Bull. Org. mond. Sante 1972, 47, 161-169Bull. Wid Hith Org.

Public health importance of rodentsin South AmericaR. B. MACKENZIE'

Indigenous South American rodents are abundant, varied, and adaptable, and occupymost ofthe available natural habitats. Knowledge oftheir taxonomy and biology is generallysuperficial. Near human habitations the introduced Rattus and Mus are common and theircontacts with man are often close. Cities in South America are expanding to keep pacewith increases in the human population and hitherto virgin land is being settled or clearedfor food production. Thus domestic rodents are brought into contact with indigenousspecies and the inevitable exchange of parasites may then produce unpredictable threatsto human health. The role ofboth wild and domestic rodents in the transmission of certaininfectious diseases, such as plague, sylvatic Venezuelan encephalitis, South Americanhaemorrhagic fevers, murine typhus, and cutaneous leishmaniasis, is well established. Theinvolvement of rodents in some other diseases, such as leptospirosis, American trypanoso-miasis, South American hydatid disease, and vesicular stomatitis, is less well understood.In certain other infections, including bartonellosis and the South American spotted fevers,a wild rodent reservoir is inferred but not yet identified.

Members of the order Rodentia are more abundantand successfully varied in form and function thanare mammals of any other order. Rodents occupyvirtually every terrestrial habitat capable of support-ing mammalian life (Hershkovitz, 1962). SouthAmerican rodents range in size from the tinyOryzomys minutus, which lives in cloud-forest andweighs only a few grams to Hydrochoerus hydro-chaeris, which is known to reach a weight of 65 kg.Twenty representative neotropical genera of rodentsare listed in Table 1, some of them groupedinto sigmodont, phyllotine, and oryzomine sub-groups as suggested by Hershkovitz (1955, 1962,1969). Some of the genera, such as those in thephyllotine group, have recently been revised butothers, such as Oryzomys, have not (Hershkovitz,1962). Cabrera (1961) listed 342 species of nativeSouth American rodent distributed among 86 genera.The biology ofsome species is reasonably well known,but fundamental data such as geographical distri-bution, range of altitude tolerated, and preferredhabitats is not available for most species, much less

1 Staff Member of the Rockefeller Foundation, andProfesor Visitante de Epidemiologia, Facultad de Medicina,Universidad del Valle, Cali, Colombia. Present address:Apartado 70, Maracay, Venezuela.

a knowledge of their biological and taxonomicrelationships.The rodent fauna may be classified ecologically

as either pastoral or sylvan (Hershkovitz, 1962).Some species have evolved along very specializedlines but adaptability and versatility are generallycharacteristic of rodents. Some primarily sylvanspecies, for example, have been reported to survivedeforestation, adapting to orchards and buildings,while pastoral species are found from deserts androcklands to coniferous forest, where they may besecondarily adapted to a tree-dwelling habit. Calomysand some species of Oryzomys have been observedto move between sylvan and pastoral habitats, andat times may become peridomestic and commensalwith man (Hershkovitz, 1962).

Population explosions, known locally in SouthAmerica as " ratadas ", occur among various speciesof neotropical rodent (Hershkovitz, 1955, 1962).Occasionally, ratadas have been associated withnatural phenomena such as the cyclic fruiting ofsome species of bamboo or the abundance of certaincultivated crops. Ratadas have been reported amongpastoral species of the genera Phyllotis, Akodon,Oryzomys, Heteromys, Calomys, Zygodontomys,Sigmodon, and Holochilus. Species able to do well

2385 - 161

Table 1. Geographical and altitudinal ranges, and preferred habitats of representative genera ofindigenous South American rodents

Family; genus Geographical range Altitudinal Ecological classification and preferredrange (in) habitat

Sciuridae (squirrels); Sciurus northern half of South American conti- 0-3 000 sylvan; arborealnent

Heromyidae (pocket mice); Colombia, Ecuador, Venezuela 0-2 500 pastoral; terrestrial; entering farm buil-Heteromys dings and granaries

Erethizontidae (porcupines); northern parts of South America 0-2 500 sylvan; arborealCoendu

Caviidae (guineapigs); Cavia from Colombia and Venezuela to north- 0-4 000 pastoral; rocky regions, forest edges, andern Argentina swamps; easily domesticated

