FACTORS INFLUENCING THE OCCURRENCE OF ILLNESS DURING NATURALLY ACQUIRED POLIOMYELITIS VIRUS INFECTIONS' CHARLES A. EVANS Department of Microbiology, University of Washington, Seattle, Washington . . . For want of knowing any other cause, epi- demics were attributed, by the ancients, to the atmosphere, without any evidence; just as polit- ical and social events were believed to be occa- sioned by the stars. Now as people are not only exposed to the atmosphere, as soldiers in battle are to bullets, but are actually immersed in it, as fishes are in the sea, it became necessary to explain why certain persons were attacked and others not attacked, and the word predisposition was used as affording an explanation. The alleged predisposi- tion, however, was nothing visible or evident: like the elephant, which supports the world, according to Hindoo mythology, it was merely invented to remove a difficulty. John Snow (1853) Paralytic poliomyelitis has always been a rela- tively rare disease in spite of the fact that polio- virus infections are common in the United States and in most other countries. More than 99 per cent of these infections cause no paralysis. This paper presents an analysis of factors that influence the course of a primary poliovirus in- fection in an unvaccinated individual. The cen- tral question is: What determines whether an infected person is like the 99 per cent who ex- perience no real harm or is one of the unfortunate few who suffer paralytic illness? The discussion is focused first on the virus, second on the status of the individual, and third on the circumstances under which infection occurs. No attempt is made to present a com- prehensive documentation of all important in- formation on the problem. Relatively little space is devoted to some of the well-established and generally accepted concepts; greater emphasis is placed on those aspects that, in my opinion, are of 1 The Presidential Address delivered in Phila- delphia on May 3, 1960, at the Annual Meeting of the Society of American Bacteriologists was a condensation of this paper. Aided by financial support from a contract with the Office of Naval Research and grants from the National Institutes of Health, the State of Wash- ington Fund for Biological and Medical Research, and the Eli Lilly Company. more significance than is indicated by the little attention accorded them in most discussions of the subject. Arguments will be made for the contention that the virulence of the virus causing the infec- tion is usually of less importance than some, as yet undefined, characteristics of the infected person. It will also be suggested that environ- mental conditions related to the conditions under which virus spreads may be of substantial im- portance. VIRULENCE OF THE INFECTING VIRUS There is imposing epidemiological evidence to support the contention that poliomyelitis virus strains as they occur in naturally infected human populations may be of relatively high virulence for man on one occasion and of much lower viru- lence at other times. One of the largest epidemics of paralytic poliomyelitis in this country was that of 1916 in New York City. More than 9,000 persons were paralyzed; this included nearly 2 per cent of all infants 1 and 2 years old (Lavinder, Freeman, and Frost, 1918). Scrutiny of the age- specific attack rates shows that, during the years preceding this epidemic, poliomyelitis virus was also prevalent; in fact, an estimated 30 per cent of all susceptible children were annually infected with virus antigenically related to that which subsequently caused the epidemic of 1916 (Sample and Evans, 1955). Yet paralytic disease was so infrequent before 1916 that poliomyelitis was not regarded as of sufficient importance to make it a reportable disease. There can be little doubt that the virus prevalent in New York City in 1916 was of high virulence and that an un- usually high proportion of those infected were paralyzed that year (Sample and Evans, 1955). Examination of the reported incidence of polio- myelitis for the entire United States shows changes from year to year that must be attrib- uted in part to shifts in virulence and/or infec- tivity of the virus. As shown in table 1, 1916 and 1952 were years of high death rates. During the 5-year period from 1948 to 1952 the annual polio- 341 on May 6, 2018 by guest http://mmbr.asm.org/ Downloaded from
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FACTORS INFLUENCING THE OCCURRENCE OF ILLNESS DURINGNATURALLY ACQUIRED POLIOMYELITIS VIRUS INFECTIONS'
CHARLES A. EVANSDepartment of Microbiology, University of Washington, Seattle, Washington
. . . For want of knowing any other cause, epi-demics were attributed, by the ancients, to theatmosphere, without any evidence; just as polit-ical and social events were believed to be occa-sioned by the stars. Now as people are not onlyexposed to the atmosphere, as soldiers in battleare to bullets, but are actually immersed in it, asfishes are in the sea, it became necessary to explainwhy certain persons were attacked and others notattacked, and the word predisposition was used asaffording an explanation. The alleged predisposi-tion, however, was nothing visible or evident: likethe elephant, which supports the world, accordingto Hindoo mythology, it was merely invented toremove a difficulty.
