Ecology of potential West Nile virus vectors in Southeastern Louisiana: enzootic transmission in the relative absence of Culex quinquefasciatus

Abstract

A study of West Nile virus (WNV) ecology was conducted in St. Tammany Parish, Louisiana, from 2002 to 2004. Mosquitoes were collected weekly throughout the year using Centers for Disease Control and Prevention (CDC) light traps placed at 1.5 and 6 m above the ground and gravid traps. A total of 379,466 mosquitoes was collected. WNV was identified in 32 pools of mosquitoes comprising four species; 23 positive pools were from Culex nigripalpus collected during 2003. Significantly more positive pools were obtained from Cx. nigripalpus collected in traps placed at 6 m than 1.5 m that year, but abundance did not differ by trap height. In contrast, Cx. nigripalpus abundance was significantly greater in traps placed at 6 m in 2002 and 2004. Annual temporal variation in Cx. nigripalpus peak seasonal abundance has important implications for WNV transmission in Louisiana. One WNV-positive pool, from Cx. erraticus, was collected during the winter of 2004, showing year-round transmission. The potential roles of additional mosquito species in WNV transmission in southeastern Louisiana are discussed.

Figure 1.

Location of study sites in St. Tammany Parish in southeastern Louisiana.

Figure 2.

Proportional abundance of the five most commonly collected species at each of seven study sites in St. Tammany Parish, Louisiana, 2002–2004.

Figure 3.

Seasonal abundance and West Nile infection in selected mosquito species collected between July 16, 2002 and November 30, 2004. Lines represent mosquitoes per TN, and bars represent WNV-positive pools.

Figure 4.

Ratios of WNV infection rates in Cx. nigripalpus collected in dry ice-baited CDC light traps set at 1.5 and 6 m above the ground expressed as a point estimate with 95% CIs within parentheses. Where CIs include the value 1.0, infection rate differences between traps heights are not significant.