months (62). Fifteen of 20 patients had hospital-acquiredinfection. Patients were more likely than controls to haveeaten tuna fish, chicken salad, and cheese, although severalbrands were used to prepare each of these foods. However,each food was served with a raw-vegetable garnish, whichmay have been the vehicle of listeriosis, although thesegarnishes were not known to be from a common supplier.Though no single common food exposure was implicated
in this outbreak, the investigation suggested that conditionsin the gastrointestinal tract may alter the risk of an individ-ual's developing listeriosis. Use of antacids or histamine-blocking drugs was more frequent among patients thancontrols. As has been observed for salmonellosis, it ispossible that the lack of gastric acidity increases the chancethat L. monocytogenes will survive passage through thestomach. In this outbreak, patients were also more likelythan controls to have undergone gastrointestinal proceduresprior to culture of L. monocytogenes. Enteritis associated
with Listeria infection might have caused gastrointestinalsymptoms which prompted gastrointestinal procedures, orunderlying gastrointestinal lesions could have increased therisk that intraluminal exposure to L. monocytogenes wouldcause invasive disease.A second large outbreak in Massachusetts occurred from
30 June to 30 August 1983 (41). Most infections occurred innonpregnant adults, all of whom had immunosuppressiveconditions. In addition, there were seven perinatal cases.The overall case fatality rate was 29%. Serotype 4b ac-counted for 32 of the 40 isolates that could be tested. Patientswere more likely than controls matched for neighborhood orunderlying disease to have drunk a specific brand of pasteur-ized whole or 2% fat milk. Additional evidence that pasteur-ized milk caused the outbreak was the presence of a dose-response effect, a protective effect from skim milkconsumption, implication of the same product in a separatestudy done in a different state, and implication of a specificphage type (2425A) of L. monocytogenes in patients who haddrunk the milk. Listeriosis was known to have occurred indairy cows that provided milk to the implicated manufac-turer. Though L. monocytogenes (not of the epidemic type)was isolated from raw milk from these farms, inspections ofthe plant where pasteurization occurred revealed no evi-dence of improper pasteurization. The outbreak raised con-cern that pasteurization may be inadequate to kill Listeriaspp. and led to extensive research concerning the heatresistance of the bacterium (see below). Postpasteurizationcontamination is one likely explanation for this large out-break.The largest epidemic of listeriosis in North America oc-
curred in 1985 in Los Angeles, Calif. (93). The majority ofinfections occurred in pregnant women or their offspring.Perinatal cases included 87 infants with early-onset infec-tions or stillborn and 6 late-onset cases. The case fatality ratewas 63% for early neonatal or fetal infections and 37% fornonneonatal infections.
Eighty-seven percent (81 of 93) of pregnancy-associatedcases occurred in Hispanic women, and a case-control studyimplicated a specific brand of Mexican-style soft cheese asthe vehicle of disease. L. monocytogenes serogroup 4b, ofthe same phage type as the epidemic strain, was isolatedfrom unopened packages of this brand of cheese. In June1985, the product was recalled and the factory was closed.Contamination of the cheese with unpasteurized milk prob-ably led to the outbreak.As calculated from food consumption histories from four
patients who ate the implicated cheese only once, theincubation period for listeriosis was a median of 31 days(range, 11 to 70 days). The interval between eating a con-taminated food and onset of symptoms appears to be muchlonger for listeriosis than for other foodborne diseases,which typically cause symptoms from hours to a few daysafter exposure to contaminated food. Investigation of spo-radic listeriosis is therefore quite complicated, since relevantfood exposures may be those which occurred several weeksprior to the onset of disease.Because the majority of cases in the Los Angeles outbreak
occurred in a single ethnic group primarily cared for at asingle medical facility, the outbreak was recognized quicklyand the implicated food was recalled during the investiga-tion. Much apparently sporadic listeriosis may be the resultof contaminated food sources, but temporal clustering ofcases may not be recognized if the vehicle is widely distrib-uted or if patients present to different medical facilities.Distinguishing sporadic disease from common-source out-
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 177
breaks can be facilitated with serotyping and subtyping ofstrains. In addition, enhanced surveillance for listeriosis canimprove early recognition of outbreaks. In 1986, the Councilof State and Territorial Epidemiologists recommended thatlisteriosis be made a reportable disease.
Soft cheese was responsible for another outbreak oflisteriosis, centered in Switzerland and recognized in 1987(12). The outbreak was traced to a regionally produced softsmear cheese which was distributed to several countries inEurope. Identification of L. monocytogenes in the impli-cated product led to an international food recall. The Listeriastrain that caused this outbreak was the same electropho-retic enzyme type as the strain responsible for the outbreakassociated with Mexican soft cheese (131). Whether certainstrains ofListeria preferentially grow in soft cheeses or otherfoods or are of enhanced virulence is not known.An unusual outbreak of listeriosis occurred in Philadel-
phia, Pa., in 1987 (148). Most (32 of 36) patients werenonpregnant adults, the majority of whom had immunosup-pressive conditions or were elderly. Only 3 of the 36 patientshad no immunosuppressive risk factor for listeriosis. Al-though the clustering of cases represented a threefold in-crease over the background incidence of listeriosis (20 casesper million population versus 7 cases per million), theinvestigation suggested that this increase was not the resultof a single contaminated food source. Patients were morelikely than controls to have eaten ice cream or salami prior todisease onset, but no single brand of either food wasimplicated. In addition, subtyping of the 22 available isolatesfrom patients in the outbreak revealed that multiple strainswere involved. Eleven different enzyme types were identi-fied, including several different enzyme types from patientswho had eaten salami and ice cream. The investigationdemonstrated the important role that subtyping may play inclarifying epidemiologic observations.The investigation revealed that patients were more likely
than controls to suffer gastrointestinal symptoms prior todiagnosis of listeriosis. One-third of patients had vomitingand diarrhea. In addition, more of the family members ofpatients than of controls were ill in the month before onset oflisteriosis in the patient. Although these symptoms in thepatients and their family members could have been due to anenteral phase of listeriosis infection, the investigators spec-ulated that the presence of a coinfecting organism such as anenteric virus might have converted asymptomatic gastroin-testinal carriage of L. monocytogenes to invasive listeriosis.An enteric virus circulating in a community might increasethe risk of individuals with sporadic exposure to L. mono-cytogenes developing disease and consequently might causean outbreak of disease associated with multiple strains of L.monocytogenes.
Nosocomial outbreaks. In addition to the large communityoutbreaks attributed to contaminated food sources, severalnosocomial clusters of neonatal listeriosis have been re-ported (21, 39, 42, 89, 98, 121, 153). Most of the reportsdescribe the delivery of an infant with early-onset infectionfollowed by the diagnosis of late-onset listeriosis in one ormore infants born subsequently (21, 42, 66, 68, 89, 121, 153).In most reports, person-to-person transmission caused byinadequate handwashing and breaks in barrier nursing tech-nique was considered to have caused late-onset infections(21, 66, 68, 98). In some reports, the affected infants hadcommon exposure to resuscitation equipment (121, 153), andin one hospital, inadequate cleaning of a shared rectalthermometer may have caused two cases of necrotizingenterocolitis due to L. monocytogenes, although transmis-
sion from person to person could not be ruled out (89).Nosocomial clusters have been small, and microbiologicefforts have failed to document any common source that wassuggested epidemiologically.
