Mosquito-transmitted encephalitis viruses: A review of their insect and vertebrate hosts and the mechanisms for survival and dispersion

Abstract

The present review deals primarily with vectors, vertebrate hosts, and mechanisms for maintenance and dispersion of the mosquito-borne encephalitis viruses (western equine, eastern equine, St. Louis, Japanese, Murray Valley, and Venezuelan). The Venezuelan virus is restricted to tropical areas, western and St. Louis viruses to temperate zones, and the eastern, Japanese, and Murray Valley viruses occur in both tropical and temperate zones, although the latter two agents are found primarily in the temperate zone. The five agents important in the temperate zones are ecologically associated with birds and, except for eastern equine virus, with Culex mosquitoes. The common factors in the maintenance and dispersion of these viruses may be sought in the association of virus, birds, and Culex mosquitoes. The eastern equine virus has been isolated from birds, and there is abundant evidence to indicate that they play some part in the circulation of this virus. There is no evidence of its occurrence in Culex mosquitoes, but epidemiological observations indicate that Aedes sollicitans may be a vector. Culex tarsalis has been definitely shown to be infected with virus throughout the area in which western equine encephalitis is found. This agent has been isolated repeatedly from birds. The bird- Culex tarsalis cycle appears to be adequately established in the case of this virus. Although the St. Louis virus has not been obtained directly from birds, there is sufficient evidence to implicate birds as being important in the ecological picture. The agent has been isolated several times from C. tarsalis and once from Aedes dorsalis and C. pipiens. On an epidemiological basis, the latter has been suggested as a vector. Japanese virus has not been isolated from birds, but serological evidence of previous infection of birds has been obtained. The majority of isolations of Japanese virus have been from Culex tritaeniorhynchus. Antibodies against Murray Valley virus have been detected in the sera of birds. On epidemiological grounds, Culex annulirostris is suspected of being important in the transmission of this virus. The evidence at hand, although it is not conclusive for the last named virus, indicates that there is an ecological relationship between the encephalitis viruses of temperate zones and mosquitoes and birds which accounts for the ability of these viruses to survive in cold climates. The single encephalitis virus (Venezuelan) restricted to tropical zones is thought to be transmitted by Mansonia tittilans, and no evidence has been obtained which would indicate that Culex species are involved. No evidence regarding the role of birds in the maintenance of this agent is available. Such mosquito-transmitted viruses as those of dengue and yellow fever, which occur primarily in tropical areas, are known to be transmitted by Aedes species and by Aedes and Haemagogus species, respectively. In general, Culex mosquitoes do not appear to be concerned with transmission of viral agents in the tropics. Much more field study is required before the ecological associations governing the presence of encephalitis viruses in the temperate zones can be considered to be firmly established. The available data suggest that there is a basic difference in the ecology of viruses of temperate and tropical zones and that further studies of ecological associations are justified.