There is some evidence which suggests weather may have played a role in the rapid spread of West Nile virus across the U.S., Canada, and Central America following its detection in New York City in 1999. A team of U.S. and Canadian researchers has looked more comprehensively at the evidence by analyzing a spectrum of weather factors for climatically diverse states, and found several significant links with the incidence of human West Nile Virus (WNV) cases. The analysis was based on 16,298 WNV cases reported to the Centers for Disease Control from 2001 to 2005, as well as year-round temperature, precipitation, and dew point data from 351 weather stations in close proximity to the infected people.
A 12°F increase in maximum daily temperature was associated with a 45–72% increase in WNV case reports within a 1-month period. Precipitation was also associated with WNV, which increased 29–66% in association with a single-day rainfall of at least 50 mm within 3 weeks of diagnosis. Smaller amounts of precipitation were associated with smaller increases in WNV cases, consistent with a related effect. Increases in cumulative weekly precipitation and mean weekly dew point temperature (a measure of relative humidity) were also associated with an increase in WNV cases.
The findings, which appear to have validity across season and location, generally agree with what is known about the biology of WNV, humans, mosquito vectors, and bird reservoir hosts. The researchers note that additional research will be needed to address some limitations of their work, specifically gaps in data from a number of geographic regions and the influence of localized interactions of factors such as bird populations, vegetation, mosquito control efforts, and acquired immunity in both humans and animals. If these weather–disease links are confirmed, and if climate changes in North America unfold as predicted with increases in temperature and precipitation, this type of research and understanding may assist public health officials with the prevention or mitigation of WNV outbreaks in the future.
The study can be found at http://www.ehponline.org/members/2009/0800487/0800487.html.