John Snow (1853)
Paralytic poliomyelitis has always been a rela-tively rare disease in spite of the fact that polio-virus infections are common in the United Statesand in most other countries. More than 99 percent of these infections cause no paralysis.
This paper presents an analysis of factors thatinfluence the course of a primary poliovirus in-fection in an unvaccinated individual. The cen-tral question is: What determines whether aninfected person is like the 99 per cent who ex-perience no real harm or is one of the unfortunatefew who suffer paralytic illness?The discussion is focused first on the virus,
second on the status of the individual, and thirdon the circumstances under which infectionoccurs. No attempt is made to present a com-prehensive documentation of all important in-formation on the problem. Relatively little spaceis devoted to some of the well-established andgenerally accepted concepts; greater emphasis isplaced on those aspects that, in my opinion, are of
1 The Presidential Address delivered in Phila-delphia on May 3, 1960, at the Annual Meeting ofthe Society of American Bacteriologists was acondensation of this paper.Aided by financial support from a contract with
the Office of Naval Research and grants from theNational Institutes of Health, the State of Wash-ington Fund for Biological and Medical Research,and the Eli Lilly Company.
more significance than is indicated by the littleattention accorded them in most discussions ofthe subject.Arguments will be made for the contention
that the virulence of the virus causing the infec-tion is usually of less importance than some, asyet undefined, characteristics of the infectedperson. It will also be suggested that environ-mental conditions related to the conditions underwhich virus spreads may be of substantial im-portance.
VIRULENCE OF THE INFECTING VIRUSThere is imposing epidemiological evidence to
support the contention that poliomyelitis virusstrains as they occur in naturally infected humanpopulations may be of relatively high virulencefor man on one occasion and of much lower viru-lence at other times. One of the largest epidemicsof paralytic poliomyelitis in this country wasthat of 1916 in New York City. More than 9,000persons were paralyzed; this included nearly 2per cent of all infants 1 and 2 years old (Lavinder,Freeman, and Frost, 1918). Scrutiny of the age-specific attack rates shows that, during the yearspreceding this epidemic, poliomyelitis virus wasalso prevalent; in fact, an estimated 30 per centof all susceptible children were annually infectedwith virus antigenically related to that whichsubsequently caused the epidemic of 1916(Sample and Evans, 1955). Yet paralytic diseasewas so infrequent before 1916 that poliomyelitiswas not regarded as of sufficient importance tomake it a reportable disease. There can be littledoubt that the virus prevalent in New York Cityin 1916 was of high virulence and that an un-usually high proportion of those infected wereparalyzed that year (Sample and Evans, 1955).Examination of the reported incidence of polio-
myelitis for the entire United States showschanges from year to year that must be attrib-uted in part to shifts in virulence and/or infec-tivity of the virus. As shown in table 1, 1916 and1952 were years of high death rates. During the5-year period from 1948 to 1952 the annual polio-
decade earlier. The causes of these differencesare not firmly established, but it seems probablethat prevailing virus strains differ significantlyin virulence and/or infectivity from year to year.
There is no reason up to the present to ascribeperiods of high poliomyelitis incidence to theappearance of new antigenic strains of virus. Theage distribution of the 1916 epidemic demon-strates that the virus prevalent in 1916 was effec-
tively controlled by inmunity induced bypoliovirus infection in earlier years.
It is of interest to inquire how virulent polio-myelitis virus can be. That is, in its most virulentform and under optimal conditions for diseaseproduction what proportion of infected persons
suffer paralytic illness? The data in table 2 are
from some of the most severe outbreaks on record.In these epidemics it is probable that close to
100 per cent of the susceptible population was
infected. It is obvious that the most virulentvirus strains infecting under optimal conditionsdo not cause paralytic illness in all infected per-
sons. In populations of moderate size, paralyticattack rates of more than 20 per cent are un-
usual but do occur. In larger populations a para-
lytic attack rate of 5 per cent is exceptional.The significance of virulence of viral strains as
a factor in the apparently low rate of paralyticpoliomyelitis in countries with primitive sanitarystandards is not clear.Hammon and his associates (1955) as a result
of their extensive studies of enteric viruses in thePhilippines were led to speculate on the possi-bility that relatively nonpathogenic strains are
more prevalent in countries where a high propor-
TABLE 2*Maximal age-specific attack rates for paralytic
poliomyelitis in some severe outbreaks
I Para- ~~AttackPlace Date Age in Popula- aytic Rate inPlace Date Years tion ytics PerCaetCent
New York City.. 1916 1 112,2001 2,062 1.8Huskerville,Nebraska..... 1952 1-10 347 16 4.6
let, Canada 1949 All 275 57 21.0Maguse River,Canada....... 1953 All 18 10 55.0
* Data are from Lavinder et al., 1918; Bancroft,Engelhard, and Evans, 1957; Moses, 1948; Peart,1949; Johnsen and Wood, 1954.
t In some cases, as in New York 1916, a smallproportion of nonparalytic cases may be includedin the available data.
t Age-specific population data are not known.Attack rate is calculated from total population.
tion of the population is immune. "If passagethrough immunes has led to selection of suchstrains, we might then speculate further that'epidemic strains' such as the type 1 strain of1952 might have been imported, perhaps fromthe United States, where many opportunities oc-cur for passage of strains from susceptible tosusceptible in series." Gear (1955) has expressedsimilar ideas.
