Monkeys in relation to yellow fever in Bwamba County, Uganda

Abstract

1. 1. For several years studies on the epidemiology of yellow fever have been conducted in Bwamba County, Uganda. The work has included the collection of blood samples from wild monkeys for examination by the protection test. In this report the results of tests on 150 Bwamba monkeys are discussed. Many other groups of animals have also been investigated, but so far immunity to yellow fever has been found in monkeys only. 2. 2. The laboratory technique employed in making the tests is briefly discussed, and the methods used in collecting bloods in the field are described. 3. 3. The natural history of the Primates of Bwamba is discussed, as their habits and distribution have been found to bear on the local epidemiology of yellow fever. So far, twelve ∗ species and subspecies of monkeys have been found to occur in Bwamba. In addition there are two members of the suborder Lemuroidea. 4. 4. In analysing immune rates by habitat, sex, and species, the crude data have been reduced to a standard population in which the frequencies in the various age grades were made proportional to those in the total sample. 5. 5. In Bwamba some species are partly terrestrial, some are mainly arboreal, and some are arboreal. Differences between the immune rates of these three groups are not significant. Immunization must occur largely in the trees, as the strictly arboreal species rarely descend to the ground. This implies that the vector is probably an arboreal mosquito, biting by night when all monkeys are in the trees.Reasons are given for suspecting that Agdes (Stegomyia) africanus Theobald may be the main local vector of monkey yellow fever. 6. 6. Immunity is widespread among the monkeys of the lowland forests. So far, no immune specimens have been found on the slopes of the Ruwenzori Mountains, which lie to the east of the lowland area. The general incidence of immunity among all Bwamba monkeys is 61 per cent. 7. 7. It is suggested that the lowland colobus monkey ( Colobus polykomos uellensis Matschie) is the main species involved in the monkey-to-monkey yellow fever cycle of the uninhabited forest areas, that the redtail ( Cercopithecus nictitans mpangae Matschie) plays an important part in bringing the virus into contact with man, and that the remaining species are of less importance in the local epidemiology. 8. 8. Analysis of the data with reference to sex shows that there is no significant difference between the immune rates in male and female monkeys. 9. 9. Similar analysis with reference to species shows that such differences as may exist between the immune rates of the important species are not significant. 10. 10. The incidence of immunity increases with increasing age. On the hypothesis that all age grades are equally exposed to infection, a curve can be computed which fits the observed data very closely indeed. It is shown that in Bwamba the susceptible monkey population becomes immunized at the rate of 27 per cent. per year. This observation, in conjunction with other evidence, leads us to believe that yellow fever is truly endemic among the monkeys of the Bwamba lowlands. Evidence is given to show that in nature the offspring of immune mothers possess passive congenitally acquired immunity to yellow fever for a few months after birth.