Arbovirus surveillance in northern Colorado, 1987 and 1991

Arbovirus surveillance was conducted during an epizootic of western equine encephalitis (WEE) during 1987 and during a nonepizootic year, 1991, in the same area in northern Colorado. Mosquitoes were collected in Larimer County, CO, during weeks 33-37 (10 August to 7 September) in 1987 and during weeks 26-35 (24 June to 26 August) in 1991. In total, 13,099 mosquitoes in 694 pools collected during 1987 and 8,672 mosquitoes in 242 pools collected during 1991 were tested for virus. WEE virus was isolated in both years from Culex tarsalis Coquillett and from Cx. pipiens L. in 1987. Infection rates and population levels of Cx. tarsalis were not significantly different in the 2 yr during weeks 33, 34, and 35 (12-26 August). St. Louis encephalitis virus was isolated in 1987 from Cx. tarsalis. Other viruses isolated included Hart Park, Turlock, and Jerry Slough, a variety of Jamestown Canyon virus.