Survey Scientists Developing WNV Forecast Tool, Illinois State Water Survey

Press Release

For Immediate Release March 28, 2005
Survey Scientists Developing WNV Forecast Tool

Kenneth Kunkel - (217) 244-1488,
Robert Novak - (217) 333-1186,
Eva Kingston - (217) 244-7270,

Scientists at the Illinois State Water Survey ( and the Illinois Natural History Survey ( have found a reliable connection between weather conditions and the threat of transmission of West Nile virus (WNV), research that eventually may lead to better forecasts of outbreaks to humans and wildlife, especially birds.

WNV first appeared in Illinois in 2001. A major statewide outbreak occurred in 2002, resulting in 66 deaths among the 884 people with clinical signs of infections after they were bitten by mosquitoes carrying the virus. Although the number of deaths and human cases declined in 2003 and 2004 (a total of 112 human cases and 5 deaths), transmission between mosquitoes and birds is still being detected throughout the state at surprisingly high levels.

Two mosquito species (Culex restuans, the white-spotted mosquito, and Culex pipiens, the northern house mosquito) are believed to maintain the natural transmission cycle between birds and mosquitoes. The population of the northern house mosquito, the primary suspect for transmitting WNV to humans, is low in spring but becomes the dominant species later in summer, especially in urban environments, as WNV in humans and horses becomes evident.

According to Illinois Natural History Survey (INHS) entomologist Robert Novak, who has been collecting data on summer changes in mosquito species in the Champaign-Urbana area since 1988, the northern house mosquito becomes the dominant species in early August, on average. Because that time frame varies substantially from year to year, from early July to mid- September, Illinois State Water Survey (ISWS) atmospheric scientist Kenneth Kunkel, together with Novak and INHS mosquito ecologists Richard Lampman and Weidong Gu, decided to examine whether those changes in vector abundance could be related to weather conditions.

They discovered that the northern house mosquito became dominant earlier in the summer in years when temperatures exceeded 81̊F more frequently than normal. Likewise dominance of the mosquito was delayed until very late in the summer in years when temperatures exceeded 81̊F less frequently than normal. That very simple relationship can be used to predict when the northern house mosquito will emerge as the dominant mosquito species.

The Survey researchers also found that the rise in WNV infection of mosquitoes parallels the rise in this species abundance, and the peak infection rate in mosquitoes occurs about 2–3 weeks after the northern house mosquito becomes the dominant species. This peak in infection obviously represents the period of greatest risk of transmission to incidental hosts (horses, humans, and other wildlife). A review of the weather and mosquito infection rate data shows that 2002 coincided with a warmer spring and summer and earlier abundance of Cx. pipiens than the two subsequent years.

Other weather factors, such as precipitation, probably also play a role in WNV transmission. The scientists are continuing their research to better understand the role of weather and climate on transmission of WNV and other vector-borne pathogens, such as St. Louis encephalitis virus, also vectored by both Culex species. They predict that if weather conditions are conducive for an early buildup of Cx. pipiens, we may see another large outbreak.

The Midwestern Regional Climate Center, housed at the ISWS, already monitors climate conditions throughout the Midwest. Kunkel says those data, together with Novak’s data, will be tested later this summer to assess the forecasting capability of a prototype model as a tool that can provide public health wildlife professionals with advance warning of periods when WNV is most likely to pose a threat.

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