Hydrochoeridae (capybaras); Colombia and Venezuela south to Rio 0-500 pastoral; gregarious, breeding in water;Hydrochoerus Parana easily domesticated

Cricetidae (Sigmodont group);Sigmodon (cotton rats) grass- and scrublands of tropical zones 0-1 550 pastoral; associated with ratadas

of northern South AmericaHolochilus (marsh rats) Venezuela, Guyana, Brazil, northern 0-2 000 pastoral; associated with ratadas

Argentina, Peru, Colombia

Cricetidae (Phyllotine group);Calomys (vesper mice) Argentina, Paraguay, Brazil, Venezuela, 0-5 000 pastoral; mountain grasslands, scrublands,

Uruguay, Bolivia, Peru forest fringes; gregarious, commensalwith other species

Zygodontomys (cane mice) Colombia, Ecuador, Venezuela, Guyana, 0-1 200 pastoral; open or brush country withBrazil, Paraguay, Bolivia thick ground cover

Phyllotis (pericotes) from Ecuadorian Andes south to Straits 0-5 500 bare ground near vegetation; some semi-of Magellan arboreal; up to limits of perpetual snow

Cricetidae (Oryzomine group);Oryzomys (rice rats) throughout most of South America -a sylvan and pastoral; great variation in

size, form, habits, and preferred habitatsNeacomys (bristly mice) Colombia, Brazil, Ecuador, Peru and 0-2 000 sylvan, terrestrial; prefer dense humid

British Guiana forestsNectomys (water rats) northern and central South America 0-1 500 palustrine or aquatic; sometimes found

in buildings

Cricetidae;Akodon (South American fieldmice) most of South America 1 050- habitats vary from arid or humid meadows

5 000 to forestsPunomys (puna mice) Peru 4 450- pastoral; prefers rocky habitats; diet

5 200 consists of two plants only

Dasyproctidae; Agouti (pacas) from Colombia to southern Brazil 0-3 000 sylvan; terrestrial, nocturnal; steals fruitand vegetables; hunted for meat

Dasyprocta (nequis) from Colombia to southern Brazil 0-3 000 sylvan;terrestrial ; adapted toa wide rangeof natural and man-made habitats;easily domesticated

Echimyidae; Proechimys northern and central South America 0-1 200 sylvan; terrestrial; it is the predominant(spiny rats) small mammal in many locationsHoplomys (armoured rats) Colombia and Ecuador west of the -a sylvan; terrestrial

AndesKannabateomys south-eastern Brazil, Paraguay, -a prefers bamboo thickets along streams

Argentina

a Tropical lowlands; maximum altitude not yet determined.

PUBLIC HEALTH IMPORTANCE OF RODENTS IN SOUTH AMERICA 163

in newly invaded territory appear to be " ratada-prone" (Hershkovitz, 1962). The number of indi-vidual animals involved in such population increasesmay be very great, resulting in extensive destructionof crops and even the invasion of households andfood stores. Population explosions are usually fol-lowed by equally dramatic population declines, per-haps because the animals overrun their food supplyand because of predation and disease.

In South America, the non-native species Rattusrattus (roof rat), R. norvegicus (Norway rat), and Musmusculus (house mouse) have generally become wellestablished wherever there is human settlement. Thelarger Norway rat is predominantly a burrowingrodent while the roof rat is scansorial. Both speciesare omnivorous and tend to be closely associatedwith man, but in tropical areas they sometimes occuras freeliving populations. The common house mousemay be either domestic' or wild. Population ex-plosions of wild Mus have occurred in the USA,but ratadas involving Mus have not yet been reportedin South America.Although it is generally felt that indigenous species

are unable to compete with the aggressive and craftyRattus or the profilic Mus, this view has seldom beenmeasured quantitatively. In rural communities, espe-cially new ones, this factor could be significant.Changes now occurring in South America mustultimately influence the rodent fauna, both quantita-tively and qualitatively. These changes chiefly involverapid increases in the human population and directmodifications of the environment, as well as theurgent efforts being made to increase the productionof food. Increases in the size and population densityof cities bring corresponding changes in the popula-tions of Rattus and Mus. While it is unlikely thatmost of the indigenous South American rodentscould survive in truly metropolitan areas, it is quitepossible that they might persist in the fringe areasof cities. In rural areas, former villages are becomingcities, and new foci of human population are appear-ing. In these places the contact between man andindigenous rodents, either directly or via theirectoparasites and/or excreta, may be close. Heretoo, a struggle between Rattus and Mus and autoch-thonous rodent species may occur and continue

I The terms " commensal ", " domiciliary ", and " domes-tic" are often used to describe introduced Old Worldrodents although none of the terms is completely satis-factory. " Domestic " is defined in Webster's Third NewInternational Dictionary as: " living near or about the habi-tations of man ". It is used here in that sense.

over a long period. In certain situations, particu-larly where the very sociable phyllotine rodentsare involved, and food and cover are abundant,a prolonged period of coexistence between the nativeand introduced species is possible.