Others have attributed the apparently lowincidence of clinical poliomyelitis in suchcountries either to under reporting or to the factthat initial poliovirus infection occurs in infantswhile they are protected from paralytic diseaseby maternally conferred passive protection(Howe and Wilson, 1959). The latter concept hasusually been supported by data showing a highincidence of primary infection during the first2 or 3 years of life. There are relatively few datashowing a high infection rate during the first 4to 6 months of life when maternal antibodies canbe expected to protect (Sabin, 1951).
Characteristics of Virulent Strains of Poliovirus
It can be presumed that naturally occurringpopulations of poliovirus in an infected individualare not genetically pure. It can be argued furtherthat disease of the nervous system occurs only inthose persons in whom viral mutations to neuro-
virulence occur early in the course of the infec-tion in the alimentary canal. The limitation ofthis concept, as a useful explanation of this prob-lem, is indicated by the following facts: 1. Virusderived from the central nervous system has notbeen shown to be of greater virulence, for mon-keys, than that obtained from feces. 2. Most in-fections resulting from feeding cynomolgus mon-keys or chimpanzees infected brain tissue failto cause paralysis. 3. A variety of individual con-ditions unlikely to alter mutation increases thelikelihood that an infected person will experienceparalytic disease. 4. Certain environmental con-ditions increase the incidence of paralytic illness.
In recent years there has been a growing in-terest in defining viral characteristics or markersthat may be correlated with virulence of polio-virus. According to Burney (1959), the markerschecked on virus strains under trial as a live-virusvaccine in 1959 included neurovirulence for mon-keys, capacity to grow when the amount ofbicarbonate in the medium is reduced (d marker),growth in a stable line of monkey cells (MS), andgrowth at 40 C (t).
It is well known that poliovirus strains, freshlyisolated from human hosts, differ in their capacityto produce disease in monkeys. Unfortunately,the evidence fails to show a good correlation be-tween monkey virulence of freshly isolated polio-virus strains and their apparent virulence forman. Although poliovirus strains of low virulencefor monkeys have been isolated from personswith inapparent infection (Ramos-Alvarez andSabin, 1954), there is no consistent difference inmonkey virulence between strains from severeepidemics and those from symptomless carriersin communities with few if any clinical cases.
Vogt, Dulbecco, and Wenner (1957) discoveredone of the earliest and most useful markers invitro related to virulence of mutant poliovirusstrains. They found that a poliovirus strain oflow virulence (LSc) failed to produce plaques orthat plaques were small and were delayed in theirappearance in tissue cultures under certain con-ditions entirely satisfactory for plaque productionby the virulent parent strain of virus, Mahoney,from which LSc was derived. The crucial factorwas the concentration of sodium bicarbonate inthe agar overlay. Marked differences were evidentif the concentration of bicarbonate was approxi-mately one-fourth of the usual amount but werecompletely lost if the bicarbonate was increased.
This characteristic, the "d" marker, has been
studied with many poliovirus strains. It is ofgreat value in following the genetic stability ofattenuated poliovirus strains used as oral vac-cines. Tests of naturally occurring virus in humanfeces indicated that those strains associated withparalytic illness usually exhibit the do character.Strains from silent infections occurring in com-munities in which there is little disease may beeither d+ or d (Hsiung and Melnick, 1958). Thereis no evidence that epidemiologically mild strainsare generally different from naturally occurringvirulent strains with respect to the d character.Kanda and Melnick (1959) showed that MS
cells, a stable strain of monkey kidney cells, canbe used to distinguish between virulent strains ofpoliovirus and attenuated substrains that havelost virulence during selection and propagationin the laboratory. Like the d marker, the MScharacter has not been shown to provide a con-sistent difference between naturally occurringstrains from paralytic cases and those obtainedfrom symptomless carriers in communities free ofclinical poliomyelitis.