Investigation of a larger nosocomial cluster of listeriosis in1989 in Costa Rica provided the opportunity to evaluate riskfactors for late-onset disease in addition to the vehicle andmodes of transmission responsible for the outbreak (146).More than 3% of infants born during the outbreak perioddeveloped listeriosis. The outbreak investigation suggestedthat transmission occurred when newborns were bathed withmineral oil from a common container, from which theoutbreak strain of L. monocytogenes was cultured. Theoutbreak occurred following the delivery of an infant withearly-onset disease born to a mother with amnionitis. The oilpresumably became contaminated during the delivery of thesource patient. Although all newborns were bathed withcontaminated oil during the subsequent period, those bornby cesarian section were more likely to develop disease.Transmission may have occurred when infants aspiratedcontaminated oil applied on the face or when oil came incontact with mucous membrane surfaces. The respiratoryportal of entry was supported by respiratory presentingsymptoms in several infants and the finding of lipid-ladenmacrophages, consistent with oil aspiration, in lung tissuefrom a patient who died of the disease. General anesthesiaused in cesarian deliveries may have increased the infants'risk of aspirating the oil they were exposed to shortly afterbirth. The outbreak underscored the ability of L. monocyto-genes to persist in the hospital environment once intro-duced, confirming the important role of multiuse materials intransmission of infectious diseases in the delivery room.
Sporadic Listeriosis
Surveillance. The numbers of cases of disease due to L.monocytogenes have been difficult to determine. Voluntaryreporting to state health departments and analysis of hospitaldischarge data have been used to attempt to estimate theincidence of disease in the United States (25, 118). However,such methods have low sensitivity and are likely to under-estimate the true rate of disease.To obtain a more precise estimate of the incidence of
listeriosis in the United States, the Centers for DiseaseControl (CDC) performed active surveillance for listeriosisin 1986 in five states and Los Angeles County (total popula-tion, 34 million) (49). The surveillance project detected anannual incidence of 0.7 cases per 100,000 population. On thebasis of this study, CDC estimates that 1,700 cases oflisteriosis occur annually in the United States and result in450 adult deaths and 100 fetal and postnatal deaths. Theoverall rate of perinatal listeriosis was 12.7 cases per 100,000live births, but Los Angeles County had a rate of perinatallisteriosis significantly higher than that of other areas (24.3/100,000 births versus 7.8/100,000 births). Race-specific ratesof perinatal listeriosis for both blacks and whites were higherin Los Angeles County than elsewhere.These differences could reflect true geographic differences
in incidence, or they might be the result of differences inobstetric practices that could lead to enhanced diagnosis ofperinatal listeriosis in Los Angeles. In 1985, the year pre-ceding the CDC surveillance project, a well-publicized out-break of listeriosis had occurred in Los Angeles (93). In-creased awareness of the disease and more aggressiveattempts at diagnosis by physicians in the area may have
VOL. 4, 1991
178 SCHUCHAT ET AL.
improved detection of listeriosis, resulting in the higher ratesof disease observed in that area.The surveillance project did not detect geographic differ-
ences in the rates of nonperinatal listeriosis (49). The inci-dence of listeriosis increased with age; 84% of the patientswith nonperinatal listeriosis were over 50 years old, and 41%were 70 years or older. In contrast to studies conducted intertiary care centers, the surveillance project found thatbacteremia was a more common presentation of nonperina-tal listeriosis than was meningitis. This difference couldreflect a true change in the presentation of illness fromearlier studies, or it could be a manifestation of differencesbetween the general population and persons referred totertiary care centers.
Dietary risks for sporadic disease. In addition to determin-ing the magnitude of listeriosis in the United States, the CDCactive surveillance project was used to evaluate dietary riskfactors for sporadic disease (147). Patients detected byactive surveillance were enrolled in a case-control study thatcompared dietary histories of persons will listeriosis andthose of controls matched for age, underlying illness orpregnancy, and health care provider (an indirect measure ofgeographic location and socioeconomic status). Persons whohad eaten undercooked chicken or uncooked hot dogs weremore likely to develop listeriosis. Though this epidemiologicstudy did not include laboratory investigation of the actualfoods eaten by patients with listeriosis, the results hadsubstantial plausibility. In various microbiologic surveys ofretail foods, L. monocytogenes was cultured from 15 to 80%of poultry samples and 30% of ready-to-eat meat products(179).Subsequent investigation of sporadic listeriosis has pro-
vided microbiologic support for the initial epidemiologicstudy. In one instance, listeriosis was associated with con-sumption of microwaved turkey franks (23). The sameelectrophoretic enzyme type of L. monocytogenes was iso-lated from a patient with listeriosis, from a package of turkeyfranks in the patient's refrigerator, and from unopenedfranks from a local store. Inspection of the factory resultedin detection of the same strain of L. monocytogenes incultures of environmental surfaces and in final containerpackages ready for sale (174).
Investigation of sporadic listeriosis has also suggested thatcross-contamination of food products occurs in the home(23). The patient's refrigerator which contained the impli-cated turkey franks also contained several other opened fooditems from which the same enzyme type of L. monocyto-genes was cultured. As is the case with salmonellosis,careful food handling may be as important as food produc-tion monitoring for the successful prevention of foodbornelisteriosis.
Possible other sources of sporadic disease. Transmission byfoodborne organisms may account for the largest portion ofsporadic disease, but other modes of transmission may playsome role in occasional cases. Although Listeria spp. havebeen isolated from urethral exudates (149), there is noevidence that sexual transmission contributes to perinatallisteriosis infection. Because L. monocytogenes can surviverefrigeration and because bacteremia may be asymptomatic,transmission of listeriosis through blood transfusion theoret-ically could occur. Such transmission has been documentedfor Yersinia enterocolitica (161), but no such occurrence hasbeen described for L. monocytogenes.
DIAGNOSIS, TREATMENT, AND PREVENTION
Listeriosis is diagnosed when L. monocytogenes is cul-tured from blood, spinal fluid, or some other normally sterilesite. Isolation of Listeria spp. from nonsterile sites such asplacenta and amniotic fluid in association with clinical symp-toms may suggest perinatal listeriosis. Culture of L. mono-cytogenes from stool is not helpful in diagnosis, sincegastrointestinal carriage of the organism may occur withoutclinical disease. Serologic tests do not contribute to diagno-sis of listeriosis, because antibody response may not bespecific (64) and because patients with culture-proven liste-riosis have had undetectable levels of antibody to L. mono-cytogenes (145).
L. monocytogenes is susceptible to a number of antibioticsin vitro, including penicillin G, ampicillin, erythromycin,trimethoprim-sulfamethoxazole, chloramphenicol, rifampin,tetracyclines, and aminoglycosides. Although controlledclinical trials have not been performed, ampicillin or peni-cillin is usually recommended for invasive listeriosis. Ampi-cillin may be superior to penicillin (90). Addition of anaminoglycoside to a beta-lactam antibiotic produces synergyin vitro (176), and this combination therapy is considered thetreatment of choice. Laboratory study suggests that chlor-amphenicol and rifampin may each antagonize the bacteri-cidal effect of penicillins (177). Case reports describe theeffectiveness of treating patients who are allergic to penicil-lin with trimethoprim and sulfamethoxazole (57, 143, 156);this therapy is bactericidal, and adequate drug levels arefound in serum and cerebrospinal fluid (178). Althoughcephalosporins may be bacteriostatic against L. monocyto-genes, treatment failures have been observed (162), andtreatment with cephalosporins is not recommended. Dura-tion of therapy for listeriosis has not been standardized.Reasonable guidelines range from 2 weeks of therapy foruncomplicated sepsis or meningitis to 4 to 6 weeks oftherapy for endocarditis or nonmeningitic disease in theimmunocompromised host (48).Recommendations to prevent listeriosis are still evolving.
