Characterization of a Newly Identified Alphavirus, Eilat Virus
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Most alphaviruses are mosquito-borne and exhibit a broad host range, infecting many different vertebrates including birds, rodents, equids, humans, and nonhuman primates as well as mosquito species encompassing at least eight genera. The ability of alphaviruses to infect both arthropods and vertebrates is essential for their maintenance in nature. Herein I describe a unique alphavirus, Eilat virus (EILV), isolated from a pool of Anopheles coustani mosquitoes from the Negev desert, Israel. Genetic analysis showed considerable sequence divergence at the both nucleotide and amino acid levels. Phylogenetic analyses placed EILV basal to the western equine encephalitis antigenic complex within the main clade of mosquito-borne alphaviruses. Electron microscopy revealed that, like other alphaviruses, EILV virions are spherical, 70 nm in diameter, and they bud from the plasma membrane of infected mosquito cells. EILV readily infects a variety of insect cells with little overt cytopathic effect. However, in contrast to all other mosquito-borne alphaviruses, EILV was not able to infect mammalian or avian cell lines. EILV host range restriction was present at attachment and entry, as well as at the RNA replication level, suggesting the involvement of multiple viral genes. Susceptibility studies showed that Aedes albopictus, A. aegypti, Anopheles gambiae, Culex quinquefasciatus were susceptible to EILV infection via the intrathoracic route, but only A. aegypti was susceptible via the oral route suggesting a narrow vector range. Lastly, superinfection experiments in an A. albopictus cell line, C7/10, demonstrated that a prior infection with EILV reduced virus replication of Sindbis, chikungunya, eastern, western and Venezuelan equine encephalitis virus by 100- to 10,000-fold indicating that EILV virus may alter host susceptibility to superinfection and therefore the ability to transmit pathogenic alphaviruses. Taken together, these data identify the first host restricted mosquito-borne alphavirus and suggest that, evolutionarily, EILV has lost the ability to infect vertebrate cells. Thus, EILV may represent a previously undescribed complex within the genus Alphavirus. Reverse genetic studies of EILV may facilitate the discovery of determinants of alphavirus host range that mediate disease emergence as well as potential vector control and transmission disruption measures.