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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