Yellow Fever Endemicity in West Africa, with Special Reference to Protection Tests

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Abstract
Summary and Discussion The results of all tests described above are summarized in table 5. It will be noted that among 125 sera, collected at random from persons living in Ibadan and Ilorin, in the region considered endemic, thirty-eight, or 30.4 per cent, protected duplicate animals. In Ife, in the same area, but where an epidemic of yellow fever had recently occurred, the percentage of protection was even higher, as seventeen among twenty-five specimens tested, or 68 per cent, protected two animals. The results of similar tests carried out with sera from persons resident in Northern Nigeria stand out in sharp contrast, for among ninety specimens tested only one was found to be positive. Twenty-four per cent of the sera of children between the ages of 4 and 5 years, taken at random in Ibadan, and a similar percentage of sera taken from children between the ages of 4 and 7 years at llorin protected duplicate animals against yellow fever virus. It appears, therefore, that at least one of every four young children in these cities has had an attack of yellow fever. Although, as will be explained below, the actual number of children who have had yellow fever is probably greater than these figures suggest, the results afford strong evidence of the existence of endemicity in Southwestern Nigeria. It should be noted here that an effort is being made to correlate the evidence for endemicity, afforded through these tests, with the identification of actual cases of yellow fever among the native population. Children in government and mission schools have been kept under fairly close observation, febrile conditions among children and adults have been carefully investigated, and large numbers of natives have been questioned as to past illnesses. As mentioned above, an extensive epidemic was observed in Ife and strains of virus were isolated from two patients. The presence of infection was also demonstrated by the mosquito catch in that town. In Ibadan and Oshogbo in 1925 and 1926 several cases were observed among Europeans and one case was seen in a native. On the other hand, it is not yet possible to identify clinically the mild cases which are presumably occurring constantly and in considerable numbers among the natives. This study is complicated by the fact that the African, being suspicious and preferring native medicine to that of the white physician, has a tendency to hide his sick. Furthermore, almost 100 per cent of children harbor malaria parasites and over 50 per cent of those in Ibadan are infected with schistosomiasis. Under the circumstances, it is difficult to eliminate malaria in febrile cases, and albuminuria loses its significance when the patient is the victim of schistosomiasis. Histories of earlier attacks of jaundice with suggestive symptomatology were obtained from a number of persons, but the percentage of these whose sera afforded protection was not materially higher than that found in persons of the same age groups taken at random. As recently reported, (4), a disease associated with jaundice and superficially resembling yellow fever is widespread in West Africa and adds greatly to the difficulty of diagnosis in individual cases and evaluation of histories of past illnesses. During the extensive epidemics studied on the Gold Coast, a few babes-in-arms with scleral icterus were observed, but no child under 5 years of age presented a clinical picture justifying the diagnosis of yellow fever. It is accordingly believed that the manifestations of this disease in native children, especially in endemic areas, are rarely sufficiently marked to permit of clinical diagnosis even under the most favorable conditions. As previously mentioned, duplicate monkeys were used in testing each specimen, and in the interpretation of the results sera were considered as positive only when both animals survived and negative when either or both died. In analyzing the findings, however, it will be noted that the percentage of cases in which only one animal died while the other survived is very much higher with specimens from Ibadan and Ilorin than with those secured from cities and towns in the north: 27 in 125 as compared with 6 in 90. The fact that rhesus monkeys are not equally susceptible and that a certain number are entirely refractory even to the highly virulent strain of virus used in these tests affords an explanation for the survival of some animals even when the sera contained no protective bodies, and where the duplicate animals died. However, as all sera in both series mentioned were tested against approximately the same amount of virus, and under similar conditions, we fell certain that the large number of sera from the south coming within this category cannot be explained on the basis of relative resistance of the Macacus rhesus alone. It is impossible to estimate with absolute accuracy the percentage of monkeys which is totally refractory or only moderately susceptible to a particular sample of yellow fever virus. But if, for purposes of comparison, the tests with the sera from Bauchi, Zaria, and Kano are taken as controls for the experiments carried out with specimens from other localities, and we accept one as protecting and the remainder as definitely negative, it appears that of the total of 178 monkeys with which the remaining eighty-nine sera were tested, only six, or slightly over 3 per cent, survived, and of these, five had definite febrile attacks from which they recovered. It is probable that some variation in susceptibility occurs with different lots of animals and at different times; but, making allowance for such variations, we fell confident that not more than 6 per cent of normal rhesus monkeys survive the injection of 0.1 cc. of virulent blood taken at the onset of fever from an animal infected with the Asibi strain, and this is in accord with the results obtained in a large number of uncomplicated experiments in which animals were injected with this dose of virus....

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