Typically, this is the first location in California and generally the western United States, where arboviral infections are discovered in mosquitoes each year, 19 – 21 and the Imperial Valley just south of this location is where WNV was first discovered west of the continental divide. ![]() The north shore of the Salton Sea in Coachella Valley is a unique area to study the introduction of arboviruses into western North America by northbound migrants. ![]() Neotropical migratory birds would seem to be the likely transport mechanism for such introductions. Temporal studies in California have shown intermittent genotype change among WEEV and SLEV isolates, 17 – 19 indicating possible introduction and replacement events. Louis encephalitis (SLEV) 15, 16 viruses, suggesting limited genetic exchange. 8 – 11 In agreement, genetic studies have shown marked separation between North and South American strains of arboviruses, such as eastern equine encephalomyelitis (EEEV), 12 western equine encephalomyelitis (WEEV), 13, 14 and St. 6, 7 Historically in the United States, large studies along the Mississippi and Atlantic flyways failed to document the introduction of encephalitis viruses by northbound migrants. 1 – 4 Proof of principal for long distance movement was shown recently in the Old World by the recovery of WNV from migrating storks 5 and WNV antibody from other migrant species. The rapid dispersal and colonization of the New World by West Nile virus (WNV) has refocused attention on the role of migratory birds as vehicles for the rapid long distance movement of arboviruses. Our results indicated that birds entering California rarely exhibited a history of infection and that most birds probably became infected after entering California. ![]() Limited experimental infection studies with WNV showed that Orange-crowned Warblers were highly susceptible and frequently died, whereas most Yellow Warblers survived. In addition, of 1,109 migrants comprised of 76 species that were reported dead by the public and necropsied, 126 (11%) were positive for West Nile virus (WNV) RNA however, only three (0.7%) of 428 birds tested during the spring were positive. A total of 5,632 sera were collected from 43 species of migrants during spring (April–June), of which 34 (0.61%) comprised of 14 species tested positive by enzyme immunoassay only 10 were confirmed by plaque reduction neutralization tests (PRNT). Each spring large numbers of neotropical migrants traversing the Pacific flyway pass through the Coachella Valley enroute to northern destinations, providing an opportunity to test the hypothesis that mosquito-borne encephalitis viruses are introduced annually into California by migratory birds.
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