Increasingly, research in South America is beingdevoted to studies of plant and animal food pro-duction in tropical conditions. Means are beingsought to increase the production of meat fromcattle, pigs, sheep, and poultry by the extension ofranges and more intensive management practices.Likewise, the production of plant foods is beingintensified by introducing exotic species and byimproving native varieties. The inevitable resultis persistent radical modification of natural habitats.Some forms of wildlife will be eliminated whileothers may thrive. Rodent populations will bequantitatively altered, and it can be expected thatchanges in their nutrition, susceptibility to disease,reproduction, ectoparasite and endoparasite faunas,predation, and, possibly most important, behaviour,will occur. These changes may be dramatic orsubtle, and the public health importance of anygroup of rodents may change in a significant way.The public health importance of rodents may be

evaluated in terms of the human pathogens theycarry, or by noting the reported incidence of rodent-associated diseases; however, these approaches wouldbe ofdoubtful value. Instead, some rodent-associateddiseases of man will be considered in relation tothe dynamics of rodent evolution and expectedenvironmental changes.

PLAGUE

Plague is a classic arthropod-transmitted, rodent-associated zoonosis, now known to be widely enzoo-tic in South America. Nearly 5 000 human caseswere reported from the continent between 1960 and1969. Table 2 lists 17 genera (comprising about50 different species) of wild, neotropical rodents thatare thought to harbour plague organisms. Amongthese wild rodents the causative agent, Yersinia(Pasteurella) pestis, has adapted sufficiently topermit its indefinite persistence.

Fleas transmit Y. pestis among rodents and attimes to other animals, including man. Some spe-cies of flea are restricted to a single host; others areeither completely indiscriminate feeders or are re-stricted to a group of related animals, the latterbeing the most common condition. The transmissionof plague by fleas may be rapid when there is

R. B. MACKENZIE

Table 2. Genera of wild South American rodents either proved to be naturally infectedwith Y. pestis or strongly incriminated through their ectoparasite fauna

CountryGenus

Argentina Bolivia Brazil Ecuador Peru Venezuela

Sciurus x x

Oryzomys x x x x x

Rhipidomys x x

Akodon x x x

Calomys x x

Phyllotis x x x x

Eligmodontia x

Holochilus x x

Graomys x

Sigmodon x

Heteromys

Cercomys x

Lagostomus x

Galea x x

Kerodon x

Cavia x x

Caviella x

mechanical transmission among dense susceptiblepopulations, or it can be slow and discontinuous,infective, nonfeeding fleas remaining alive in animalburrows for several weeks or months. The patho-genicity for man of the plague organism in sylvanand domestic rodents varies widely and this is thecause of many different epizootologic situations.However, in the long term there is tendency for theparasite not to destroy its natural host, therebysurviving permanently in a variety of hosts andhabitats.Human plague transmitted by direct contact with

infected wild animals or via fleas from sylvaticsources is usually of the bubonic type, sporadic, anddoes not initiate outbreaks. However, when Y.pestis escapes from its natural self-regulating sylvaticcycles and enters populations of domestic ratsexplosive human epidemics may occur. In suchsituations, infected fleas-namely, Xenopsylla cheopis,which is an efficient vector species-leave dying ratsand seek other vertebrate hosts, sometimes initiatinga man-to-man transmission cycle of the plague

organism involving species of flea that bite man, oraerosol transmission-a characteristic feature ofpneumonic plague (Meyer, 1965; Stark et al., 1966).