Lwoff (1959) has studied the effect of tempera-ture on the rate and extent of multiplication ofpoliovirus strains. In general, strains able tomultiply rapidly and to produce high yields ofvirus at 40 C were virulent when injected intothe brains of monkeys. Strains not able to multi-ply rapidly and extensively at this temperaturewere of low virulence.To my knowledge this "t" marker has not
been tested with the virus strains essentially asthey occur in nature and selected on the basis ofepidemiological histories indicating low virulencein man.
Sabin (1955), using an entirely different ap-proach, provided what appears to be an excellentexplanation for the inability of some laboratorystrains of low virulence to cause paralytic infec-tion. He showed that the LSc strain and someother strains derived by selection for low viru-lence during prolonged passage in the laboratorydid not spread through the body of infected ani-mals in the usual fashion. Chimpanzees inocu-lated intramuscularly did not develop viremiaand did not excrete virus in their feces. Virus un-able to spread cannot reach the central nervoussystem and, therefore, is harmless.We have been interested to see whether
markers of the sort described in the precedingparagraphs would differentiate between polio-virus strains isolated during a severe epidemic and
strains obtained when there was little or no illnessin a community. Our studies and those of othershave given essentially negative results. In thiswork we used strains of type 1 poliovirus from theChesterfield Inlet epidemic, which was shown intable 2 as one of the world's most severe out-breaks.
For our studies we used virus in extracts offeces collected several weeks after the epidemicfrom patients with paralytic illness and theirclose contacts. The feces were sent to us by Dr.F. P. Nagler of the Canadian Laboratory ofHygiene. These virus strains were compared withtype 1 poliovirus in extracts of fecal specimenssupplied to us by Drs. John Fox and HenryGelfand, who collected them from healthy sub-jects in their family studies in several communi-ties in Louisiana. The feces were collected duringa year when there was very little clinical polio-myelitis in the community.The virus strains from Chesterfield Inlet
showed no consistent difference from the Louisi-ana strains in plaque-forming ability on monkeykidney cells in the presence of medium withvarious concentrations of bicarbonate. Controltests with the Mahoney and LSc strains gave theresults expected from the report of Vogt,Dulbecco, and Wenner (1957).
In view of Sabin's observations on the failureof attenuated strains to spread readily throughthe bodies of infected animals, we looked into thepossibility that our Louisiana strains might be-have similarly. For these studies we took ad-vantage of the fact that poliovirus will localizein skin wounds of infected cynomolgus monkeys.One square centimeter of skin was excised from
each of 5 sites (figure 1). Three days later polio-virus was injected into the 3 lesions on one thigh,and the animal was placed in a restraining deviceto prevent him from transferring virus with hisfingers from one skin site to another. Sub-sequently virus was assayed at appropriate in-tervals by collecting small amounts of the wateryexudate that appeared under the scabs (figure 2).As expected from Sabin's work, the avirulent
LSc strain failed to spread from inoculated lesionsto uninoculated lesions (figure 2). Tests were thenmade on virus from the Chesterfield Inlet epi-demic. As expected, it multiplied readily andspread promptly to distant lesions.
For our present purposes the most interestingstudies were those with the "Iberia" strain of
04k1IQra
Figure 1. One square centimeter of skin wasremoved from the locations shown, three sites onthe left thigh, one on the right thigh, and one onthe head. Three days later poliovirus was injectedinto the base of the three lesions on the left thigh.Subsequent tests were made for local increase ofvirus and for spread of virus to the lesions on theright thigh and the head.
virus. This was one of the Louisiana strains.Although this virus was avirulent epidemiologi-cally, it was fully as active as other strains inmultiplying and spreading in the body, as shownin figure 2. Viremia occurred in all four animalsand lasted several days. The "Iberia" strain wasin no way like the LSc strain in this regard. Ofthe four animals infected, one developed typicalparalytic illness.
In summary, one may state that naturallyoccurring strains of poliovirus may be of high orlow virulence for men; the few pertinent studieshave failed to reveal viral characteristics thataccount for these differences or that are consist-ently correlated with high or low epidemiologicvirulence.
Paralysis at 10 daysNo illnessFigure 2. Concentration of virus in healing skin wounds and in blood of two cynomolgus monkeys,
one inoculated with LSc strain of poliovirus and one with the Iberia strain. Virus was inoculated intothree lesions on the left leg in the amounts shown. Note that the Iberia virus was recovered in relativelyhigh concentration from all five lesions and from the blood. The LSc virus multiplied locally only in thelesion given the largest inoculum. It did not spread even to the other lesions on the same leg. Virus was
assayed in monkey-kidney tissue cultures by the tube dilution method.Dark columns indicate presence of virus at the times indicated at the top of each graph; 0 = test for
virus was negative in all tissue cultures (usually 2, more if test was repeated); = test for virus was
positive in all tissue cultures (usually 2, more if the test was repeated); L = test for virus was positivein at least one tissue culture and negative in at least one tissue culture. Vertical axis = negative log ofdilution of exudate; 0.1 ml was used in each test.
