Transmission of La Crosse Virus in Southwest Virginia: Role of Accessory Vectors, Microfilarial Coinfection and Canine Seroprevalence

Abstract

Southwest Virginia has recently become an emerging focus of activity for La Crosse (LAC) virus, a mosquito-transmitted arbovirus in the California serogroup of Bunyaviruses. In 2005 and 2006, ovitrap surveys were conducted to access the spatiotemporal oviposition activity of LAC virus vectors Aedes triseriatus, Ae. albopictus and Ae. japonicus across a wide region of southwest Virginia. Egg abundance and oviposition patterns of these vectors were significantly different across the three study areas. The primary LAC virus vector, Ae. triseriatus, was collected in the greatest abundance from all three areas, and favored forested habitats. Aedes albopictus was the second most abundant species collected, and was found to favor urban environments. Aedes japonicus also has a preference for urban habitats, and is actively expanding its range throughout southwest Virginia. Dogs were used to determine their efficacy as sentinels for assessing the distribution of LAC virus in southwest Virginia. Canine serum samples were tested using plaque reduction neutralization (PRNT) assays. Of the 436 collected canine serum samples, 21 (4.8%) were positive for LAC virus antibodies. LAC virus seroprevalence was evident in dogs from each study region, including areas where LAC virus human cases and LAC virus positive mosquito isolates have not been reported. As a result, this study provided documentation of horizontal transmission of LAC virus throughout southwest Virginia, demonstrating that dogs make useful sentinels for assessing the distribution of LAC virus in an area. The final objective examined the effects of coinfection with D. immitis microfilariae and LAC virus in three species of Aedes mosquitoes. No significant differences were found between mosquitoes fed dually infected bloodmeals (i.e. D. immitis microfilariae and LAC virus) and those fed bloodmeals containing LAC virus only. A follow-up study found low mosquito midgut penetration rates by D. immitis, despite using biologically significant doses of microfilariae. Failure to demonstrate enhancement of LAC virus in vector mosquitoes suggests that D. immitis does not have a significant impact on LAC virus epidemiology in areas where these organisms co-exist.