Mammalogy in an Epidemiological Study of Jungle Yellow Fever in Brazil

Abstract

Prior to 1932 yellow fever had been considered as strictly an urban disease, carried by the domestic mosquito, Aedes aegypti, in the classic cycle of manmosquito-man. In 1932, however, the Brazilian Yellow Fever Service, an organization maintained by the Brazilian Ministry of Education and Health with the cooperation of the International Health Division of The Rockefeller Foundation, discovered the disease in a rural area in the absence of Aedes aegypti (Soper, et al., 1933). This site, now historic from the standpoint of yellow fever, was the Valle de Chanaan, in the State of Espirito Santo, on the east coast of Brazil just north of Rio de Janeiro. The discovery marked a departure from the then current epidemiological beliefs regarding the disease, by calling for serious consideration of a different insect vector and a different vertebrate host (a jungle inhabitant) and also raised the question of possible control once the wild species in the newly discovered cycle were found. In late 1935, I joined the staff of the Yellow Fever Service and participated in the field investigations, involving clinical, laboratory, entomological, and vertebrate zoological aspects, which were begun as a joint operation at a field laboratory at Anapolis in the State of Goyaz. Studies were continued there throughout 1936 and 1937; another field laboratory was set up in the southern Matto Grosso in 1937; and in 1938 Fordlandia on the Rio Tapajoz near the Amazon, and the vicinity of Rio de Janeiro were the scenes of continued intensive field work. The solution of the problem of explaining all the epidemiology of jungle yellow fever is difficult, and is not yet complete; but it seems probable now (1941) that the disease is perpetuated by a cycle involving several species of wild mosquitoes and various mammals, especially monkeys and opossums. Control of these forms is not feasible, and an easily administered and effective vaccine has been developed as a solution to the problem of public health protection. The demand on the zoologist in the beginning was for the collection of specimens of blood serum for an immunity (mouse-protection) test from as many vertebrates as possible, particularly primates. For it had been shown by laboratory and field investigations, on monkeys and man, that recovery from yellow fever infection conferred a specific and lasting immunity in these species. Therefore, it was concluded that any species of wild animal whose blood sera showed a high percentage of immunes, could be seriously suspected of complicity in the natural jungle cycle of the disease. This was an indirect method to be sure, but the only direct way to pin guilt on a species of vertebrate would be to isolate the virus from the circulating blood of a wild-taken individual. This would not be as easy, however, as it sounds, because (1) no susceptible animals are known to harbor the virus more than a week or so (generally 3-4 days) after infectionthat is, there is no persistent infection, and (2) it would be necessary to guess several weeks ahead where yellow fever would occur in the human population 144