In any case, one can assume that properties ofthe virus itself cannot account for the chief differ-ence between those who suffer paralysis andthose who have poliovirus infection withoutparalysis. This conclusion is evident from thefact that even the most virulent virus straincauses paralysis of only a small proportion of thepersons it infects. Therefore, the characteristics
of the infecting virus must be less important thansome other factors in determining the occurrence
of paralytic disease.
STATUS OF THE INFECTED INDIVIDUAL
Let us next consider how the status of the in-fected individual may influence the infectiousprocess. A number of conditions likely to involve
only isolated individuals rather than whole popu-lations are considered to increase the likelihoodof paralytic illness. Among these may be listedpregnancy, previous tonsillectomy, and injec-tions during the month before infection. Age ofthe infected person, fatigue, and genetic constitu-tion may also influence the course of some polio-virus infections.
In considering how factors of this sort mayaffect poliovirus infections, it is helpful to reviewbriefly some of our ideas on the mechanismsthat suppress the viral growth and spread inpoliomyelitis. Antibody formed in the infectedperson undoubtedly plays an important role. Inthe usual infection, poliovirus first multiplies inthe tissues of the pharynx and the intestinal wall.While this is occurring, viral antigen reachesantibody-forming tissues. In due course neu-tralizing antibody appears in the tissue fluid andblood stream. This antibody is presumably effec-tive in preventing the spread of virus to thenervous system, if this has not previously oc-curred.
Obviously then, early spread of poliovirus tothe central nervous system would make paralyticillness more likely. Likewise, delay in the formationof antibody might have the same effect. The sig-nificance of these factors has been well describedin the case of several other viruses. Overman andKilham (1953), for example, have documentedwell the crucial importance of the time and ex-tent of antibody formation in determining theoutcome of experimental mumps virus infectionsof the central nervous system of young hamsters.A shift from high susceptibility to resistance oc-curred with increasing age and was shown tocorrelate with an increased rate of antibody for-mation with aging. Precise data of this sort arenot available for poliovirus infections. To myknowledge there are no comparable data on therate of antibody formation as a factor in naturalpoliovirus infections of man. It is possible thatthe relatively high rates of paralytic diseaseassociated with pregnancy, tonsillectomy, andinjection of vaccines with certain poliovirusstrains reflect conditions that favor invasion ofthe central nervous system by virus before anti-body response is adequate to arrest viral spread.The same general concept can be extended tospread of virus within the central nervous system,with the qualification that substantially moreantibody is required to be effective.
Although one can state with confidence thatseveral specific conditions affecting the statusof the individual may increase his chances ofsuffering paralytic disease, it remains true thatthese factors cannot be shown to apply in mostcases of poliomyelitis. A relatively small pro-portion of the paralytic infections occur in preg-nant women, or persons with a history of recentinjections, for example. It seems that our ignor-ance of the host-controlled factors that influencethe course of poliovirus infections is probablymuch greater than our understanding of this sub-ject and that the personal conditions within theinfected individual that are most important indetermining the course of a poliovirus infectionhave eluded investigators up to the present time.Furthermore, there is evidence that circum-stances outside the infected person are important.
ENVIRONMENTAL CONDITIONS
We are here shifting to what might be calledan ecological study of the virus and of the in-fections it causes. We are engaged in a searchsomewhat like that of the early naturalists whonoted that mud and a warm sun generated in-sects in profusion. We are looking for the counter-parts of mud and warmth that generate paralyticpoliomyelitis. If we find them, it may be presumedthat further research will permit us ultimately totrace the evidence back to a rational explanationof our findings.
It is well known that environmental conditionscan increase the incidence of certain infectiousdiseases. This is brought about by either orboth of two general mechanisms: by increasingthe number of persons infected, as in water-borne typhoid; or by causing more of those whoare infected to develop illness, as in the activa-tion of latent psittacosis in parrots subjected tomistreatment. In poliomyelitis both kinds ofenvironmental mechanisms are important.
SeasonThe most obvious environmental influence on
the incidence of poliomyelitis is season. Thisdisease occurs predominantly in the summer andearly autumn months. Turner and his associates(1950) showed many years ago that the spread ofpoliovirus in Baltimore was largely restricted tothat time of yvear when paralytic cases occurred.The mechanism that serves to restrict the
spread of poliovirus in winter or to facilitate its
spread in summer is unknown. There are, how-ever, plenty of winter outbreaks to show thatman is susceptible to poliovirus infection and toneurological involvement during any season ofthe year.