Persons at increased risk, such as pregnant women andimmunosuppressed adults, should be advised to avoid eatingunpasteurized milk products and raw or partially cookedmeats (18). Consumption of raw eggs may be a risk factor forlisteriosis (as well as salmonellosis) in some populations(147), and persons at risk should avoid these products. Inaddition, foods should be prepared without cross-contami-nation between raw and cooked foods, and vegetablesshould be washed carefully. Foods prepared in a microwaveoven may be unevenly heated, and sufficient standing timeafter cooking may be important to permit even distributionof heat by conduction without overcooking (99). Internaltemperatures alone may not be adequate to ensure the safetyof all microwaved foods, and new cooking guidelines mayneed to be established to minimize the risk of infectionassociated with eating microwaved foods (92).
ISSUES FOR THE FOOD INDUSTRY
Unusual growth and survival properties of L. monocyto-genes contribute to the complexity of producing Listeria-free foods. The ability of the organism to survive refrigera-tion and freezing has important implications. A low initialinoculum in a food at the time of manufacturing can translateinto a substantial dose of listeriae for the consumer, depend-ing on the shelf life and handling of a particular product. Asnew methods of food preparation are introduced to the
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 179
public (e.g., cook-chill methods [51, 78]), the potential fortransmission of Listeria spp. needs to be considered. On thebasis of epidemiologic and laboratory implications of proc-essed meats as a cause for listeriosis (23, 147) and concernregarding growth of the organism in processed foods beforeuse by the consumer, the USDA in 1989 instituted a zerotolerance policy for L. monocytogenes in ready-to-eat prod-ucts.
Early reports suggested that Listeria spp. were relativelyheat resistant (8). Because L. monocytogenes can be isolatedfrom approximately 5% of unpasteurized milk samples (97,179), the adequacy of pasteurization in eliminating the or-ganism from milk is of more than academic interest. Theissue became important when investigation of an outbreak oflisteriosis in 1983 implicated pasteurized whole and 2% milkas the source of infection (41). Procedures at the milk-processing plant in that outbreak were consistent with reg-ulatory guidelines for pasteurization, raising questions aboutthe adequacy of pasteurization itself. Several studies havesubsequently shown that L. monocytogenes is inactivated bystandard pasteurization (9, 16, 22, 33, 35), although investi-gators have continued to study the effects of growth temper-ature and anaerobic conditions on the survival of L. mono-cytogenes (85, 97). Postpasteurization contamination anddeviation from time and temperature guidelines for pasteur-ization may explain the presence of L. monocytogenes inpasteurized products. Efforts to prevent listeriosis requirethe enforcement of regulatory guidelines for pasteurizationprocedures and the elimination of sources of postpasteuriza-tion contamination.
Species of Listeria other than L. monocytogenes can beisolated frequently from foods and food plants, but suchorganisms do not hold the same pathogenic potential as L.monocytogenes. Therefore, the refinement of molecularmethods for the rapid detection of L. monocytogenes thatare specific at the species level will have important applica-tions in the food industry.The frequency with which L. monocytogenes can be found
during environmental surveys has raised doubts that foodmanufacturers can effectively eliminate L. monocytogenesfrom the workplace. However, investigation of a turkey-processing plant associated with a case of human listeriosisyielded encouraging results (174). Listeria contamination ofthe plant was not widespread; the strain associated withdisease was restricted to the peeler used to process turkeyfranks. Additional industrial studies may improve the abilityto identify areas where Listeria clean-up will be most effec-tive.
SUMMARY
During the 1980s, investigation of several large epidemicsof listeriosis confirmed that transmission of L. monocyto-genes in food causes human disease. Progress in laboratorydetection and subtyping of the organism has enhanced ourability to compare human and environmental isolates of L.monocytogenes. Transmission by foodborne organisms isnow recognized as causing both epidemic and sporadiclisteriosis. Continued study of dietary risk factors associatedwith listeriosis is needed in order to develop dietary recom-mendations for the expanding population at increased risk ofdisease. Current research application of new molecularmethods to the study of L. monocytogenes may improve theability to diagnose pregnancy-associated disease and permitthe rapid detection and control of L. monocytogenes in thefood supply.
REFERENCES1. Anonymous. 1987. Virulence of L. monocytogenes. Third
Forum in Microbiology. Ann. Inst. Pasteur Microbiol. 138:241-284.
2. Anonymous. 1988. Listeria and listeriosis. Infection 16(Suppl.2):S77-5178.
3. Ansbacher, R., K. A. Borchardt, M. W. Hannegan, and W. A.Boyson. 1966. Clinical investigation of L. monocytogenes as apossible cause of human fetal wastage. Am. J. Obstet. Gyne-col. 94:386-390.
4. Audurier, A., R. Chatelain, F. Chalons, and M. Piechaud. 1979.Lysotypie de 823 souches de L. monocytogenes isolees enFrance de 1958 a 1978. Ann. Microbiol. (Paris) 130B:179-189.
5. Audurier, A., J. Rocourt, and A. L. Courtieu. 1977. Phagetyping system for L. monocytogenes, p. 108-121. In I. Ivanov(ed.), Seventh International Symposium on Problems in List-eriosis. National Agroindustrial Union Center for ScientificInformation, Sofia, Bulgaria.
6. Bagnarello, A. G., A. J. Berlin, A. J. Weinstein, M. C.McHenry, and P. S. O'Connor. 1977. L. monocytogenes en-dophthalmitis. Arch. Ophthalmol. 84:337-340.
7. Bassan, R. 1975. Bacterial endocarditis produced by L. mono-cytogenes: case presentation and review of literature. Am. J.Clin. Pathol. 63:522-527.
8. Bearns, R. E., and K. F. Girard. 1958. The effect of pasteur-ization on L. monocytogenes. Can. J. Microbiol. 4:55-61.
9. Beckers, H. J., P. S. S. Soentoro, and E. H. M. Delfgou-vanAsch. 1985. The occurrence of L. monocytogenes in milk. J.Food Prot. 4:249-256.
10. Berche, P., J. L. Gaillard, and S. Richard. 1988. Invasivenessand intracellular growth of L. monocytogenes. Infection16(Suppl. 2):S145-S148.
11. Bibb, W. F., B. Schwartz, B. G. Gellin, B. D. Plikaytis, andR. E. Weaver. 1989. Analysis of L. monocytogenes by multi-locus enzyme electrophoresis and application of the method toepidemiologic investigations. Int. J. Food Microbiol. 8:233-239.
12. Bille, J. 1990. Epidemiology of human listeriosis in Europe,with special reference to the Swiss outbreak, p. 71-74. In A. J.Miller, J. L. Smith, and G. A. Somkuti (ed.), Foodbornelisteriosis. Elsevier, Amsterdam.
13. Bojsen-Moller, J. 1972. Human listeriosis: diagnostic, epidemi-ologic and clinical studies. Acta Pathol. Microbiol. Scand.Sect. B Suppl. 229:72-92.
14. Bortulussi, R., A. Issekutz, and G. Faulkner. 1986. Opsoniza-tion of Listeria monocytogenes type 4b by human adult andnewborn sera. Infect. Immun. 52:493-498.
