Researchers at the Illinois State Water Survey (ISWS) and the Illinois Natural History Survey (INHS) report a reliable link between weather conditions and an abundance of two mosquito species linked to outbreaks in humans and wildlife, especially birds.
The West Nile virus first appeared in Illinois in 2001. A major statewide outbreak occurred in 2002, with 66 deaths among the 884 people showing clinical signs of infection after being bitten by mosquitoes carrying the virus. Although the number of cases in humans declined in 2003 and 2004 (112 human cases and five deaths), transmission between mosquitoes and birds continues 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 northern house mosquitoes, the primary suspect for transmission to humans, is low in spring but grows to become the dominant species later in summer, especially in urban areas.
The northern house mosquito becomes the main species in early August, on average, said INHS entomologist Robert Novak, who also is an adjunct professor of veterinary pathobiology in the College of Veterinary Medicine. He has been collecting data on summer changes in mosquito species in the Champaign-Urbana area since 1988.
Because that time period varies each year, ISWS atmospheric scientist Kenneth Kunkel, along with Novak and INHS mosquito ecologists Richard Lampman and Weidong Gu, examined whether changes in vector abundance may be related to weather conditions.
The northern house mosquito became dominant earlier in the summer in years when temperatures exceeded 81 degrees Fahrenheit more frequently than normal, they discovered.
Likewise, the dominance of the mosquito was delayed until late summer in years when temperatures exceeded 81 degrees 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 scientists say.
The researchers also found that a rise of West Nile infection in mosquitoes parallels the rise in species abundance. The peak infection rate in mosquitoes occurs about two to three 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 such as horses, humans and other wildlife, they concluded.
A review of the weather and mosquito-infection-rate data shows that 2002, the major outbreak year, coincided with a warmer spring and summer and earlier abundance of Cx. pipiens than the two subsequent years.
Other weather factors, such as precipitation, also may play a role in the transmission of West Nile virus. The scientists are continuing their research to better understand the role of weather and climate on the transmission and of other vector-borne pathogens, such as the St. Louis encephalitis virus, also carried by both Culex species.
The Midwestern Regional Climate Center, housed at the ISWS, monitors climate conditions in the Midwest. That data, with Novak's data, will be tested this summer to assess the forecasting capability of a prototype model that can provide public health wildlife professionals with warning of periods when West Nile virus is most likely to pose a threat.