Children in the HouseholdA second environmental factor worthy of
careful attention is the presence of young childrenin the household. Several reports (Rindge, 1957;Poos and Nathanson, 1956; Siegel, Greenberg,and Bodian, 1957) indicate that paralytic polio-myelitis occurs with a higher incidence amongadults living in households in which youngchildren are present than among other adults.Siegel and his associates, for example, studiedthe distribution of paralytic poliomyelitis casesin New York City. They stated that the presenceor absence of children in the household is "themost significant factor influencing the rates ofparalytic poliomyelitis in adults." Among preg-nant women the attack rate was twice as greatin those with children as in those not havingchildren.
Proximate Fecal Pollution of Water SuppliesA third environmental factor worthy of careful
evaluation came to my attention during a studyof an interesting epidemic in Nebraska in 1952(Bancroft, Engelhard, and Evans, 1957). Thepoliomyelitis cases occurred in one part of acommunity called Huskerville but spared theremainder of this community.
Dr. Paul Bancroft, a local pediatrician with aspecial interest in bacteriology, carried outdetailed epidemiological studies. He later enlistedthe help of Dr. Warren Engelhard of the Uni-versity of Nebraska to carry out serological tests,and they subsequently invited me to participatein the analysis of the data and the completion ofcertain laboratory tests.
Huskerville was a former military hospitalwhere married World War II veterans attendingthe University of Nebraska lived with theirfamilies. It was an unusually homogeneouscommunity. Barracks-like buildings, clustered infour rows on the open prairie, had been sub-divided into apartments suitable for one family.An abrupt, severe outbreak of poliomyelitisoccurred among residents of two and one-halfrows of buildings. This section of the village willbe called Area A, and the balance of the com-munity will be called Area B.
In Area A there were 347 children. A clinicaldiagnosis of poliomyelitis was made in 11.5 percent of these children; 4.6 per cent of themsuffered paralysis lasting at least two years. InArea B there were 256 children. Among themthere were no cases of paralytic poliomyelitisduring the epidemic; one case developed threeweeks later.There were no geographical or social barriers
between Area A and Area B. Nor were there anybiological or other environmental factors corre-lated with the distribution of cases. The spreadof measles and chickenpox showed no similarrestriction to parts of the community.The important questions in this case were as
follows: Why were there no cases of paralyticpoliomyelitis in Area B? Was the virus somehowrestricted to Area A? Or did infection reachchildren in all parts of the village but causeserious illness only in Area A? Answers to thesequestions were obtained in terms of the mostprobable situation. Available evidence was re-markably good but not adequate to be com-pletely certain in answering them.Serum specimens available were collected
approximately 18 months after the epidemic. Inspite of this delay it was possible to deducevaluable information from tests for antibody.Serological tests established that the epidemicwas caused by poliovirus of type 1. It furtherappeared that about 85 per cent of children ofpreschool age had been infected in Area A andabout 65 per cent in Area B. There was an obviousdisparity between the high infection rate in AreaB and the absence of paralytic cases in that partof the community. From the serological evidenceone might have expected a substantial number ofparalytic poliomyelitis cases in Area B. Thelack of such cases suggests that poliovirus infec-tion was more dangerous in Area A than in AreaB.The probability that chance alone could
account for the distribution of poliomyelitiscases with paralysis among those infected wasless than 1 in 100. If we include cases that did notshow residual damage after two years (and mostbona-fide paralytic cases recover completely),the probability that chance alone caused thedifference between Area A and Area B is reducedto 1 in 100,000.A careful epidemiologic study revealed one
be related to the distribution of paralytic cases.This was fecal pollution of the water supplyclose to the apartments where paralytic polio-myelitis occurred. Toilets with defective plumb-ing were present in 12 apartments in Area A andin one apartment in Area B. Aspiration of toiletcontents into the water line was favored byseveral episodes of negative pressure in the weekspreceding the epidemic. Chlorination during andprior to the epidemic was insufficient to kill fecalbacteria.The most probable source of infection in Area
A appears to have been polluted water obtainedclose to the point at which fecal pollution oc-curred. In Area B infection was also prevalent,but we do not know how the virus spread.From the available evidence we were led to
suggest an hypothesis to explain the peculiardistribution of poliomyelitis in Huskerville. Inour hypothesis we proposed two related concepts:First, that the mechanism of spread of virus mayhave been the crucial difference between Area Aand Area B; second, that the specific dangerousmechanism that occurred in Area A may havebeen ingestion of poliovirus in polluted waterobtained close to the point of pollution. We haveused the phrase proximate pollution in referringto this particular kind of viral spread.The concept that mode of spread might be
crucial in determining whether man developsparalytic illness or latent immunizing infection isnot new. Lepine (1955) and Wilson (1955)suggested that inhalation of infected pharyngealsecretions might be more dangerous than infectionfrom ingestion of "excretal material." Wilsonpointed out the difficulty of accounting for pre-dominant occurrence of the disease in summer ifsignificant infection is chiefly by the respiratoryroute.
