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Yellow Fever Surveillance -- Africa

The epidemics of yellow fever (YF) in Africa in recent years have stimulated research on the survival mechanisms of the YF virus during interepidemic phases. Virologic surveillance in West and Central Africa has led to the isolation of numerous YF virus strains, particularly from Aedes africanus, A. opok, A. furcifertaylori, and A. luteocephalus, outside of any declared epidemic. Forest-savanna mosaics, undifferentiated savannas of relatively moist type, differentiated savannas with abundant Isoberlinia, and equatorial moist forest belong to the "endemic area" in which the primary sylvatic circulation of YF virus can occur. In these transitional savannas, the circulation of YF virus fluctuates, giving way to intense epizootics that favor sporadic human infection. The term "emergence zone" has been suggested to designate this geographic belt of major epidemiologic importance. The "epidemic area," which generally seems to be inaccessible to primary sylvatic YF virus circulation, stretches beyond the limits of the emergence zone. Here, YF virus can occasionally be introduced by viremic humans and the threat of major epidemics is maximal. The emergence zone is probably the main source of initial contaminations. Certain outbreaks occurring on the southern border of the epidemic area (such as in Gambia in 1978) could be considered the outcome of occasional northward extensions of the emergence-zone borders in certain climatic situations.

Transovarial transmissions, recently demonstrated in A. aegypti, have apparently been corroborated in the field by the isolation of YF virus from male mosquitoes of the A. furcifer group. They also account for the survival of the virus during the dry season in emergence zone, as well as for the occurrence of pluriannual epizootic phases whose intensity and duration seems to be correlated to climatic factors and to the size of the monkey population. Yellow fever virus has also been isolated from the adults and eggs of Amblyomma variegatum ticks collected in the field. The notion of transovarial transmission supports the concept of "reservoir vector" formulated in previous studies. Nevertheless, the regular recurrence of the amplification process made possible by the mosquito-veterbrate cycle appears to be essential to survival of the YF virus. Reported by WHO Weekly Epidemiologic Record 1982;57:197-8.

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