Distribution and Incidence

In the Americas epidemic yellow fever has been in rapid decline during the twentieth century, essentially due to efforts aimed at eradicating A. aegypti from major population centers. The last outbreak of urban yellow fever took place in Trinidad in 1954.

Nonetheless the yellow fever virus remains very much alive in the monkeys that inhabit the forests of Central and South America, and consequently a small number of human cases (between 50 and 300) continue to be reported annually among individuals who work in or live in close proximity to those forests. The vast majority of the cases occur in regions of Brazil, Ecuador, Venezuela, Colombia, and Peru that are drained by river networks contributing to the Orinoco, Magdalena, and Amazon systems. A bit earlier in the century, human cases of the disease were also reported with some regularity in Central America, Bolivia, Argentina, and Paraguay.

In Africa, severe epidemics of urban yellow fever still occur from time to time; a notable recent example is the Ethiopian epidemic of 1961, which cost thousands of lives. Still more recently a major outbreak in western and southwestern Nigeria claimed thousands of lives. Isolated human cases, however, are not reported with systematic regularity despite the vast belt of endemic yellow fever that stretches across much of that continent, and despite the presence of antibodies in a great number of the inhabitants of that belt, indicating past yellow fever infection.

One mystery surrounding yellow fever is that it has never occurred in Asia, despite the presence there of the Aedes mosquito vector. Some think that the mosquitoes themselves in that part of the world are resistant to infection. Others suspect that a population may be able to support only so many group B arboviruses and that entrenched illnesses in this category such as dengue and Japanese encephalitis may have forestalled the advance of the yellow virus.

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