15. Boucher, M., and M. L. Yonekura. 1984. Listeria meningitisduring pregnancy. Am. J. Perinatol. 1:312-318.
16. Bradshaw, J. G., J. T. Peeler, J. J. Corwin, J. M. Hunt, J. T.Tierney, E. P. Larkin, and R. M. Twedt. 1985. Thermalresistance of L. monocytogenes in milk. J. Food Prot. 48:743-745.
17. Breckenridge, R. L., L. Buck, E. Tooley, and G. W. Doublas.1980. L. monocytogenes in septic arthritis. Am. J. Clin. Pathol.73:140-141.
18. Buchdahl, R., M. Hird, H. Gamsu, A. Tapp, D. Gibb, and C.Tzannatos. Listeriosis revisited: the role of the obstetrician.1990. Br. J. Obstet. Gynecol. 97:186-189.
19. Butman, B. T., M. C. Plank, R. J. Durham, and J. A. Mat-tingly. 1988. Monoclonal antibodies which identify a genus-specific Listeria antigen. Appl. Environ. Microbiol. 54:1564-1569.
20. Cain, D. B., and V. L. McCann. 1986. An unusual case ofcutaneous listeriosis. J. Clin. Microbiol. 23:976-977.
21. Campbell, A. N., P. R. Sill, and J. K. Wardle. 1981. Listeriameningitis acquired by cross-infection in a delivery suite.Lancet ii:752-763.
23. Centers for Disease Control. 1989. Listeriosis associated withconsumption of turkey franks. Morbid. Mortal. Weekly Rep.38:267-268.
VOL. 4, 1991
180 SCHUCHAT ET AL.
24. Chenevert, J., J. Mengaud, E. Gormley, and P. Cossart. 1989.A DNA probe specific for Listeria monocytogenes in the genusListeria. Int. J. Food Microbiol. 8:317-320.
25. Ciesielski, C. A., A. W. Hightower, S. K. Parsons, and C. V.Broome. 1988. Listeriosis in the United States 1980-1982.Arch. Intern. Med. 148:1416-1419.
26. Crawford, R. G., C. M. Beliveau, J. T. Peeler, C. W. Donnelly,and V. K. Bunning. 1989. Comparative recovery of uninjuredand heat-injured Listeria monocytogenes cells from bovinemilk. Appl. Environ. Microbiol. 55:1490-1494.
27. Curtis, G. D. W., R. G. Mitchell, A. F. King, and E. J. Griffin.1989. A selective differential medium for the isolation ofListeria monocytogenes. Lett. Appl. Microbiol. 8:95-98.
28. Datta, A. R., B. A. Wentz, and W. E. Hill. 1987. Detection ofhemolytic Listeria monocytogenes by using DNA colony hy-bridization. Appl. Environ. Microbiol. 53:2256-2259.
29. Datta, A. R., B. A. Wentz, D. Shook, and M. W. Trucksess.1988. Synthetic oligodeoxyribonucleotide probes for detectionof Listeria monocytogenes. Appl. Environ. Microbiol. 54:2933-2937.
30. Dee, R. R., and B. Lorber. 1986. Brain abscess due to L.monocytogenes: case report and literature review. Rev. Infect.Dis. 8:968-977.
31. Dominguez Rodriguez, L., G. S. Fernandez, J. F. F. Garayza-bal, and E. R. Ferri. 1984. New methodology for the isolationof Listeria microorganisms from heavily contaminated envi-ronments. Appl. Environ. Microbiol. 47:1188-1190.
32. Donnelly, C. W., and G. J. Baigent. 1986. Method for flowcytometric detection of Listeria monocytogenes in milk. Appl.Environ. Microbiol. 52:689-695.
33. Donnelly, C. W., E. H. Briggs, and L. S. Donnelly. 1987.Comparison of heat resistance of L. monocytogenes in milk asdetermined by two methods. J. Food Prot. 50:14-17.
34. Doyle, M. P., and J. L. Schoeni. 1987. Comparison of proce-dures for isolating Listeria monocytogenes in soft surface-ripened cheese. J. Food Prot. 50:4-6.
35. Farber, J. M., G. W. Sanders, D. B. Emmons, and R. C.McKellar. 1987. Heat resistance of L. monocytogenes in arti-ficially-inoculated and naturally-contaminated raw milk. J.Food Prot. 50:893.
36. Farber, J. M., G. W. Sanders, and J. I. Speirs. 1988. Method-ology for isolation of listeria from foods: a Canadian perspec-tive. J. Assoc. Off. Anal. Chem. 71:675-678.
37. Farber, J. M., and J. I. Speirs. 1987. Monoclonal antibodiesdirected against flagellar antigens of Listeria species and theirpotential in EIA-based methods. J. Food Prot. 50:479-484.
38. Felsenfeld, 0. 1951. Diseases of poultry transmission to man.Iowa State Coll. Vet. 13:89-92.
39. Filice, G. A., H. F. Cantreli, A. B. Smith, P. S. Hayes, J. C.Feeley, and D. W. Fraser. 1978. Listeria monocytogenes infec-tion in neonates: investigation of an epidemic. J. Infect. Dis.138:17-23.
40. Flamm, R. K., D. J. Hinrichs, and M. F. Thomashow. 1989.Cloning of a gene encoding a major secreted polypeptide ofListeria monocytogenes and its potential use as a species-specific probe. Appl. Environ. Microbiol. 55:2251-2256.
41. Fleming, D. W., S. L. Cochi, K. L. MacDonald, J. Brondum,P. S. Hayes, B. D. Plikaytis, M. B. Holmes, A. Audurier, C. V.Broome, and A. L. Reingold. 1985. Pasteurized milk as avehicle of infection in an outbreak of listeriosis. N. Engl. J.Med. 312:404-407.
42. Florman, A. L., and V. Sundararajan. 1968. Listeriosis amongnursery mates. Pediatrics 41:784-788.
43. Fraser, J. A., and W. H. Sperber. 1988. Rapid detection ofListeria spp. in food and environmental samples by esculinhydrolysis. J. Food Prot. 51:762-765.
44. Gaillard, J. L., P. Berche, J. Mounier, S. Richard, and P.Sansonetti. 1987. Penetration of L. monocytogenes into thehost: a crucial step of the infectious process. Ann. Microbiol.138:259-264.
45. Gaillard, J. L., P. Berche, J. Mounier, S. Richard, and P.Sansonetti. 1987. In vitro model of penetration and intracellulargrowth of Listeria monocytogenes in the human enterocyte-
like cell line Caco-2. Infect. Immun. 55:2822-2829.46. Gaillard, J. L., P. Berche, and P. Sansonetti. 1986. Transposon
mutagenesis as a tool to study the role of hemolysin in thevirulence of Listeria monocytogenes. Infect. Immun. 52:50-55.
47. Gallagher, P. G., C. A. Amedia, and C. Watanakunakorn.1986. L. monocytogenes endocarditis in a patient on chronichemodialysis, successfully treated with vancomycin-gentami-cin: case report. Infection 14:125-128.
48. Gellin, B. G., and C. V. Broome. 1989. Listeriosis. J. Am. Med.Assoc. 261:1313-1320.
49. Gellin, B. G., C. V. Broome, W. F. Bibb, R. Weaver, S.Gaventa, L. Mascola, and the Listeriosis Study Group. Am. J.Epidemiol., in press.
50. Geoffroy, C., J. L. Gaillard, J. E. Alouf, and P. Berche. 1987.Purification, characterization and toxicity of the sulfhydryl-activated hemolysin listeriolysin 0 from Listeria monocyto-genes. Infect. Immun. 138:369-371.