LEPTOSPIROSIS

Leptospirosis in man has been recognized inSouth America for over 60 years, attention firstbeing drawn to the icteric form on account of itsclinical similarity to yellow fever (Alexander, 1960).Forty years after the discovery of leptospirosis inSouth America workers studying the virus diseaseknown as Argentine haemorrhagic fever (AHF)encountered enough evidence of leptospiral infec-tions among suspected cases of AHF in man to leavethe real etiology of AHF in doubt for some time.The inadequacy of information about leptospirosis

in man in South America is probably the result ofseveral factors: (1) it is not a reportable disease;(2) the overall case fatality rate appears to be low;(3) leptospiral infections in man result in clinical

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Table 3. Isolation of leptospires from small forest animals associated with a humanleptospirosis outbreak in Panama in 1961

No. of No. withSpecies No. examined No. infected different mixed

serotypes infections

Proechimys semispinosus 27 11 6 4

Liomys adspersus 14 2 3 1

Didelphis marsupialls 3 1 1 0

Philander opossum 3 1 1 0

total 47 15 7 5

syndromes that range widely in severity and sympto-matology; and (4) the detection of cases is contingenton the availability of suitable diagnostic laboratoryfacilities.Mammals, including man, that are infected with

organisms of the genus Leptospira often excrete theinfective agent in the urine for long periods afterthe initial infection. Persons become infected, eitherthrough broken skin or across intact mucous

membranes, by contact with water contaminatedby leptospire-excreting animals.Therelative importance ofSouthAmerican rodents,

domestic or sylvatic, to domesticated animals (espe-cially cattle, horses, pigs, and dogs) in the epidemio-logy of leptospirosis is still uncertain. The view iswidespread that feral animals, including rodents, doserve as reservoirs of leptospires in other parts of theworld, but it is not clear whether native wildmammalsare sources of indigenous leptospires or simply serve

as hosts for introduced organisms; both eventsare possible.The ability of leptospire-excreting forest mammals

to contaminate water and thereby secondarily infectman was observed quantitatively in Panama during1961. Altogether, 9 of 196 soldiers became infectedwith at least 4 different leptospire serotypes, 2 ofwhich were isolated from the soldiers' urine a monthafter their illness (Mackenzie et al., 1966); 7 differ-ent serotypes (including the 2 serotypes from humanurine) were isolated from small mammals trappednear a riverbank after the outbreak. It can be seen

from Table 3 that mixed infections among rodentswere common (Gale et al., 1966). A total of 8 new

serotypes of Leptospira and 1 new serogroup were

discovered as a result investigating this singleoutbreak.

Representatives of 13 ofthe 18 currently designatedserotypes of Leptospira have been reported fromSouth America, even in the absence of broad syste-matic surveys of feral animals (Szatalowicz et al.,1970). Such studies, in addition to intensive in-vestigations of outbreaks of leptospirosis in man ordomesticated animals, would undoubtedly be re-warding.

VENEZUELAN ENCEPHALITIS

Venezuelan encephalitis (VE), also known asVenezuelan equine encephalitis (VEE), is an acuteinfectious mosquito-transmitted viral disease affect-ing horses, mules, and burros, and also man. Anoutbreak in Venezuela beginning late in 1962 andcontinuing for 2 years is believed to have resultedin over 23 000 human cases, 960 of which hadneurological manifestations, and 156 deaths. During1967 an epidemic in Colombia killed between 70 000and 100 000 equine animals and probably affectedbetween 200 000 and 400 000 persons, causing symp-toms in about half of them. Since 1967, 100 000-150 000 equine animals have died of VEE in Southand Central America. During equine outbreaks,the virus is transmitted by mosquitos from horse tohorse and man is secondarily infected-also bymosquitos.

There appear to be 2 general classes of VE virusstrains-namely, those that are not pathogenic forequine animals, and those that are pathogenic. Bothclasses of virus cause illness and sometimes mortalityin man. The epidemiology of the infection and therole of feral animals with regard to strains causingepizootics among equine animals are not known,

165

R. B. MACKENZIE

but strains not pathogenic for equine animals are en-demic in sylvatic rodents, being transmitted by mos-quitos of the genus Culex, subgenus Melanoconion.

