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Fig. 26-3 Epidemiological investigation of yellow fever. (A) Serological survey being conducted among fndians of the Ama zon forest. (B) Monkey being bled in Trinidad (Courtesy of the World Health Organization and Cambridge University Press Prom F. M. Burnet and D. O. White, "Natural History of Infectious Disease " Cambridge University Press, London, 1972 )

numbers of /^afrs aegypti have greatly increased again throughout much of Central and South America and southern United States, and urban epidemics could readily recur.

In endemic regions of West Africa infection of children is very common; although disease is usually subclinical or mild, most of the deaths nevertheless occur in children. Major epidemics continue to occur every few years in Africa—Sudan 1940, Ethiopia 1960-1962, Nigeria 1969, Gambia 1978-1979, Ghana/Burkina Faso 1983—an estimated 100,000 human cases with 30,000 deaths having occurred during the Ethiopian epidemic alone. Since 1985 there has been a resurgence of yellow fever in Africa, where 33 countries are currently at risk of outbreaks, and thousands of cases have occurred in Nigeria.

Urban yellow fever can be prevented by eliminating or drastically reducing the population of Aedcs aegi/pti mosquitoes in the vicinity of towns; the means of achieving this are discussed in Chapter 15. Vector control is not applicable to jungle yellow fever, but infection of the scattered human populations can be prevented by vaccination.

A highly effective yellow fever vaccine was developed by Theiler in the 1940s, long before the birth of modern virology. Intuitively, he derived an avirulent strain by isolating yellow fever virus from a monkey and passaging it successively through mice, various primary cell cultures, and finally chick embryos. This 17D strain is still the seed virus for today's chick embryo grown vaccine, which is supplied freeze-dried, stored refrigerated or frozen, and reconstituted for subcutaneous injection. Only 5% of vaccinees report mild constitutional side effects, and the only contraindications are known allergy

Monkey

Enzootic zone Zone of emergence

Rain forest Moist savanna/Gallery forest

Ae bromehae Ae lurciter-taylori other spp

Monkey Human Verlical Monkey Human

y ' Ae bromehae

Ambtyomma\ ^e (uraler-taylon ! ^y \ other spp [ Vertical | ' *\\ i ' Amblyomma

. Ae africanus

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