In the present studies ten common species of Western North American mosquitoes have been tested for their ability to act as vectors of Japanese B encephalitis virus (see summary Table XII). The strain of Japanese B encephalitis virus which was used was adapted to direct mouse brain passage, probably a disadvantage, but no freshly isolated strain was available. Of the ten species of mosquitoes tested, seven were demonstrated to be laboratory vectors. These seven species represent three genera (Culex, Aedes, and Culiseta). In previously reported work Japanese and Russians had only incriminated five species of two genera (Aedes and Culex) (1–3). Transmission was made to mice 21 times and to a chicken once. Two attempts to infect mosquitoes from an infected chicken were unsuccessfui, but no significance is attached to so few experiments. Repeated tests for virus in the eggs, or in imagines reared from eggs of infected female mosquitoes have been negative. In this we failed to confirm results claimed by Japanese investigators (5, 6).
These data, in addition to the published accounts by Japanese and Russian workers of the natural epidemiology of this disease lead us to believe that this virus might well establish itself in North America, especially if introduced in those areas where our native encephalitides are now endemic. These studies also indicate that species of mosquitoes (Culex tarsalis, Culex pipiens,
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Aedes dorsalis, and Culiseta inornata) now known to be fully incriminated vectors of the Western equine or St. Louis encephalitis viruses can also serve as laboratory vectors of the Japanese B virus. Methods for the effective abatement of these species should be further developed and put into practice if future epidemics of encephalitis of the Western equine, St. Louis, or Japanese B types in Western North America are to be prevented or brought under control.