SOUTH AMERICAN HAEMORRHAGIC FEVERS

The suddenness with which attention may bedrawn to a group of rodents that previously attractedonly limited notice was dramatically illustrated bythe outbreaks of viral haemorrhagic fevers inArgentina and Bolivia. AHF probably made itsappearance as an epidemiological entity about30 years ago when outbreaks of " malignant grippe "were first noticed in man in the pampas of theProvince of Buenos Aires, Argentina. The annualepidemics were subsequently observed to occurseasonally among workers engaged in agriculturalharvesting, particularly of maize (Arribalzaga, 1955;Martinez, 1962; Mettler, 1969). Some of the earlylocal names for the disease, mal de los rastrojos,for example, emphasized this rural/harvest associ-ation. During a 10-year interval beginning in1958, over 8 000 cases were recorded in Argentina,and the case fatality rate is believed to have beenbetween 5% and 10%. A previously unknown virus,later named Junin virus, was isolated from victimsof the 1958 epidemic. It was confirmed that thisvirus was the causative agent of AHF.

Bolivian haemorrhagic fever (BHF) struck in thesparsely populated tropical prairies ofthe Departmentof Beni in north-eastern Bolivia, where it was firstknown as el tifo negro (Mackenzie et al., 1964).During a 6-year period ending in 1964, about1 300 persons were infected, and the case fatalityrate was about 200%. A virus, distinct from butserologically related to Junin virus, was establishedas the cause of BHF and named Machupo virus in1963 (Johnson et al., 1965).

Junin, Machupo, and several other viruses havesince been found to be serologically and morphologi-cally related. All seem to be strongly associated intheir natural history with New World rodents.The complex includes Tamiami virus (from Sigmodonhispidiis in the USA), Pichinde virus (from Oryzomysalbigularis in Colombia), Parana virus (from Oryzo-mys buccinatus in Paraguay), and Amapari virus(from Oryzomys capito and Neacomys guianae inBrazil). Only Junin and Machupo viruses have sofar been associated with disease in man (Murphyet al., 1969, 1970).

Intensive epidemiological studies in Bolivia esta-blished that many rodents of the pastoral species

Calomys callosus had become commensal with man,and they were found to be chronically infected withMachupo virus. The virus was excreted by the ro-dents in the urine and thereby transmitted to man,probably by the direct contamination of food(Johnson et al., 1966). It was further shown thatcontrol of epidemics could be achieved by controllingthese rodents and that no other vertebrate or arthro-pod host was involved (Kuns, 1965).

In Argentina, the association of Junin virus witha single rodent species is less definite. Isolations ofvirus are reported from several species of rodentas well as from hares. However, there has beena strong epidemiological association between Juninvirus and the indigenous rodent species C. lauchain the endemic areas for many years.

Evidence from Bolivia suggests that Machupovirus and C. callosus have been associated witheach other for a very long time. Other evidencesuggests that the present human population ofnorth-eastern Bolivia has not previously experiencedthe BHF virus. Whether this " new " human diseaseis the result of an extension of the range of Calomys,or of a modification of its spatial or behaviouralrelationship with man (perhaps caused by humanactivities), has not yet been established.

CUTANEOUS LEISHMANIASIS

Several clinical forms of cutaneous leishmaniasisoccur in Latin America, all being caused by a groupof protozoa of the family Trypanosomidae knownas the Leishmania braziliensis complex (Lainson& Shaw, 1971). With the exception of a specialsituation in Peru, forest rodents of the tropics andsubtropics appear to be natural reservoirs of theleishmanias. The Peruvian disease known as uta isattributed to a separate species of leishmania namedL. peruviana, and it occurs in arid valleys of the Andesas high as 3 000 m where dogs serve as a reservoir.The clinical forms of cutaneous leishmaniasis rangefrom simple self-limiting " chiclero ulcer " to asevere mucocutaneous form that is sometimes fatal.These clinical types tend to be associated with cer-tain geographical areas and differ in their epidemio-logy, but all are transmitted to man by the biteof phlebotomid sandflies belonging to the genusLutzomyia.Accumulating evidence indicates that forest ro-

dents are primary host-reservoirs for the leish-manias, and that other animals, such as marsupials,play a secondary role. Members of several genera of

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PUBLIC HEALTH IMPORTANCE OF RODENTS IN SOUTH AMERICA

South and Central American rodents are implicatedin leishmanial epidemiology; they include: Oryzomys,Zygodontomys, Neacomys, Proechimys, Hoplomys,Coendu, Ototylomys, Heteromys, Dasyprocta, Kanna-bateomys, and Agouti. Infection in rodents is oftenrevealed by distinct Leishmania-containing lesionsat the base of the tail (Lainson & Shaw, 1971).Although the epidemiological associations of theserodents with one another and with other animalshave not yet been evaluated, rodents are proving tobe of major importance in the life cycle of NewWorld leishmanias.

