A new study finds reliable ways to predict outbreaks of West Nile virus to guide local health officials in preventing epidemics.

In a new study published in The Journal of the American Medical Association, epidemiologists in Dallas, Texas, say they can better predict where outbreaks of West Nile virus will strike. Predictors include unusually warm winters, a history of past outbreaks, and the number of West Nile-infected mosquitoes collected in traps.

Robert Haley, M.D., lead study author and an epidemiologist from the University of Texas Southwestern Medical Center in Dallas, explains, “The mosquito vector index tracks perfectly with human cases, giving us a much earlier warning.”

This means that the number of infected mosquitoes collected on a given day is a predictor of how many people may need to be hospitalized one or two weeks later.

Lyle Peterson, M.D., is director of the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases in Fort Collins, Colo. The CDC recently released new recommendations for controlling West Nile virus, based on 12 years of data gathered since the first outbreak, in 1999.

“West Nile virus is remarkably ubiquitous throughout the U.S.,” Peterson says. “But that doesn’t mean every place is at the same risk. What we found is that certain areas, particularly in the Midwest—all the way from the Dakotas, Minnesota, Wisconsin, and Chicago, on down to Texas, Louisiana, and Mississippi—that whole central swath of the country seems to be at a higher risk of getting West Nile.”

The Dallas researchers also looked at data on 173 cases of West Nile neuroinvasive disease (WNND), 225 cases of West Nile fever, 17 cases of West Nile virus-positive blood donors, and 19 deaths during Dallas’s 2012 epidemic. They found that WNND cases often cluster in neighborhoods with:

  • Higher-density housing
  • Less-forested areas
  • More unoccupied houses
  • Older populations
  • Higher income and property values (perhaps because some swimming pools provide breeding grounds for mosquitos)

The best way to avoid West Nile virus is to protect yourself from mosquito bites. “Be aware of the West Nile virus activity in your area, and avoid being outside from dusk to dawn,” Haley says.

Wear long sleeves, long pants, and socks—and spray your clothes with an insect repellent containing permethrin or another EPA-registered repellent. But don’t apply repellents that contain permethrin directly to skin. Products containing the chemical DEET can be applied to the skin; just be sure to follow the instructions on the label for safe use.

Dead birds may be a sign that West Nile virus is passing between birds and mosquitoes in your area, so report dead birds to local health authorities. (Check with your state health department to learn more about reporting dead birds in your area.)

Since West Nile was first detected in 1999, there have been an estimated 780,000 illnesses caused by the virus. According to the CDC, 16,196 people have been made seriously ill and more than 1,500 have died. Nearly 3,000 cases of WNND were recorded each year in 2002, 2003, and 2012.

Only about 20 to 30 percent of people who become infected develop symptoms, according to the CDC. About one in five will develop a fever with other flu-like symptoms, such as body aches and diarrhea. Most people recover completely, but many notice fatigue and weakness for weeks or months afterward.

Serious neurological illness (WNND) occurs in less than one percent of people who get the virus, with those over age 60 at the greatest risk. Symptoms range from high fever, neck stiffness, and disorientation to coma, tremors, seizures, and paralysis. Hospitalization is necessary in most cases of WNND, and some neurological problems may be permanent.

There is currently no effective treatment for West Nile virus infection, only medications to ease symptoms.