Vertical transmission of the yellow fever virus by Aedes aegypti (Diptera, Culicidae): dynamics of infection in F1 adult progeny of orally infected females.

Abstract

Abstract. Vertical transmission of yellow fever virus from orally infected females to their progeny was experimentally demonstrated in 2 Aedes aegypti colonies from the Dakar and Koungheul regions in Senegal. A total of 10,530 F 1 adult mosquito progeny were tested. The overall vertical transmission rate was 0.97%, with no significant difference between the Dakar and Koungheul colonies. The infection rates were significantly higher in females (1.15%) than in males (0.74%) in both colonies. The virus was not isolated from the progeny of the first oviposition cycle (OVC1). The true infection rates were 0.27% and 1.99%, respectively, for the OVC2 and OVC3 progeny in the Dakar colony, and 1.1% and 1.48%, respectively, for the OVC2 and OVC3 progeny in the Koungheul colony. The infection rates increased with extrinsic incubation in both male and female offspring of the 2 colonies, reaching 5.2% in 20-day-old OVC3 female progeny in the Dakar colony. The epidemiologic consequences of these results are discussed.In 1995, a yellow fever epidemic occurred in Koungheul, Senegal. 1 Entomologic surveys provided evidence of vertical transmission of the yellow fever virus by its mosquito vector, Aedes aegypti. The virus was isolated from wild males and recently emerged adults from larvae collected in the field. 2 Vertical transmission of yellow fever virus has already been proved experimentally in Ae. aegypti, 3,4 and 3 strains of this virus have been isolated from male Ae. furcifer-taylori mosquitoes. 5 The capacity of other mosquito species to vertically transmit yellow fever virus has also been shown experimentally in Ae. mascarensis and Haemagogus equinus. 6,7 The studies conducted in Koungheul on Aedes mosquitoes collected in the field had shown that the infection rate in recently emerged adults was lower than that in males caught on humans. This observation suggested that the viral titer in emerging adults infected by vertical transmission was low and hard to detect with conventional isolation methods, whereas the older mosquitoes had a higher titer. The objective of this study was to study the dynamics of viral infection in emerging adult mosquitoes vertically infected, using the same isolation methods as those used during the Koungheul epidemic. MATERIALS AND METHODSMosquitoes. Two geographic strains of Ae. aegypti from Dakar, and Koungheul, Senegal were used. These colonies were reared at 27 Ϯ 1ЊC and a relative humidity of 70-80% with a 12-hr photoperiod. Adult mosquitoes were regularly fed on guinea pig blood or 10% sugar and larval stages were provided with Tetra Baby Fish Food (TetraWerke, Melle, Germany).Virus. The viral strain used was ArD 114891. It was isolated from a pool of 10 Ae. aegypti females caught on humans in the village of Koung-Koung during the epidemic that occurred in Koungheul, Senegal in 1995. This strain was passed twice on AP61 cell lines (Ae. pseudoscutellaris) and 3 times on suckling mice. The viral stock that we used in our experiments was prepared with triturated, infected suckling mice bra...

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