AMERICAN TRYPANOSOMIASIS

Although sylvatic rodents serve as major naturalhost-reservoirs for the infective agents of Americancutaneous leishmaniasis, this is not true of anotherSouth American representative of the familyTrypanosomidae-namely, Trypanosomia cruzi, whichcauses the infection known as Chagas' disease.Canids, procyonids, marsupials, edentates, and sev-eral other groups of mammal serve as natural reser-voirs for the infective agent and reduviid bugs arethe arthropod vectors. Although wild rodents areonly occasionally found to be infected, informationfrom Brazil, and more recently from Panama,suggests that R. rattus may be an important reservoirin both those countries (De Alencar, 1965; Edgcomb& Johnson, 1970). Since man usually takes Rattusinto newly colonized territory, it is not difficult toimagine that with Rattus serving as an amplifyinghost, " haloes " of hyperendemicity might be pro-duced around rural communities in areas that pre-viously supported only low levels of trypanosomalactivity.

It has also been established that guineapigs,which are domesticated for. food purposes in house-holds of Ecuador, Peru, and Bolivia, often serve asreservoirs for household-dwelling reduviid bugs(Herrer, 1959; Torrico, 1959).

HYDATID )DISEASE

Human unilocular hydatid disease, which is wide-spread in South America and is caused by Echino-coccus granulosus, is not a rodent-associated disease.Sheep are the intermediate hosts and dogs thedefinitive hosts of this species of tapeworm. How-ever, two rodent-associated species of Echinococcusare known-namely, E. multilocularis and E. oligar-thrus. E. multilocularis is found in Canada, the USA

(in the states of North and South Dakota, Washing-ton, and Alaska), Europe, Russia, and Japan.Microtine rodents serve as intermediate hosts, thedefinitive hosts being dogs, cats, foxes, and wolves.Eggs passed in the faeces of the definitive hosts maybe ingested by man and cause infection and multi-locular disease. The presence of E. multilocularishas not yet been demonstrated in South America.

In 1965 attention was called to a fatal case ofmultilocular hydatid disease in Panama that wascaused by the third species of Echinococcus-E.oligarthrus. It is believed that wild felids are thedefinitive hosts and that feral rodents, includingDasyprocta punctata, Proechimys guyannensis, andAgouti paca are intermediate hosts (Sousa & That-cher, 1969; Thatcher & Sousa, 1966; Thatcher,1970). Those workers also reported that domesticcats readily become infected by eating hydatidmaterial from infected rodents and the cats subse-quently shed E. oligarthrus eggs in their faeces(Sousa & Thatcher, 1969). Since the eggs couldbe infective for man, this observation is of consider-able interest.

VESICULAR STOMATITIS

Vesicular stomatitis is a viral disease that causesillness in man, cattle, horses, and pigs. Attentionis usually called to outbreaks among cattle, whereit causes extensive and severe vesicular lesions ofmouth, teats, and feet that are often clinicallyindistinguishable from those of foot and mouthdisease. Infection in dairy cattle frequently resultsin a permanent fall in milk production, and in beefcattle a failure to gain weight. Man is sometimesinfected from sick animals and experiences a gene-ralized febrile illness, not infrequently with vesicularlesions of the hands, feet, or mouth, the site dependingon the route of infection.The disease is caused by two main serotypes of

vesicular stomatitis virus known as New Jersey(VSV-NJ) and Indiana (VSV-Ind). At least two otherviruses related to VSV-Ind are known-namely,Alagoas in Brazil, and Cocal in Trinidad (and possi-bly in Argentina also). All these viruses are consi-dered by some workers to be serological subtypesof VSV-Ind (Federer et al., 1967).While there is increasing evidence that the vesicular

stomatitis viruses may be transmitted to large ani-mals or man by haematophagous arthropods, theirnatural reservoirs are unknown (Tesh et al., 1971).The Cocal subtype of VSV-Ind has been isolated

