The Impact of Cycling Temperature on the Transmission of West Nile Virus

Abstract

West Nile virus (WNV) is an important cause of disease in humans and animals. Risk of WNV infection varies seasonally, with the greatest risk during the warmest parts of the year due in part to the accelerated extrinsic incubation rate of the virus in mosquitoes. Rates of extrinsic incubation have been shown in constant-temperature studies to increase as an approximately linear function of temperature, but for other vector-borne pathogens, such as malaria or dengue virus, nonlinear relationships have been demonstrated under cycling temperatures near the thermal limits of pathogen replication. Using typical daily air temperature profiles from three key periods of WNV amplification in a hyperendemic area of WNV activity in California's Central Valley, as well as a fourth temperature profile based on exposures that would result from daily mosquito host-seeking and resting behavior, we explored the impacts of cycling temperatures on WNV transmission by Culex tarsalis Coquillett, one of the principal vectors in the western United States. The daily cycling temperature ranges studied were representative of those that occur across much of California, but they did not significantly alter the extrinsic incubation period of WNV compared with estimates from mean temperatures alone. This suggests that within the relatively broad range we studied, WNV incubation rates are a simple function of mean temperature. Realistic daily temperature patterns that reflected mosquitoes' avoidance of daytime high temperatures during summer reduced transmission over time compared with air temperatures, indicating that adjustment for mosquito exposure temperatures would be prudent for calculating risk.

Keywords: Culex tarsalis; West Nile virus; extrinsic incubation period; mosquito-borne disease; vector competence.

Fig. 1.

Daily temperature profiles for each monthly setting. Daily means for treatments are indicated in matched colors (dashed lines).

Fig. 2.

Percent of mosquitoes transmitting WNV on each day of observation by temperature setting.

Fig. 3.

Distribution of EIPs for mosquitoes reared under typical July air temperatures, in red, versus those reared under the same July conditions accounting for daytime mosquito resting in sheltered habitats, in blue.