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

Abstract

It is concluded from a review of the literature on the transmission of St. Louis encephalitis and western equine encephalitis that, at least in the Yakima Valley, Washington [R.A.E., B 29 194; 30 190; 31 73], both viruses seem to be transmitted by mosquitos and that the source of infection in the mosquitos must be a very large inapparent vertebrate reservoir, probably chiefly among domestic fowls. As all known vertebrate infections have been short and laboratory infections in mosquitos have occasionally been lifelong, a mosquito that overwinters in the adult stage, such as Culex tar salis Coq., in the Yakima Valley, is considered a likely vector. C. tarsalis moreover has been found infected with both viruses in nature [31 73], frequents dwellings and feeds readily on domestic fowls. In tests of its ability to acquire infection from fowls and transmit it to them, mosquitos and fowls were confined in a temporary field laboratory in Yakima during the epidemic season in the summer of 1942. The St. Louis virus was repeatedly transmitted from fowls to fowls and from ducks to fowls after an incubation period in the mosquito of 8-16 days at 78-90°F. and about 85 per cent, relative humidity. C. tarsalis also transmitted the western equine virus from a duck and guineapigs to fowls after incubation periods of 10-19 days and transmitted it repeatedly to fowls after acquiring it by feeding on blood-virus suspension. In addition to Culex pipiens L. [cf. 30 162], C. coronator D. & K. [31 50] and Aëdes lateralis Mg. [31 74], A. taeniorhynchus, Wied., A. vexans Mg., and Theobaldia incidens Thos., transmitted the St. Louis infection after feeding on blood-virus suspensions, and T. incidens infected in the same way, also transmitted the western equine virus. The results given are thought to confirm that C. tarsalis is the vector and that fowls are the reservoir in the Yakima Valley, but the probability that this does not apply in other foci is stressed. It appears that control should be directed against mosquitos, with vaccination against the western equine virus in special circumstances. A vaccine for the St. Louis virus is not yet available. The classification " Arthropod-borne virus encephalitides " is suggested to include St. Louis encephalitis, western and eastern equine encephalitis, Japanese B encephalitis [32 2-4] and Russian spring-summer (tick-borne) encephalitis. The name is selected to avoid confusion with forms of infectious encephalitis that are not Arthropod-borne, such as rabies, or are not caused by a virus, such as sleeping sickness (African trypanosomiasis).