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Since an epidemic of western equine encephalitis in Manitoba in 1941, attempts have been made each year to isolate the virus from mosquitoes collected during the summer months. In 1942, 94 pools of mosquitoes were tested: 43 of the genus Aedes, 36 of Culex, and 15 of Culiseta. Suspensions made from each pool, which consisted of from 1 to 30 insects, were injected intracerebrally into guinea pigs. No isolation of virus was made. In 1943, 69 pools were tested: 49 of Aedes, 6 of Culex, 1 of Culiseta, 1 of Anopheles, and 12 of mixtures of different genera. Each suspension was derived from 10 to 75 mosquitoes and was injected intracerebrally into Rockefeller strain Swiss mice. There was no recovery of virus. In 1944, 90 pools were tested: 10 of Culex, 74 of Aedes, and 6 of Culiseta. Suspensions were usually made from 70 to 100 insects, and injected intracerebrally into Swiss mice of the Rockefeller strain. From one pool of Culex restuans (Theo.), a strain of western equine encephalitis virus was recovered.

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