During studies on the emission of Sindbis virus from the mouthparts of experimentally infected Aedes aegypti, virus was detected in the anal discharge. This report deals with the quantity and source of virus expelled from the anus and with the possible significance of these findings with respect to the transmission of arboviruses from mosquitoes to vertebrate hosts. Anal fluid was collected from mosquitoes by several methods during and after the mosquitoes fed on either infant mice, chick embryos, or artificial feeding solutions contained beneath a membrane or within a capillary tube. Infected mosquitoes frequently discharged droplets or spray of virus-containing anal fluid during and after feeding. The probable source of the anal virus was the Malpighian tubules. Infant mice were infected by applying virus to broken skin or nasal mucosa. Sealing the anus of infected mosquitoes did not, however, alter the frequency of virus transmission, by bite, to infant mice. Transmission from infected mosquito to vertebrate host by this “posterior” route deserves further investigation with other arboviruses and appropriate insect intermediate hosts.
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