In attempting to assess the hypothesis thatproximate fecal pollution of water can increasethe paralytic attack rate of poliovirus, it is ofinterest to consider how this might alter thecourse of the infection. At least three possibilitiescan be mentioned. First, transfer of unusuallylarge amounts of virus might cause the virus tomultiply and spread more rapidly than usual inthe infected person, before an immune responsedeveloped. Second, ingestion of unusually largeamounts of virus with water might cause primary,infection of tissues at an unusual location in thealimentary canal, a location from which inva-
sion of virus was favored. A third possibility isthat there was transfer of other fecal viruses ormicrobes with potentiating effects on the invasionof poliovirus. It is conceivable that any one ofthese 3 factors might make a poliovirus infectionmore hazardous. There is only a limited amountof information of use in evaluating them.
Sabin and Winsser (1953) summarized dataindicating that the amount of virus fed to orallyinfected monkeys had a substantial effect on thenature of the proportion developing neurologicaldisease. Rhodes and van Rooyen (1958) suggestthat exposure to a large amount of virus increasesthe likelihood of infection but add, "There is noevidence that the actual variety of illness develop-ing is directly dependent on the amount of virusentering the body."
Dalldorf and Wiegand (1958) have presentedexperimental evidence that some Coxsackieviruses of group A may act synergistically withpoliovirus to increase the likelihood of paralyticillness. Epidemiological evidence of an associationof group A Coxsackie viruses with poliovirus inparalytic cases has been reported repeatedly(Melnick et al., 1951) and has been noted specifi-cally not to occur on other occasions (Curnenand Melnick, 1951). If a second infectious agentis indeed an important factor in making "proxi-mate" pollution a dangerous mode of spread ofpoliovirus, it may be presumed that more distantpollution is less dangerous because the agent islost by dilution or by instability in cold water.Both Coxsackie viruses and poliovirus survivemany days in cold water.
Re-examination of reported poliomyelitisoutbreaks reveals many in which proximatefecal pollution of water can be presumed to haveoccurred. However, in nearly all cases, it isimpossible to assess the significance of this factor,for suitable data are not available. Furthermore,it is impossible to rule out the occurrence ofproximate pollution in situations in which it isnot suspected. Plumbing cross connections andother sanitary defects are undesirably common inwater distribution systems of our cities. Suchcross connections might be a factor in the well-known clustering of paralytic cases in some epi-demics, some outbreaks in hospitals and otherinstitutions (Sims-Roberts and Thomson, 1953;Ingalls and Aycock, 1951), or for single or multi-ple cases in private homes. The many studies ofmultiple cases in single families have failed to
give a clear picture of the factors involved (Siegeland Greenberg, 1954; Littell and Smith, 1955;Pierce, 1958; Strom, 1959).One wonders whether a common denominator
exists between proximate fecal pollution of waterand the importance of young children in thehousehold in relation to paralytic poliomyelitisin adults. All parents are fully aware that fecalcontamination from young children is stillproximate, frequent, and ample.
Epidemics in Virgin PopulationsIt is well known that infectious diseases may
cause unusually severe epidemics in previouslyunexposed populations. The paralysis of 55 percent of the Eskimos in the Maguse River out-break and 21 per cent of those in the ChesterfieldInlet epidemic are good examples of this phenom-enon. The explanation is, however, not clear.
It has been suggested that epidemics in isolatedvirgin populations are severe because prior selec-tion for resistance has not occurred. A geneticsusceptibility might be established in this manner.This explanation for the severity of epidemics invirgin populations is open to serious question.
In the devastating measles epidemic amongFiji Island natives in 1875, 40,000 died. This wasone-fourth of the total population. If this highfatality rate were due to a genetically determinedsusceptibility, it might be expected that measleswould continue to be exceptionally severe over aperiod of several generations during which selec-tion for resistance to measles would occur.Contrary to this prediction, measles has notshown unusual severity among Fijians since thatdate.2 Others have pointed out (Pickles, 1939)that the concept of genetically determined highsusceptibility of Fijians in 1875 is contradictedby the low fatality rate among those who, asmembers of the police force, lived under condi-tions quite different from other natives of theisland and received some care during theirillness. It seems desirable to keep an open mindon the explanation of the severity of viral epi-demics in virgin populations.
