First evidence of natural vertical transmission of yellow fever virus in Aedes aegypti, its epidemic vector.

Entomological investigations were conducted in 1995 in Senegal, following a yellow fever (YF) outbreak. A total of 1125 mosquitoes collected in the field, including males, females and 12-48 h old newly emerged adults reared from wild-caught larvae, were tested for YF virus. Among the 22 species captured, Aedes aegypti was the most common. 'Wild' vectors of YF were also captured, including A. furcifer, A. metallicus and A. luteocephalus. In all, 28 YF virus isolations were made: 19 from A. aegypti females, including 2 from newly emerged specimens; 5 were obtained from A. aegypti males, including one from a pool of newly emerged specimens, 2 from A. furcifer females, and one each from a female A. metallicus and a female A. luteocephalus. The true infection rates (TIRs) were much higher in adult A. aegypti than in specimens reared from larvae--8.2% and 31.4% for female and male A. aegypti captured on human volunteers, respectively (P < 0.0001). The TIRs for A. aegypti reared from larvae were 1.4% and 0.5% for females and males, respectively (P > 0.05). This outbreak was an intermediate YF epidemic, involving 4 vector species. Our data provide the first evidence of vertical transmission of YF virus in nature by A. aegypti, its main vector to humans, and strongly suggest that vertical transmission played a major role in the spread of the epidemic.