West Nile Virus Cases Continue to Climb

CDC: Infections Up 40% in Past Week; Death Toll Rises

Medically Reviewed by Louise Chang, MD on August 29, 2012
From the WebMD Archives

Aug. 29, 2012 -- West Nile virus cases in the U.S. continue to climb, increasing 40% in just the past week, according to the CDC.

As of Aug. 28, 48 states have reported West Nile virus infections in people, mosquitoes, or birds (which carry the virus). A total of 1,590 cases in people have now been reported, up from 1,118 last week.

Sixty-six people have died this year from complications of the virus, Lyle R. Petersen, MD, MPH, director of the CDC's division of vector-borne diseases, said today in a news teleconference. The death toll last week was 41.

Complications include infections of the brain (encephalitis) or the spinal cord and connecting nerves (meningitis). Most people have a less severe form of the disease, West Nile fever, but it can include severe fatigue that can drag on for weeks or months.

"The 1,590 cases reported thus far is the highest number of West Nile virus disease cases reported to the CDC through the last week in August since West Nile virus was first detected in the U.S. in 1999," Peterson said.

"This looks like to us it is going to be our worst year for West Nile," Petersen said.

He and other public health experts urged people to follow precautions against mosquito bites, including wearing long sleeves and long pants when outdoors at dusk or dawn, using insect repellents, and getting rid of standing water outdoors, which provides a breeding ground.

"In light of this ongoing risk, it is important for people to protect themselves from mosquito bites," Petersen said.

Although the rate of transmission is low in some states, "it is still not zero," Petersen said.

West Nile Virus: Tracking the Outbreak

Texas and five other states have been particularly hard hit. More than 70% of all cases are in six states, Petersen said. The other five are:

  • South Dakota
  • Mississippi
  • Oklahoma
  • Louisiana
  • Michigan

In Dallas County, there are 309 reported cases, including 152 with neuroinvasive disease, said David L. Lakey, MD, commissioner of the Texas Department of State Health Services, who also spoke at the news teleconference. Dallas County alone has had 12 deaths, he said.

Peterson said he expects the outbreak to peak by the end of this month. There are always reporting time lags, because of the time it takes for people to go to the doctor, get a diagnosis, and then for the health departments to report to the CDC. Because of that lag, the number of cases reported to the CDC are expected to continue and increase through September and early October, said CDC spokesman Tom Skinner.

West Nile Virus: Beyond the Numbers

Why this year is especially bad remains a mystery, Petersen said. The heat wave blanketing many states may have played a role. That has been the case in some previous years.

"But other heat waves have not produced outbreaks," he said.

Hurricane Isaac, which hit southern Louisiana on Wednesday, is not expected to have an effect on the outbreak, Petersen said.

About one in five people who get infected with West Nile virus get West Nile fever. Their symptoms are flu-like and usually appear three to 14 days after the bite by an infected mosquito.

About 1 in 150 people bitten have more severe problems, including meningitis and encephalitis. Some also get a condition called acute flaccid paralysis, in which the person is unable to move his arms or legs.

There's no reason to believe that severe cases are increasing in relation to the total number of infections, Petersen said.

No vaccine is available for the West Nile virus, Peterson said, although some are being studied.