Yellow Fever

Published on 22/03/2015 by admin

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Last modified 22/03/2015

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Chapter 262 Yellow Fever

Yellow fever is an acute infection characterized in its most severe form by fever, jaundice, proteinuria, and hemorrhage. The virus is mosquito-borne and occurs in epidemic or endemic form in South America and Africa. Seasonal epidemics occurred in cities located in temperate areas of Europe and the Americas until 1900, and epidemics continue in West, Central, and East Africa.

Epidemiology

Human and nonhuman primate hosts acquire the yellow fever infection by the bite of infected mosquitoes. After an incubation period of 3-6 days, virus appears in the blood and may serve as a source of infection for other mosquitoes. The virus must replicate in the gut of the mosquito and pass to the salivary gland before the mosquito can transmit the virus. Yellow fever virus is transmitted in an urban cycle—human to A. aegypti to human—and a jungle cycle—monkey to jungle mosquitoes to monkey. Classic yellow fever epidemics in the USA, South America, the Caribbean, and parts of Europe were of the urban variety. Since 2000, West Africa has experienced 5 urban epidemics, including in the capital cities of Abidjan (Cote d’Ivoire), Conakry (Guinea), and Dakar (Senegal). Most of the approximately 200 cases reported each year in South America are jungle yellow fever. In colonial times, attack rates in white adults were very high, suggesting that subclinical infections are uncommon in this age group. Yellow fever may be less severe in children, with subclinical infection:clinical case ratios ≥2 : 1. In areas where outbreaks of urban yellow fever are common, most cases involve children because many adults are immune. Transmission in West Africa is highest during the rainy season, from July to November. The migration of nonimmune laborers into endemic regions is a significant factor in some outbreaks.

In tropical forests, yellow fever virus is maintained in a transmission cycle involving monkeys and tree hole-breeding mosquitoes (Haemagogus in Central and South America, Aedes africanus in Africa). In the Americas, most cases involve men who work in forested areas and are exposed to infected mosquitoes. In Africa, the virus is prevalent in moist savanna and savanna transition areas where other tree hole–breeding Aedes vectors transmit the virus between monkeys and humans and between humans.