Mosquitoes are important vectors of many viral diseases in the United States, including but limited to western equine encephalomyelitis, St. Louis encephalitis, and most recently West Nile virus (WNV) infection. Mosquito-related encephalitis outbreaks in North America are delineated by temperature, with western equine encephalitis being more common in the northern United States and St. Louis encephalitis being more common in the southern United States. The time from ingestion of an infectious blood meal until the mosquito can transmit the disease is known as the extrinsic incubation period (EIP). Ambient temperatures play a key role in virus replication within a vector and affect the duration of the EIP. This is expressed as the degree-day number (DD). The emergence of WNV in the United States is new, and the authors of this study investigated the affect of temperature on transmission of the virus. Known infected birds were exposed to uninfected, unfed female mosquitoes of the Culex tarsalis species. The C. tarsalis were allowed to feed for 1 to 2 hours during the bird's peak viremic period. Immediately after feeding, the mosquitoes were transferred to incubators set at temperatures of 10°, 14°, 18°, 22°, 26°, and 30° C. Mosquitoes were removed from the incubators at 2 to 110 days after infection and tested for the virus (ie, transmission). At temperatures ranging from 14° to 30° C, the mean EIP was 109 days. Below 14° C, little to no virus developed. When temperatures in the United States during the outbreaks of WNV from 2002 to 2004 were evaluated, it was found using this model that epicenters of infection were associated with above-average temperatures. Subsequent cooler summers were associated with decreased or delayed virus activity. Funded in part by the National Institute of Allergy and Infectious Diseases, National Institute of Health, Climate Variability and Human Health Program, Office of Global Programs, National Oceanic & Atmospheric Administration

COMMENTARY: This article provides an interesting preview to yet another serious potential consequence of global warming and rising environmental temperatures-the emergence, alteration, and maintenance of infectious disease cycles. It is critical that all scientific specialties work together to address the issues of global climate change.

Effects of temperature on the transmission of West Nile virus by Culex tarsalis (Diptera: Culicidae). Reisein WK, Fang Y, Martinez M. J Med Entomol 43: 309-317, 2006.