Mosquito Vectors and Inapparent Animal Reservoirs of St. Louis and Western Equine Encephalitis Viruses

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Abstract
A review and discussion. Field and laboratory investigations furnished data which indicated that in at least 1 epidemic area both St. Louis and western equine encephalitis were mosquito-borne, and that the source of mosquito infection was a huge inapparent reservoir among vertebrates, particularly domestic fowl. Since both viruses had been found in naturally infected Culex tarsalis, this species of mosquito was tested in the laboratory for its ability to acquire infection from fowl, and in turn to transmit it. This reproduction in the laboratory of the suspected natural cycle was effected for both viruses. Both types of encephalitis are evidently mosquito-borne and there probably exists an inapparent animal reservoir. However, other mosquitoes representing 3 genera and other arthropods are capable of transmitting the infection in the laboratory. In some areas these might serve as vectors, and in this case other vertebrate hosts might be involved as reservoirs. Control apparently should be directed primarily at the arthropod vectors, with human vaccination employed under special circumstances.