West Nile Virus (WNV) is an RNA arbovirus that cycles between mosquitoes and birds, but also infects and causes disease in humans, horses and vertebrate species. Although most infections are asymptomatic, WNV has emerged as the most common cause of epidemic meningoencephalitis in North America and the leading cause of arboviral encephalitis in the United States.
Mosquitoes of the genus Culex (Cx.) are the most active vectors of WNV in North America. Approximately 85% of human WNV infections in the United States occur in late summer with a peak number of cases in August and September. Vertical transmission is defined as the passage of virus from an infected female parent mosquito to her F1 progeny. Although vertical transmission of WNV has been demonstrated in the laboratory, confirmation of vertical transmission of WNV in wild mosquito populations has been elusive.
WNV was first identified in Clark County mosquito populations in 2004. The Southern Nevada Health District’s Vector Control office conducts surveillance and control of mosquitoes in rural and urban locations, including washes, drainage ditches, parks, and abandoned residential swimming pools. Since 2004, WNV positive mosquitoes, primarily Culex spp., have been collected June through September, the months with the hottest temperatures and longest daylight periods. Although WNV positive adult mosquitoes have been identified in Clark County, there has been no surveillance of larvae within these sites looking for vertical transmission of WNV. This study represented the first attempt to identify WNV in Culex spp. mosquito larvae of Clark County.
Using convenience sampling, larval and adult Culex spp. mosquitoes were collected between July 1st and September 31 st at locations throughout Clark County, including areas historically positive for WNV. Larval mosquitoes were maintained in emergence containers and allowed to mature into adults before being identified. Adult mosquitoes were identified to the genus and species level and submitted to the Nevada State Department of Agriculture, Animal Disease Laboratory, Sparks for West Nile Virus analysis.
A total of 3,171 emerged adults and 2,898 trapped adults were submitted from 67 corresponding sites during the study period. All samples were negative for WNV. SNHD tested an additional 13,000 adult mosquitoes from sites not part of the study; however these samples were negative for WNV. Additionally no human WNV cases were reported, representing the first year since 2004 that WNV was not identified in Clark County.