51. Gilbert, R. J., K. L. Miller, and D. Roberts. 1989. L. monocy-togenes and chilled foods. Lancet i:383-384.
52. Golden, D. A., L. R. Beuchat, and R. E. Brackett. 1988.Evaluation of selective direct plating media for their suitabilityto recover uninjured heat-injured and freeze-injured Listeriamonocytogenes from foods. Appl. Environ. Microbiol. 54:1451-1456.
53. Gray, M. L., and A. H. Killinger. 1966. Listeria monocyto-genes and listeric infections. Bacteriol. Rev. 30:309-382.
54. Gray, M. L., H. J. Stafseth, F. Thorp, Jr., L. B. Sholl, andW. F. Riley, Jr. 1948. A new technique for isolating listerellaefrom the bovine brain. J. Bacteriol. 55:471-476.
55. Gregorio, S. B., and W. C. Eveland. 1975. Isolation of L.monocytogenes from inapparent sources in Michigan, p. 87-93. In M. Woodbine (ed.), Problems of listeriosis. LeicesterUniversity Press, Leicester, England.
56. Grimont, F., and P. A. D. Grimont. 1986. Ribosomal ribonu-cleic acid gene restriction patterns as potential taxonomictools. Ann. Inst. Pasteur/Microbiol. (Paris) 137B:165-175.
57. Gunther, G., and A. Philipson. 1988. Oral trimethoprim asfollow-up treatment of meningitis caused by L. monocyto-genes. Rev. Infect. Dis. 10:53-55.
58. Harvey, R. L., and P. H. Chandrasekar. 1988. Chronic menin-gitis caused by Listeria in a patient infected with humanimmunodeficiency virus. J. Infect. Dis. 157:1091.
59. Hayes, P. S., J. C. Feeley, L. M. Graves, G. W. Ajello, andD. W. Fleming. 1986. Isolation of Listeria monocytogenesfrom raw milk. Appl. Environ. Microbiol. 51:438-440.
60. Hayes, P. S., L. M. Graves, G. W. Ajello, and B. Swaminathan.1990. Abstr. Annu. Meet. Inst. Food Technol., no. 406, p. 180.
61. Helsel, L., S. Johnson, W. DeWitt, and W. Bibb. 1990. Char-acterization of monoclonal antibodies to the hemolysin ofListeria monocytogenes and use of the antibodies in affinitypurification of the protein, abstr. B-168, p. 54. Abstr. 90thAnnu. Meet. Am. Soc. Microbiol. 1990. American Society forMicrobiology, Washington, D.C.
62. Ho, J. L., K. N. Shands, G. Friedland, P. Eckind, and D. W.Fraser. 1986. An outbreak of type 4b Listeria monocytogenesinfection involving patients from eight Boston hospitals. Arch.Intern. Med. 146:520-524.
63. Houang, E. T., C. J. Williams, and P. F. M. Wrigley. 1976.Acute L. monocytogenes osteomyelitis. Infection 4:113-114.
64. Hudak, A. P., S. H. Lee, A. C. Issekutz, and R. Bortolussi.1984. Comparison of three serological methods: enzyme-linkedimmunosorbent assay, complement fixation, and microaggluti-nation in the diagnosis of human perinatal L. monocytogenesinfection. Clin. Invest. Med. 7:349-354.
65. Hume, 0. S. 1976. Maternal L. monocytogenes septicemia withsparing of the fetus. Obstet. Gynecol. 48(Suppl.):33S-34S.
66. Isaacs, D., and M. N. Libermann. 1981. Babies cross-infectedwith L. monocytogenes. Lancet ii:940.
67. Jacobs, J. L., and H. W. Murray. 1986. Why is Listeriamonocytogenes not a pathogen in the acquired immunodefi-ciency syndrome? Arch. Intern. Med. 146:1299-1300.
68. Kachel, W., and H. G. Leonard. 1981. Babies cross-infectedwith L. monocytogenes. Lancet ii:939-940.
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 181
69. Kales, C. P., and R. S. Holzman. 1990. Listeriosis in patientswith HIV infection: clinical manifestations and response totherapy. J. Acquired Immune Defic. Syndr. 3:139-143.
70. Kampelmacher, E. H., W. T. Huysinga, and L. M. van NoorleJansen. 1972. The presence of Listeria monocytogenes in fecesof pregnant women and neonates. Zentralbl. Bakteriol. Mikro-biol. Hyg. Abt. I Orig. Reihe A 222:258-262.
71. Kampelmacher, E. H., and D. A. A. Mossel. 1989. Listeriamonocytogenes: attributes and prevention of transmission byfood. In Culture 10. Oxoid, London.
72. Kampelmacher, E. H., and L. M. van Noorle Jansen. 1969.Isolation of Listeria monocytogenes from feces of clinicallyhealthy humans and animals. Zentralbi. Bakteriol. Mikrobiol.Hyg. Abt. 1 211:353-359.
73. Kampelmacher, E. H., and L. M. van Noorle Jansen. 1972.Further studies on the isolation of Listeria monocytogenes inclinically healthy individuals. Zentralbl. Bakteriol. Mikrobiol.Hyg. Abt. I Orig. Reihe A 221:70-77.
74. Kathariou, S., P. Metz, H. Hof, and W. Goebel. 1987. Tn916-induced mutations in the hemolysin determinant affectingvirulence of Listeria monocytogenes. J. Bacteriol. 169:1291-1297.
75. Kathariou, S., and L. Pine. 1990. Laboratory studies of viru-lence of L. monocytogenes, p. 55-60. In A. J. Miller, J. L.Smith, and G. A. Somkuti (ed.), Foodborne listeriosis.Elsevier, Amsterdam.
76. Kathariou, S., J. Rocourt, H. Hof, and W. Goebel. 1988. Levelsof Listeria monocytogenes hemolysin are not directly propor-tional to virulence in experimental infections of mice. Infect.Immun. 56:534-536.
77. Kempton, G. 1989. Listeria monocytogenes: a protocol forrapid identification. Med. J. Austr. 150:607.
78. Kerr, K., S. F. Dealler, and R. W. Lacey. 1988. Listeria incook-chill food. Lancet ii:37-39.
79. Kim, C. 1990. Ph.D. dissertation. Purdue University, WestLafayette, Ind.
80. Kim, C., L. M. Graves, B. Swaminathan, L. W. Mayer, andR. E. Weaver. 1991. Evaluation of hybridization characteris-tics of a cloned pRF106 probe for Listeria monocytogenesdetection and development of a nonisotopic colony hybridiza-tion assay. 57:289-294.
81. Kim, C., B. Swaminathan, B. Holloway, R. Pinner, and V. A.Varma. 1990. Detection of Listeria monocytogenes in forma-lin-fixed paraffin-embedded tissues using a 2-stage nested poly-merase chain reaction, abstr. D-54, p. 89. Abstr. 90th Annu.Meet. Am. Soc. Microbiol. 1990. American Society for Micro-biology, Washington, D.C.
82. King, W., S. Raposa, J. Warshaw, A. Johnson, D. Halbert, andJ. D. Klinger. 1989. A new colorimetric nucleic acid hybrid-ization assay for Listeria in foods. Int. J. Food Microbiol.8:225-232.
83. Klinger, J. D. 1988. Isolation of Listeria: a review of proce-dures and future prospects. Infection 16(Suppl.):S98-S105.
