Impact of aerial spraying of pyrethrin insecticide on Culex pipiens and Culex tarsalis (Diptera: Culicidae) abundance and West Nile virus infection rates in an urban/suburban area of Sacramento County, California.

In response to an epidemic amplification of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), the Sacramento and Yolo Mosquito and Vector Control District (SYMVCD) sprayed ultralow-volume (ULV) formulations of pyrethrin insecticide (Evergreen EC 60-6: 6% pyrethrin insecticide, 60% piperonylbutoxide; MGK, Minneapolis, MN, applied as 0.003 kg/ha [0.0025 lb/acre] ) over 218 km2 in north Sacramento and 243.5 km2 in south Sacramento on three consecutive evenings in August 2005. We evaluated the impact of this intervention in north Sacramento on the abundance and WNV infection rates of Culex pipiens L. and Culex tarsalis Coquillett. Mortality rates of caged Cx. tarsalis sentinels ranged from 0% under dense canopy to 100% in open fields. A comparison of weekly geometric mean mosquito abundance in CO2-baited traps in sprayed and unsprayed areas before and after treatment indicated a 75.0 and 48.7% reduction in the abundance of Cx. pipiens and Cx. tarsalis, respectively. This reduction was statistically significant for Cx. pipiens, the primary vector of WNV, with highest abundance in this urban area, but not for Cx. tarsalis, which is more associated with rural areas. The infection rates of WNV in Cx. pipiens and Cx. tarsalis collected from the spray zone were 8.2 and 4.3 per 1,000 female mosquitoes in the 2 wk before and the 2 wk after applications of insecticide, respectively. In comparison, WNV infection rates in Cx. pipiens and Cx. tarsalis collected at same time interval in the unsprayed zone were 2.0 and 8.7 per 1,000, respectively. Based on the reduction in vector abundance and its effects on number of infective bites received by human population, we concluded that the aerial application ofpyrethrin insecticide reduced the transmission intensity of WNV and decreased the risk of human infection.

Detection of chicken antibodies to mosquito salivary gland antigens by enzyme immunoassay.

Sentinel chickens are used to detect western equine encephalomyelitis, St. Louis encephalitis, and West Nile virus activity. Flocks that receive high mosquito exposure will be most effective for surveillance purposes. However, mosquito population indices at the flock sites may only provide an indirect measure of potential exposure. Therefore, we developed an indirect enzyme immunoassay to detect chicken antibodies to salivary gland antigens (SGAs) from Culex tarsalis, the primary arbovirus vector in California. Chickens fed upon by Cx. tarsalis developed an antibody response that was proportional to the amount of exposure. Cross-reactivity between sera from Cx. tarsalis-exposed chickens and SGAs from Culex pipiens quinquefasciatus, Culex pipiens pipiens, Ochlerotatus melanimon, and Ochlerotatus sierrensis was likely due to shared SGAs among these species. This serologic assay for mosquito exposure could be used to evaluate the sensitivity of sentinel flocks for detecting arboviral activity.