3

167

R. B. MACKENZIE

from rodents in Trinidad while prototype VSV-Indhas been associated with arboreal animals, includingrodents, in Panama (Jonkers et al., 1964, 1965;Tesh et al., 1969). Serological surveys in Colombiahave shown the widespread presence of neutralizingantibody to VSV-NJ and prototype VSV-Ind virusesas well as to a VSV-Ind subtype among rodents ofthe genera Proechimys, Oryzomys, Nectomys, Sciurus,Hydrochoerus, Dasyprocta, and Zygodontomys (CaliVirus Laboratory, unpublished data). At one loca-tion in Colombia, 17 of 31 Proechimys guyannensiswere found to have neutralizing antibody to VSV-NJand the virus itself was isolated from the liver tissueof captured Proechimys (Cali Virus Laboratory,unpublished data). The prevalence of neutralizingantibody for VSV-NJ among 227 specimens ofProechimys semispinosus collected in Panama was23% while among 1 729 other wild animals the pre-valence was only 4% (Tesh et al., 1969). Althoughthese data do not incriminate rodents as essentialagents in the natural cycles of vesicular stomatitisviruses, they do suggest that rodents becomeinfected.

DISCUSSION

While it is possible to estimate the public healthimportance ofa single group ofrodents at a particulartime with respect to a single infective agent, it isimportant to remember that in response to naturaland human influences rodent populations are con-tinually changing. Improved agricultural methodshave resulted in widespread changes of habitat inSouth America. Formerly, for example, only onecrop of maize could be produced annually on theColombian north coast but it is now possible toharvest three crops. Forests are being removed, notonly from the flat lowlands, but also from steepmountainsides. Sometimes this land is convertedto productive arable land, but often it becomes

eroding wasteland after unsuccessful attempts to growcrops or to raise sheep or cattle. These types ofchange can be expected to continue at an acceleratingrate, and it must be expected that modifications ofthe natural cover and food supply of rodents andtheir predators will lead to continuing changes inthe rodent fauna. Some species of rodent will thrive(even to the point of becoming domesticated), whileothers will disappear, and the same will be true ofthe arthropods that serve as vectors for organismsinfecting both rodents and man.A number of diseases have been omitted from this

review. It is believed that a wild vertebrate reservoirprobably exists for some of these, but the animalsinvolved are not yet known; this group of diseasesmight include the spotted fevers of rickettsial etio-logy, and bartonellosis. The distribution and frequen-cy of some other diseases, such as turalaemia inSouth America, have still to be assessed.

Meanwhile, it must be emphasized that ordinaryhousehold rats and mice, that is, Old World speciesadapted to man and taken to the New World asmaritime passengers, are the most abundant rodentsin close contact with man. Although the rate ofintroduction of Old World murines has been slowedor stopped, the threat of renewed importation isalways present. Modem fast cargo ships and " con-tainerized " cargoes may increase this risk unlessadequate measures are taken. The possibility ofaccidental introduction of exotic rodents in aircargoes should also be borne in mind.The dangers of murine typhus, ratbite fevers,

lymphocytic choriomeningitis, and rickettsial poxhave not disappeared since they are linked intimatelywith human poverty, unsatisfactory housing, andthe presence of rats and mice. Our knowledgeof the indigenous South American rodent fauna issuperficial and the real public health importance ofthe native rodents is still unknown.

UME

IMPORTANCE DES RONGEURS POUR LA SANTE PUBLIQUE EN AMERIQUE DU SUD

On trouve en Amerique du Sud un grand nombre etune grande variete d'especes autochtones de rongeurs,capables de s'adapter aux conditions changeantes dumilieu et occupant la plupart des habitats naturels dis-ponibles. Si certaines d'entre elles ont W assez bienetudiees, on ne possede par contre que peu de rensei-gnements sur la biologie et la taxonomie du plus grand

nombre. A proximite des habitations humaines, lesespeces non indigenes (Rattus rattus, R. norvegicus etMus musculus) sont tres fr6quemment rencontrees etvivent souvent en contact intime avec l'homme.En Amerique du Sud, l'accroissement rapide de la

population a amene une expansion des zones urbaines.Les hommes, entrainant a leur suite les rongeurs domes-