84. Klinger, J. D., A. Johnson, D. Croan, P. Flynn, K. Whippie, M.Kimball, J. Lawrie, and M. Curiale. 1988. Comparative studieson nucleic acid hybridization assay for Listeria in foods. J.Assoc. Off. Anal. Chem. 71:669-673.
85. Knabel, S. J., H. W. Walker, P. A. Hartman, and A. F.Mendonca. 1990. Effects of growth temperature and strictlyanaerobic recovery on the survival of Listeria monocytogenesduring pasteurization. Appl. Environ. Microbiol. 56:370-376.
86. Kokubo, Y., T. lida, S. Kaneko, and T. Maruyama. 1990.Evaluation of enrichment and plating media for isolating List-eria monocytogenes from raw meat. J. Food Hyg. Soc. Jpn.31:51-56.
87. Lammerding, A. M., and M. P. Doyle. 1989. Evaluation ofenrichment procedures for recovering Listeria monocytogenesfrom dairy products. Int. J. Food Microbiol. 9:249-268.
88. Lamont, R. J., and R. Postlethwaite. 1986. Carriage of Listeriamonocytogenes and related species in pregnant and non-pregnant women in Aberdeen, Scotland. J. Infect. 13:187-193.
89. Larsson, S. 1978. Listeria monocytogenes causing hospital-acquired enterocolitis and meningitis in newborn infants. Br.
Med. J. 2:473-474.90. Lavetter, A., J. L. Leedom, A. W. Mathies, D. Ivler, and P. F.
Wehrle. 1971. Meningitis due to L. monocytogenes. A reviewof 25 cases. N. Engl. J. Med. 285:598-603.
91. Lee, W. H., and D. McClain. 1986. Improved Listeria mono-cytogenes selective agar. Appl. Environ. Microbiol. 52:1215-1217.
92. Lindsay, R. E., W. A. Krissinger, and B. F. Fields. 1986.Microwave vs. conventional oven cooking of chicken: relation-ship of internal temperature to surface contamination by Sal-monella typhimurium. J. Am. Diet. Assoc. 86:373-374.
93. Linnan, M. J., L. Mascola, X. D. Lou, V. Goulet, S. May, C.Salminen, D. W. Hird, L. Yonekura, P. Hayes, R. Weaver, A.Audurier, B. D. Plikaytis, S. L. Fannin, A. Kieks, and C. V.Broome. 1988. Epidemic listeriosis associated with Mexican-style cheese. N. Engl. J. Med. 319:823-828.
94. Louria, D. B., T. Hensle, D. Armstrong, H. S. Collins, A.Blevins, D. Krugman, and M. Buse. 1967. Listeriosis compli-cating malignant disease: a new association. Ann. Intern. Med.67:261-281.
95. Lovett, J. 1988. Isolation and identification of Listeria mono-cytogenes in dairy products. J. Assoc. Off. Anal. Chem.71:658-660.
96. Lovett, J., I. V. Wesley, M. J. Vandermaaten, J. G. Bradshaw,D. W. Francis, R. G. Crawford, C. W. Donnelly, and J. W.Messer. 1990. High-temperature short-time pasteurization in-activates Listeria monocytogenes. J. Food Prot. 53:734-738.
97. Lovett, J., D. W. Francis, and J. M. Hunt. 1987. L. monocy-togenes in raw milk: detection, incidence, and pathogenicity.J. Food Prot. 50:188-192.
98. Lubani, M. M., D. C. Sharda, T. Al-Shab, and S. Sethi. 1987.Neonatal listeriosis: a report of seven cases. Ann. Trop.Paediatr. 7:42-46.
99. Lund, B. M., M. R. Knox, and M. B. Cole. 1989. Destruction ofL. monocytogenes during microwave cooking. Lancet i:218.
100. MacDonald, T. T., and P. B. Carter. 1980. Cell-mediatedimmunity to intestinal infection. Infect. Immun. 28:516-523.
101. MacGowan, A. P., R. J. Marshall, and D. S. Reeves. 1989.Evaluation of API 20 STREP system for identifying Listeriaspecies. J. Clin. Pathol. 42:548-550.
102. Mackaness, G. B. 1962. Cellular resistance to infection. J. Exp.Med. 116:381-406.
103. Mackaness, G. B. 1969. The influence of immunologicallycommitted lymphoid cells on macrophage activity in vivo. J.Exp. Med. 129:973-992.
104. Mackaness, G. B. 1969. The effect of anti-lymphocyte globulinon cell-mediated resistance to infection. J. Exp. Med. 129:993-1012.
105. Manev, C., M. Yanakieva, E. Ivanova, R. Slavova, R. Kirov,and G. Planski. 1975. Healthy Bulgarians as Listeria carriers,p. 84-86. In M. Woodbine (ed.), Problems of listeriosis.Leicester University Press, Leicester, England.
106. Mascola, L., L. Lieb, J. Chiu, S. L. Fannin, and M. J. Linnan.1988. Listeriosis: an uncommon opportunistic infection inpatients with acquired immunodeficiency syndrome. Am. J.Med. 84:162-164.
107. Mattingly, J. A., B. T. Butman, M. C. Plank, R. J. Durham,and B. J. Robison. 1988. Rapid monoclonal antibody-basedELISA for detection of Listeria in food products. J. Assoc.Off. Anal. Chem. 71:679-681.
108. McBride, M. E., and K. F. Girard. 1960. A selective mediumfor the isolation of Listeria monocytogenes from mixed bacte-rial populations. J. Lab. Clin. Med. 55:153-157.
109. McClain, D., and W. H. Lee. 1988. Development of USDA-FSIS method for isolation of Listeria monocytogenes from rawmeat and poultry. J. Assoc. Off. Anal. Chem. 71:660-664.
110. McLauchlin, J. 1990. Human listeriosis in Britain, 1967-85, asummary of 722 cases. 1. Listeriosis during pregnancy and inthe newborn. Epidemiol. Infect. 104:181-189.
111. McLauchlin, J. 1990. Human listeriosis in Britain, 1967-85, asummary of 722 cases. 2. Listeriosis in non-pregnant individ-uals, a changing pattern of infection and seasonal incidence.Epidemiol. Infect. 104:191-201.
VOL. 4, 1991
182 SCHUCHAT ET AL.
112. McLauchlin, J., A. Audurier, and A. G. Taylor. 1986. Theevaluation of a phage-typing system for L. monocytogenes foruse in epidemiological studies. J. Med. Microbiol. 22:357-365.
113. McLauchlin, J., A. Black, H. T. Green, J. Q. Nash, and A. G.Taylor. 1988. Monoclonal antibodies show Listeria monocyto-genes in necropsy tissue samples. J. Clin. Pathol. (London)41:983-988.
114. McLauchlin, J., D. Samuel, and A. G. Taylor. 1989. The use ofmonoclonal antibodies to demonstrate Listeria monocytogenesin post mortem tissue using a direct immunofluorescencetechnique and to detect a soluble antigen in CSF supernatantsusing an ELISA. Acta Microbiol. Hung. 36:303-308.
115. Mengaud, J., M. I. Vicente, J. Chenevert, J. M. Pereira, C.Geoffrey, B. Gicquel-Sanzey, F. Baquero, J. C. Perez-Diaz, andP. Cossart. 1988. Expression in Escherichia coli and sequenceanalysis of the listeriolysin 0 determinant of Listeria monocy-togenes. Infect. Immun. 56:766-772.
