Etiology and Epidemiology

Generally speaking, humans suffer most from those illnesses for which they are not the intended host. Bubonic plague, for example, is normally a disease of rodents that has, from time to time, incidentally infected humans with devastating consequences. Similarly, yellow fever is normally a disease of non-human primates, particularly monkeys. The disease is transmitted among them by mosquito vectors, but not mosquitoes that are ordinarily attracted to human beings. In this form the disease is called sylvan or jungle yellow fever, and is enzootic, meaning that the pattern of transmission is from nonhuman primate to mosquito to nonhuman primate.

When the disease leaves the treetops (as, for example, when a tree is felled), and when mosquitoes such as Aedes africanus and Aedes simpsoni in Africa and

Haemogogus genera in the Americas link jungle yellow fever with humans and begin a cycle of transmission from nonhuman primate to mosquito to human, the disease is often called endemic; when the yellow fever virus is taken in the blood of an infected human to heavily populated areas where the cycle is one of human to the female A. aegypti mosquito to human, the disease is termed epidemic or urban yellow fever.

The habits of the female A. aegypti have much to do with shaping the characteristics of an epidemic. She is a domestic mosquito that lives close to humans, depending on them for blood meals and breeding in puddles or containers of water in and around their places of dwelling. Her range is very short, generally a few hundred yards at most, which means that A. aegypti requires a fairly closely packed human population. Because A. aegypti can survive only a few days without water (although her eggs can survive for years in dehydrated form) and requires water in which to breed, adequate rainfall is a prerequisite for urban yellow fever, and indeed, many of the classic epidemics have taken place shortly after a period of extended rainfall. Warm weather is another prerequisite, for A. aegypti will not bite when the temperature falls below 62°F (or 17°C), and extended chilly weather will send her into hibernation.

The virus also has some distinctive requirements, especially for transmission - a process in which humans are best thought of as the site where the virus changes mosquitoes. This exchange can take place only during the first 3 to 6 days of infection of the yellow fever victim while the virus still remains in the blood (viremia); after the virus has entered the mosquito, it must incubate for another 9 to 18 days before the mosquito can infect another human being. After this period of extrinsic incubation, however, the mosquito will remain infective for the remainder of its life, which could be upward of 180 days, although generally the life-span of the female A. aegypti is closer to a month or two.

Although it has generally been thought that monkeys serve as the reservoir for the virus, other mammals (marsupials and armadillos) are also suspected of carrying the virus in endemic areas, and for short periods of time the mosquito populations can act as a reservoir.

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