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PUBLIC HEALTH IMPORTANCE OF RODENTS IN SOUTH AMERICA 169

tiques, se sont installes dans des regions jusqu'alorsinoccupees, et les uns et les autres sont entres en contactavec les especes autochtones de rongeurs. II en est resulteun echange inevitable de parasites dont les consequencespour la sante humaine sont imprevisibles. A ces modifi-cations directes de l'environnement, se sont ajout6escelles causees par les efforts en vue d'exploiter au maxi-mum les ressources vegetales et animales. Cette evolutiona profond6ment influence la structure et le comportementdes populations autochtones de rongeurs.On connait le r6le des rongeurs, domestiques ou sau-

vages, dans la transmission de certaines maladies commela peste, l'encephalite equine venezuelienne, les fievreshemorragiques sud-americaines, le typhus murin et laleishmaniose cutan6e. Pour d'autres affections, commela leptospirose, la trypanosomiase americaine, l'hydati-dose sud-americaine et la stomatite vesiculeuse, les moda-lites de leur intervention ne sont qu'imparfaitementelucidees. Enfin, en ce qui concerne des maladies tellesque la bartonellose et les fievres pourpr6es d'Ameriquedu Sud, on soupconne l'existence d'un reservoir animalqui n'a cependant pas encore ete identifie.

REFERENCES

Alexander, A. D. (1960) Bull. Wld Hlth Org., 23, 113-125Arribalzaga, R. A. (1955) DIa med., 27, 1204-1210Cabrera, A. L. (1961) Rev. Mus. argent. Cienc. nat.

Bernardino Rivadavia, 4, 309-732De Alencar, J. E. (1965) Rev. bras. Malar., 17, 149-158Edgcomb, J. H. & Johnson, C. M. (1970) Amer. J. trop.Med. Hyg., 19, 767-769

Federer, K. E. et al. (1967) Res. vet. Sci., 8,103-117Gale, N. B. et al. (1966) Amer. J. trop. Med. Hyg., 15,

64-70Herrer, A. (1959) Rev. goiana Med., 5, 389 409Hershkovitz, P. (1955) Fieldiana, Zool. Mem., 37, 639-673Hershkovitz, P. (1962) Fieldiana, Zool. Mem., 46,13-524Hershkovitz, P. (1969) Quart. Rev. Biol., 44, 1-70Johnson, K. M. et al. (1965) Proc. Soc. exp. Biol. (N.Y.),

118, 113-118Johnson, K. M. et al. (1966) Amer. J. trop. Med. Hyg.,

15, 103-106Jonkers, A. H. et al. (1964) Amer. J. trop. Med. Hyg.,

13, 613-619Jonkers, A. H. et al. (1965) Amer. J. vet. Res., 26,

758-763Kuns, M. L. (1965) Amer. J. trop. Med. Hyg., 14,

813-816Lainson, R. & Shaw, J. J. (1971) Epidemiological con-

siderations of the leishmaniasis with particular referenceto the New World. In: Fallis, A. M., ed., Ecologyand physiology of parasites, University of TorontoPress, pp. 21-57

Mackenzie, R. B. et al. (1964) Amer. J. trop. Med. Hyg.,13, 620-625

Mackenzie, R. B. et al. (1966) Amer. J. trop. Med. Hyg.,15, 57-63

Martinez Pintos, I. (1962) An Comn. Invest. cient. Prov.B. Aires, 8, 9-102

Mettler, N. E. (1969) Argentina hemorrhagic fever:current knowledge, Washington, D.C., Pan AmericanHealth Organization (Scientific Publication No. 183)

Meyer, K. F. (1965) In: Dubos, R. J. & Hirsch, J. C., ed.,Bacterial and mycotic infections of man, 4th ed.,Philadelphia, Lippincott, pp. 659-677

Murphy F. A. et al. (1969) J. Virol., 4, 535-541Murphy, F. A. et al. (1970) J. Virol., 6, 507-517Sousa, 0. A. & Thatcher, V. E. (1969) Ann. trop. Med.

Parasit., 63, 165-175Stark, H. E. et al. (1966) Plague epidemiology, Atlanta,

Ga., US Department ofHealth, Education, and WelfareSzatalowicz, F. T. et al. (1970) The international dimensionsof leptospirosis, Washington, D.C., US Armed ForcesInstitute of Pathology

Tesh, R. B. et al. (1969) Amer. J. Epidem., 90, 255-261Tesh, R. B. et al. (1971) Amer. J. Epidem., 93, 491495Thatcher, V. E. & Sousa, 0. E. (1966) Ann. trop. Med.

Parasit., 60, 405Thatcher, V. E. (1970) J. Parasitol., 56, 342Torrico, R. A. (1959) Rev. Goiana Med., 5, 375-3&7