EXPERIMENTAL INFECTION OF HORSES WITH THREE STRAINS OF VENEZUELAN EQUINE ENCEPHALOMYELITIS VIRUS II. EXPERIMENTAL VECTOR STUDIES1

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Abstract
Sudia, W. D., V. F. Newhouse and B. E Henderson (CDC, Atlanta, Ga. 30333). Experimental infection of horses with three strains of Venezuelan equine en-cephatomyelitis virus. II. Experimental vector studies. Amer J Epidem 93: 206–211, 1971.—Attempts were made to infect Aedes triseriatus, a mosquito species known to be highly susceptible to the Trinidad strain of Venezuelan equine encephalo-myelitis (VEE) virus, by feeding it upon horses previously Infected with the Fort Detrick live attenuated VEE virus (TC-83) and a Florida VEE virus strain (Fe3-7C). In neither case did the mosquitoes become infected since the viremias produced in the horses were below their threshold of infection. These findings suggest that either virus strain might be used for field vaccination of equines without particular danger of further propagation by mosquitoes; however, the studies were too limited to be accepted as proof in this regard. The low viremias produced in horses by the prototype Florida strain suggest that small mammals serve as the main source of infection for mosquitoes in south Florida. The restriction of VEE virus in south Florida to local enzootic foci may, in part, be attributed to the absence of the horse from its amplification cycle. Four of eight species of mosquitoes tested, all indigenous to the southeastern United States, became infected after feeding on horses which had been inoculated with a virulent Guatemalan strain of VEE virus. The infection and transmission rates for Psorophora confinnis were 28% and 33%, respectively. Thresholds of infections were determined to be 103.2 to 103.5 SMICLD50/.O2 ml for P. confinnis and Aedes aegypti and 105.0 to 105.4 for Aedes taeniorhynchus. The importance of host viremia levels, mosquito susceptibility, mosquito abundance and association of the mosquito with pertinent vertebrate hosts in VEE outbreaks is discussed.

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