Vertical t !
Epidemic zone Dry savanna
Vertical 1 I I I
Fig. 26-4 Ecologic relationships of yellow fever virus in tropical Africa Established parts of the cycle are drawn in solid lines, unproved or rare cycles in broken lines In the rain forest zone of central and western Africa, the virus is maintained tn a continuous cycle of low-level transmission, involving monkeys and Aedes africanus mosquitoes In savanna areas with gallery forest along the watercourses, high rates of transmission to both monkeys and humans may occut during the rainy season, associated with high densities of treehole-breeding Acdcs vectors Humans may be infected in both of these cycles In the very dry savanna, and in urban areas, wheie domestic water storage is practiced, domestic Aedes aegypti may be responsible for inferhuman spread when the virus is introduced from endemic zones. Vertical transmission in mosquitoes probably occurs in each of these cycles. [From T. P. Monath, tn "Fields Virology" (B. N Fields, D. M. Knipe, R M Chanock, M S. Hirsch, J L. Melnick, T. P Monath, and B. Roizman, eds.), 2nd Ed , P 802 Raven, New York, 1990.]
to eggs, age younger than 6 months, and severe immunosuppression (e.g., AIDS). Over 95% of recipients develop solid immunity, which generally lasts for decades. Nevertheless, most yellow fever-free countries require that travelers through endemic areas have been vaccinated within the past 10 years. Vaccination policy within countries in which yellow fever is endemic is still not uniform. Some South American nations have undertaken routine vaccination for many years. Following the recent strengthening of the Expanded Programme of Immunization and the upsurge of yellow fever in tropical Africa, many African nations have incorporated yellow fever vaccine into routine childhood immunization programs; it is usually given at the same time as measles vaccine.
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