116. Mielke, M., S. Ehlers, and H. Hahn. 1988. The role of T cellsubpopulations in cell mediated immunity to facultative intra-cellular bacteria. Infection 16(Suppl. 2):S123-S127.
117. Miller, J. K., and M. Hedberg. 1965. Effects of cortisone onsusceptibility of mice to L. monocytogenes. Am. J. Clin.Pathol. 43:248-250.
118. Moore, R. M., and R. B. Zehmer. 1973. Listeriosis in theUnited States 1971. J. Infect. Dis. 127:610-611.
119. Morrison, R. E., J. Brown, and R. S. Goodling. 1980. Spinalcord abscess caused by L. monocytogenes. Arch. Neurol.37:243-244.
120. Murray, E. G. D., R. A. Webb, and M. B. R. Swann. 1926. Adisease of rabbits characterized by a large mononuclear leuco-cytosis, caused by a hitherto undescribed bacillus, Bacteriummonocytogenes. J. Pathol. Biol. 29:407-439.
121. Nelson, K. E., D. Warren, A. M. Tomasi, T. N. Raju, and D.Vidyasagar. 1985. Transmission of neonatal listeriosis in adelivery room. Am. J. Dis. Child. 139:903-905.
122. Nieman, R. E., and B. Lorber. 1980. Listeriosis in adults: achanging pattern. Report of eight cases and review of theliterature, 1968-1978. Rev. Infect. Dis. 2:207-227.
123. Nocera, D. A., M. H. Fonjallaz, E. Bannerman, J. C. Piffaretti,J. Rocourt, and J. Bille. 1989. Abstr. Annu. Meet. Am. Soc.Microbiol. 1989, D-233, p. 121.
124. Northolt, M. D. 1989. Recovery of Listeria monocytogenesfrom daily products using the TNCB-TNSA method and draftIDF methods. Neth. Milk Dairy J. 43:299-310.
125. Notermans, S., T. Chakraborty, M. Leimeister-Waechter, J.Dufrenne, K. J. Heuvelman, H. Maas, W. Jansen, K. Wernars,and P. Guinee. 1989. Specific gene probe for detection ofbiotyped and serotyped Listeria strains. Appl. Environ. Micro-biol. 55:902-906.
126. Okwumabua, O., B. Swaminathan, P. Edmonds, M. Alden, andJ. Hogan. 1990. Presented at the Sixth International Congresson Rapid Methods and Automation in Microbiology and Im-munology, Helsinki, Finland.
127. Owen, C. R., A. Meis, J. W. Jackson, and H. G. Stoenner. 1960.A case of primary cutaneous listeriosis. N. Engl. J. Med.262:1026-1028.
128. Parish, M. E., and D. P. Higgins. 1989. Extinction of Listeriamonocytogenes in single-strength orange juice: comparison ofmethods for detection in mixed populations. J. Food Safety9:267-278.
129. Perez-Diaz, J. C., M. F. Vicente, and F. Baquero. 1982.Plasmids in Listeria. Plasmid 8:112-118.
130. Petit, J. C., B. Burghoffer, and G. L Daguet. 1985. Suppressionof cellular immunity to Listeria monocytogenes by activatedmacrophages: mediation by prostaglandins. Infect. Immun.49:383-388.
131. Piffaretti, J. C., H. Kressebuch, M. Aeschbacher, J. Bile, E.Bannerman, J. M. Musser, R. K. Selander, and J. Rocourt.1989. Genetic characterization of clones of the bacterium L.monocytogenes causing epidemic disease. Proc. Natl. Acad.Sci. USA 86:3818-3822.
132. Pini, P. N., and R. J. Gilbert. 1988. A comparison of twoprocedures for the isolation of Listeria monocytogenes from
raw chickens and soft cheeses. Int. J. Food Microbiol. 7:331-338.
133. Potel, J. 1953-1954. Atiologie der granulomatosis Infantisep-tica. Wiss. Z. Martin Luther Univ. 3:341-364.
134. Rabau, E., and A. David. 1963. L. monocytogenes in abortion.J. Obstet. Gynecol. 70:481-482.
135. Rappaport, F., M. Rabinovitz, R. Toaff, and N. Krochic. 1960.Genital listeriosis as a cause of repeated abortion. Lanceti:1273-1275.
136. Read, E. J., J. M. Orenstein, T. L. Chorba, A. M. Schwartz,G. L. Simon, J. H. Lewis, and R. S. Schulof. 1985. Listeriamonocytogenes sepsis and small cell carcinoma of the rectum:an unusual presentation of the acquired immunodeficiencysyndrome. Am. J. Clin. Pathol. 83:385-389.
137. Real, F. X., J. W. Gold, S. E. Krown, and D. Armstrong. 1984.Listeria monocytogenes bacteremia in the acquired immuno-deficiency syndrome. Ann. Intern. Med. 101:883.
138. Redline, R. W., and C. Y. Lu. 1987. Role of local immunosup-pression in murine fetoplacental listeriosis. J. Clin. Invest.79:1234-1241.
139. Riedo, F. X., R. W. Pinner, M. Tosca, M. L. Cartter, L. M.Graves, M. W. Reeves, B. D. Plikaytis, and C. V. Broome. 1990.A point source foodborne listeriosis outbreak: documentedincubation period and possible mild illness, abstr. no. 972, p.248. Program Abstr. 30th Intersci. Conf. Antimicrob. AgentsChemother. American Society for Microbiology, Washington,D.C.
140. Robison, B. J., T. Donlery, S. Keelan, and R. S. Flowers. 1988.Abstr. Annu. Meet. Am. Soc. Microbiol. 1988, P35, p. 279.
141. Saunders, N. A., A. M. Ridley, and A. G. Taylor. 1989. Typingof Listeria monocytogenes for epidemiological studies usingDNA probes. Acta Microbiol. Hung. 36:205-210.
142. Schaffner, A., and P. Rellstab. 1988. Gamma-interferon re-stores listericidal activity and concurrently enhances release ofreactive oxygen metabolites in dexamethasone-treated humanmonocytes. J. Clin. Invest. 82:913-919.
143. Scheer, M. S., and S. Z. Hirschman. 1982. Oral and ambulatorytherapy of listeria bacteremia and meningitis with trimetho-prim-sulfamethoxazole. Sinai J. Med. (New York) 49:411-414.
144. Schlech, W. F. 1990. Listeria, animals and man: aspects ofvirulence, p. 51-54. In A. J. Miller, J. L. Smith, and G. A.Somkuti (ed.), Foodborne listeriosis. Elsevier, Amsterdam.
145. Schlech, W. F., P. M. Lavigne, R. A. Bortolussi, A. C. Allen,E. V. Haldane, A. J. Wort, A. W. Hightower, S. E. Johnson,S. H. King, E. S. Nicholls, and C. V. Broome. 1983. Epidemiclisteriosis-evidence for transmission by food. N. Engl. J.Med. 308:203-206.
146. Schuchat, A., C. Lizano, C. V. Broome, B. Swaminathan, C.Kim, and K. Winn. Pediatr. lnfect. Dis. J., in press.
147. Schwartz, B., C. A. Ciesielski, C. V. Broome, S. Gaventa, G. R.Brown, B. G. Gellin, A. W. Hightower, L. Mascola, and theListeriosis Study Group. 1988. Association of sporadic listeri-osis with consumption of uncooked hotdogs and undercookedchicken. Lancet ii:779-782.
