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I determined whether transovarial passage of Lyme disease spirochetes occurred more efficiently in Ixodes scapularis Say fed on reservoir-competent dogs. Unlike deerfed ticks, spirochetes are not diminished in dog-fed ticks, and significantly higher transovarial infection rates have been reported for the latter. Females of field-infected I. scapularis (North Salem strain) were used to induce experimental Lyme disease in specific pathogen-free dogs as part of another study. Seventy-four (48 of 65) percent of the females examined were infected with spirochetes, and larvae from infected females were tested for Borrelia burgdorferi within 1 mo of eclosion. No spirochetes were found in 14,700 larvae (225 larvae per female [n = 12] and 750 larvae per female [n = 16]) examined individually by fluorescent antibody (FA) tests or in triturated larval suspensions (approximately 1,500 larvae per female (n = 20]) examined by FA and polymerase chain reaction. Larvae remain an unlikely source of B. burgdorferi infection in the peridomestic setting because transovarial transmission is highly inefficient in I. scapularis.
Outbreaks of Rift Valley fever (RVF) in Egypt, Yemen, and Saudi Arabia have indicated the potential for this disease to spread from its enzootic areas in sub-Saharan Africa. Because little is known about the potential for most African mosquito species to transmit RVF virus (family Bunyaviridae, genus Phlebovirus, RVFV), we conducted studies to determine the vector competence of selected African species of mosquitoes for this virus. All eight species tested [Aedes palpalis (Newstead), Aedes mcintoshi Huang, Aedes circumluteolus (Theobald), Aedes calceatus Edwards, Aedes aegypti (L.), Culex antennatus (Becker), Culex pipiens (L.), and Culex quinquefasciatus Say], were susceptible to infection, and all except Ae. calceatus, Ae. aegypti and Cx. quinquefasciatus transmitted RVFV by bite after oral exposure. Estimated transmission rates for mosquitoes that successfully transmitted RVFV by bite ranged from 5% for Ae. mcintoshi to 39% for Ae. palpalis for mosquitoes that fed on a hamster with a viremia > or = 10(8) plaque-forming units of virus/ml. We did not recover RVFV from any of 3,138 progeny of infected female mosquitoes. RVFV is unusual among arboviruses in that it has been isolated in nature from a large number of species and that numerous mosquitoes and other arthropods are able to transmit this virus in the laboratory. The recent introduction and spread of West Nile virus into the Americas and the spread of RVFV to the Arabian Peninsula illustrates the potential for viruses, once enzootic in Africa, to spread to other parts of the world.
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