West Nile Virus Kills 41 People in Biggest U.S. Outbreak

West Nile virus has killed 41 people in the U.S. and infected 1,118 in the biggest outbreak of the mosquito-borne disease since it was first confirmed in the country 13 years ago.

Through Aug. 21, the number of West Nile cases had increased 55 percent over last year’s total, the Centers for Disease Control and Prevention’s Lyle Petersen said today on a conference call with reporters. While the illnesses have been reported in 47 states, most of the cases have been concentrated in five -- Texas, Mississippi, Louisiana, South Dakota and Oklahoma -- with more than half in Texas alone.

Spread by mosquito bites, West Nile can cause symptoms ranging from fever, headaches and rashes to coma, tremors and paralysis in the rarest cases. While it’s unclear why more cases have occurred, hotter-than-normal weather across the U.S. may have contributed by improving breeding conditions for mosquitos, Petersen said. Northern states are usually affected after southern ones, he said on the call.

“Often times West Nile virus is a very local disease,” said Petersen, director of the Atlanta-based CDC’s Division of Vector-Borne Infectious Diseases. The extent of an outbreak “has a lot to do with the local ecology of that area.”

Severe Cases

The most dangerous cases, called neuroinvasive because of the resulting inflammation in or around the brain and spinal cord, total 629 nationwide, CDC officials said on the call. Only about 1 in 150 people will develop the most severe symptoms and 4 of 5 suffer no effects from an infection, according to the agency’s website.

Texas has accounted for 537 human infections this year. Eleven West Nile deaths have been recorded in Dallas County, the state’s health commissioner, David Lakey, said on the CDC’s conference call.

Aerial pesticide spraying has begun in Dallas to kill mosquitos, Lakey said. CDC officials said they couldn’t say what other states are doing, as they don’t track local efforts.

Officials in Louisiana, South Dakota and Michigan, among other states, are gearing up to spray, a step they take every year, said Joe Carlson, an entomologist with the American Mosquito Control Association. The Mount Laurel, New Jersey-based nonprofit represents mosquito control and public health officials in the U.S. and abroad.

Mild weather during the U.S. winter this year and a rainy spring probably boosted the mosquito population and added to the outbreak, Carlson said. Budget cuts may have played a role as well, as many agencies reduced surveillance and public education efforts, he said.

“This is a relatively recent epidemiological phenomenon,” Carlson said. “It’s only been around in this country for 13 years, so there’s a lot we don’t know about this virus, about how it transmits, how it overwinters, and so on.”

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