148. Schwartz, B., D. Hexter, C. V. Broome, A. W. Hightower,R. B. Hirschhorn, J. D. Porter, P. S. Hayes, W. F. Bibb, B.Lorber, and D. G. Faris. 1989. Investigation of an outbreak oflisteriosis: new hypothesis for the etiology of epidemic Listeriamonocytogenes infections. J. Infect. Dis. 159:680-685.
149. Seeliger, H. P. R. 1961. Listeriosis. Hafner Publishing Co.,New York.
150. Seeliger, H. P. R., and K. Hohne. 1979. Serotyping of L.monocytogenes and related species, p. 31-49. In T. Bergan andJ. R. Norris (ed.), Methods in microbiology, vol. 13. AcademicPress, Inc., New York.
151. Seeliger, H. P. R., and D. Jones. 1986. Listeria, p. 1235-1245.In P. H. Sneath et al. (ed.), Bergey's manual of systematicbacteriology, vol. 2. The Williams & Wilkins Co., Baltimore.
152. Selander, R. K., D. A. Caugant, H. Ochman, J. M. Musser,M. N. Gilmour, and T. S. Whittman. 1986. Methods of multi-locus enzyme electrophoresis for bacterial population geneticsand systematics. Appl. Environ. Microbiol. 51:873-884.
153. Simmons, M. D., P. M. Cockcroft, and 0. A. Okubadejo. 1986.
CLIN. MICROBIOL. REV.
EPIDEMIOLOGY OF HUMAN LISTERIOSIS 183
Neonatal listeriosis due to cross-infection in an obstetrictheatre. J. Infect. 13:235-239.
154. Smith, J. L., and D. L. Acher. 1988. Heat-induced injury inListeria monocytogenes. J. Industr. Microbiol. 3:105-110.
155. Smith, J. L., and S. E. Hunter. 1988. Heat injury in Listeriamonocytogenes. Lebensm.-Wiss. Technol. 21:307-311.
156. Spitzer, P. G., S. M. Hammer, and W. Karchmer. 1986.Treatment of L. monocytogenes infection with trimethoprim-sulfamethoxazole: case report and review of the literature.Rev. Infect. Dis. 8:427-430.
157. Stanley, N. F. 1950. Studies on L. monocytogenes. III. Thefailure to isolate the organism from the human throat. Aust. J.Exp. Biol. 28:117-119.
158. Swaminathan, B., L. M. Graves, G. M. Carlone, and B. D.Plikaytis. 1988. Abstr. Annu. Meet. Am. Soc. Microbiol. 1988,D-61, p. 81.
159. Swaminathan, B., and P. S. Hayes. 1989. Sampling, isolationand rapid detection, p. 21-37. In G. L. Jones (ed.), Isolationand identification of Listeria monocytogenes. U.S. Depart-ment of Health and Human Services, Centers for DiseaseControl, Atlanta.
160. Sword, C. P. 1966. Mechanisms of pathogenesis in Listeriamonocytogenes. I. Influence of iron. J. Bacteriol. 92:536-542.
161. Tipple, M. A., L. A. Bland, J. J. Murphy, M. J. Arduino, A. L.Panlilio, J. J. Farmer, M. A. Tourault, C. R. Macpherson, J. E.Menitove, A. J. Grindon, P. S. Johnson, R. G. Strauss, J. A.Bufill, P. S. Ritch, J. R. Archer, 0. C. Tablan, and W. R.Jarvis. 1990. Sepsis associated with transfusion of red cellscontaminated with Yersinia enterocolitica. Transfusion 30:207-213.
162. Trautmann, M., J. Wagner, M. Chahin, and T. Weinke. 1985.Listeria meningitis: report of ten recent cases and review ofcurrent therapeutic recommendations. J. Infect. 10:107-114.
163. Tripp, C. S., P. Needleman, and E. R. Unanue. 1987. Indo-methacin in vivo increases the sensitivity to Listeria infectionin mice: a possible role for macrophage thromboxane A2synthesis. J. Clin. Invest. 79:399-403.
164. U.S. Food and Drug Administration. 1988. Bacteriologicalanalytical manual, chapter 29-Listeria isolation; revisedmethod of analysis: notice. Fed. Regist. 53:44147-44153.
165. Van-Netten, P., I. Perales, A. Van-De-MoosdUk, G. D. W.Curtis, and D. A. A. Mossel. 1989. Liquid and solid selectivedifferential media for the detection and enumeration of Listeriamonocytogenes and other Listeria spp. Int. J. Food Microbiol.8:299-316.
166. Vogt, R. L., C. Donnelly, B. Gellin, W. Bibb, and B. Swami-
nathan. 1990. Linking environmental and human strains ofListeria monocytogenes with isoenzyme and ribosomal RNAtyping. Eur. J. Epidemiol. 6:229-230.
167. Watkins, J., and K. P. Sleath. 1981. Isolation and enumerationof L. monocytogenes from sewage, sewage sludge, and riverwater. J. Appl. Bacteriol. 50:1-9.
168. Weaver, R. E. 1989. Serotyping and other typing techniques, p.45-49. In G. L. Jones (ed.), Isolation and identification ofListeria monocytogenes. U.S. Department of Health and Hu-man Services, Centers for Disease Control, Atlanta.
169. Weinberg, E. D. 1984. Pregnancy-associated depression ofcell-mediated immunity. Rev. Infect. Dis. 6:814-830.
170. Weis, J. 1975. The incidence of L. monocytogenes on plantsand in soil, p. 61-65. In M. Woodbine (ed.), Problems oflisteriosis. Leicester University Press, Leicester, England.
171. Welshimer, H. J. 1960. Survival of Listeria monocytogenes insoil. J. Bacteriol. 80:316-320.
172. Welshimer, H. J. 1968. Isolation of Listeria monocytogenesfrom vegetation. J. Bacteriol. 95:300-303.
173. Wenger, J. D., R. Pinner, A. Schuchat, R. Pierce, and C. V.Broome. 1990. Epidemiology of human listeriosis, abstr. S-45,p. 324. Program Abstr. 30th Intersci. Conf. Antimicrob.Agents Chemother. American Society for Microbiology,Washington, D.C.
174. Wenger, J. D., B. Swaminathan, P. S. Hayes, S. S. Green, M.Pratt, R. W. Pinner, A. Schuchat, and C. V. Broome. 1990.Listeria monocytogenes contamination of turkey franks: eval-uation of a production facility. J. Food Prot. 53:1015-1019.
175. Wesley, I., R. Ruby, J. Heisick, and F. Ashton. 1990. Restric-tion enzyme analysis of epidemic strains of Listeria monocy-togenes, abstr. P-54, p. 287. Abstr. 90th Annu. Meet. Am. Soc.Microbiol. 1990. American Society for Microbiology, Wash-ington, D.C.
176. Wiggins, G. L., W. L. Albritton, and J. C. Feeley. 1978.Antibiotic susceptibility of clinical isolates of L. monocyto-genes. Antimicrob. Agents Chemother. 13:854-860.
177. Winslow, D. L., J. Damme, and E. Dieckman. 1983. Delayedbactericidal activity of beta-lactam antibiotics against L. mono-cytogenes: antagonism of chloramphenicol and rifampin. An-timicrob. Agents Chemother. 23:555-558.
178. Winslow, D. L., and G. A. Pankey. 1982. In vitro activities oftrimethoprim and sulfamethoxazole against L. monocyto-genes. Antimicrob. Agents Chemother. 22:51-54.
179. World Health Organization. 1988. Report of the informal work-ing group on foodbome listeriosis. Bull. W.H.O. 66:421-428.