2 In a personal communication dated November,10, 1952, Dr. G. Loison, of the South PacificCommission, provided statistics on the occurrenceof measles in Fiji 1875 to 1951. This informationcame from the "Annual Reports of the MedicalDepartment" and was sent to Dr. Loison by theInspector General of the South Pacific HealthService at Suava.
There is great need for careful, on-the-spotinvestigation of severe epidemics in previouslyunexposed populations. These should include anevaluation of the interrelated effects of thequality of care provided and the evidence con-cerning the route and circumstances of viraltransmission. A microbiologist would also wonderwhether the so-called normal flora of isolatedpopulations might be different and might insome way influence the growth and spread ofpathogens.
VIRAL INFECTIONS IN VITRO AS MODELS FORNATURAL INFECTIONS
In concluding this discussion I wish to shiftaway from a consideration of specific investiga-tions of poliomyelitis and present a broader viewof some of the main lines of virological researchthat bear on our problem.
Exciting new knowledge of viruses and theirmechanisms of infecting cells appears almostdaily. We have good reason to hope that we willsoon possess a much-improved understanding ofthe physiological processes involved in viralinfections of a cell either with or without destruc-tion of the cell.
There is now great interest in virus infectionsthat persist for months in healthy, thrivingtissue cultures. A related field of fruitful researchis the study of systems in which virus-infectedcells produce substances which will confer onother cells protection against viral infection. Inour laboratory, Chambers (1957), Lockart(1960), and Wilcox (1959) have studied two"carrier culture" systems in which most of thecells are protected by "auto-interference." Thework of Isaacs and Lindeman (1957) and otherswith interferon and the studies of Ho and Enders(1959) represent major contributions in this field.We are probably only on the threshold of acomprehensive understanding of such systems.The healthy, infected cultures have an obvious
resemblance to the person who remains wellthroughout the course of an infection with polio-virus. However, one can predict with reasonableassurance that this broad area of research willfall short of providing a full understanding ofthe reasons why most persons infected withpoliovirus have no illness and proportionatelyfew are paralyzed.
It seems clear that there must ultimately bean explanation for such questions as the following:
Why does paralytic poliomyelitis occur morecommonly in adults who share a home withchildren? Why can poliovirus cause a high rateof paralysis in one part of a community andspread without such effects in another part ofthe same community Why are the highestparalytic attack rates restricted to rather smalland sharply defined population groups?
Solution of the problems posed by the virus asit manifests itself in nature will undoubtedlyrequire that studies focused on the course ofvirus infections in cell populations in vitro belinked with further studies of epidemiology andattempts to define more precisely the successiveevents that occur during infection of experimentalanimals.
CONCLUSIONS
From the evidence presented, the followingconclusions appear justified:
1. Naturally occurring poliovirus strains differin their paralytogenic capability for man. Noviral characteristics or markers have as yet beenshown to have general validity as indicators ofthe degree of virulence of these "wild" virusstrains as they occur in naturally acquiredinfection.
2. Most naturally acquired human infectionswith even the most virulent strains of virusresult in no paralytic illness. Therefore, factorsother than the nature of the infecting strain ofpoliovirus must be crucial in determining theoutcome of most infections.
3. A number of personal characteristics suchas pregnancy, tonsillectomy, or recent injectionsincrease the likelihood that poliovirus infectionwill lead to paralytic disease. Most cases ofparalytic poliomyelitis occur in persons to whomthe known factors of this sort do not apply. Itmay be presumed that the individual conditionsthat are of greatest significance in determiningthe course of a poliovirus infection are still notidentified.
4. A number of environmental circumstanceshave an important influence on the occurrence ofparalytic poliomyelitis. Of these, only the obviouseffect of season appears to be generally acknowl-edged at this time. In addition, the presence ofchildren in the family, the ingestion of fecallypolluted water close to the place of pollution, andinfection of isolated populations under undefinedcircumstances appear to merit serious attention.
5. Research on chronically infected and ap-parently healthy tissue cultures and on sub-stances derived from virus-infected cells thatconfer protection on other cells can be expectedto have significance in resolving some problemsin the pathogenesis of poliovirus infections.Additional epidemiological studies and investiga-tions of the sequence of events in experimentalinfections of animals are also needed if we are tounderstand what determines whether a personinfected with poliovirus develops paralytic illnessor suffers no significant harm.
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