Toward a climate index for West Nile Virus

This study explored possible approaches to the development of a climate index related to the risk of West Nile Virus (WNV) through mosquito transmission. The primary mosquito vector for WNV is Culex pipiens. In the Midwest U.S., Cx. restuans is considered an amplification vector of West Nile virus in bird populations early in the summer. As summer progresses, the Cx restuans population diminishes and Cx pipiens population increases until pipiens is dominant. Outbreaks of WNV in horses and humans appear to begin about the time of the crossover in population from restuans to pipiens. The crossover date is highly variable from year to year. We investigated whether this variability was related to climate variations and thus could be predicted during a warm season. This investigation was based on a dataset of mosquito numbers and species collected since the late 1980s in the Champaign-Urbana area. Culex egg rafts were collected daily, reared and identified as to species. Species abundance was graphed and used to identify two dates: the date of the first capture of Cx. pipiens and the date when Cx. pipiens exceeded 50% of the total. Analysis of these data resulted in a simple model for the number of days from first pipiens to crossover. This is a simple linear model based on the number of occurrences of minimum temperatures below 60°F. Work is ongoing to develop a complete model such that the date of crossover can be estimated from climate data alone.