DALLAS -- West Nile deaths were mounting quickly this summer as mosquitoes spread the virus across the country.
The situation was especially dire in Texas, where some authorities resorted to aerial spraying for the first time in decades to curb what became one of the worst such outbreaks in U.S. history.
Nationally, more than 240 people died of the mosquito-borne illness -- about a third of them in Texas.
Now that the mosquito population has been decimated by the cooler weather, health experts have turned their attention to learning lessons from the latest round of deadly cases. Federal health authorities are collecting data and examining potential factors, while Dallas County -- the epicenter of the outbreak -- has begun year-round mosquito surveillance and testing.
What remains unclear is whether experts will be able to shed light on what caused the outbreak, why parts of Texas were so severely affected and whether they can forecast the next major surge in the illness.
"I don't think that we're ever going to totally be able to sort it out," said Dr. Lyle Petersen of the U.S. Centers for Disease Control and Prevention. "For one reason, the ecology in the United States is extremely varied, so a factor that may affect an outbreak in Colorado may be different than a factor that causes an outbreak in Louisiana. The conditions in an urban area may be different than a suburban area."
West Nile virus is thought to have first appeared in the U.S. in 1999 in the New York City area and then gradually spread across the country. Mosquitoes get the virus from feeding on infected birds and then spread it to people they bite. Most people infected show no symptoms, but the most severe form of the disease, called neuroinvasive, can cause a coma, convulsions, muscle weakness, paralysis and death.
The Texas Department of State Health Services reported more than 835 neuroinvasive cases this year and 86 deaths, led by Dallas County's 18 fatalities. The national death toll this year approached historic numbers from 2002, when 284 people died of the disease.
Petersen said the CDC is trying to determine what caused the latest outbreak by looking at factors such as heat, rainfall and the number of migrating birds that transmit the virus to mosquitoes. The agency is also researching the genetics of the virus to see whether it might have mutated, but that doesn't seem to have happened.
Petersen added that while a warm spring with ample rainfall in North Texas was probably a factor, it's not known exactly what caused so many cases in the area.
"Probably, there was just the right combination of warmer weather and enough rainfall and probably a good dose of bad luck as well," Petersen said. "These outbreaks are subject to a fair amount of random variation."
Fight the virus
To help prevent becoming infected with the West Nile virus, during summer residents are advised to:
- Apply insect repellent.
- Dress in long sleeves and long pants when outside.
- Stay indoors from dusk to dawn.
- Drain standing water on their property.
Source:"Dallas County Health and Human Services