Aug. 22, 2012 — West Nile virus now has killed 47 people and infected an estimated 95,000 in 38 states in what is almost certain to be the worst West Nile outbreak since the virus hit the U.S. in 1999.

“The number of West Nile disease cases in people has risen dramatically. We are in the midst of one of the largest West Nile outbreaks ever seen,” Lyle R. Petersen, MD, MPH, director of the CDC’s division of vector-borne diseases, said today in a news teleconference.

It’s far from over. Right now, the U.S. is in the middle of mosquito season — and nearly all West Nile virus infections come from mosquito bites. Case counts usually rise through September.

“The number of cases is trending upward in most areas,” Petersen said, noting that 47 states have detected West Nile virus circulating in mosquitoes, birds, or people. Only Alaska, Hawaii, and Vermont have not yet detected the virus.

Half the cases and more than half of the deaths have been in Texas. David L. Lakey, MD, Texas state health commissioner, characterized the situation as a “disaster.”

“It is not just about the numbers. This disease impacts the lives of hundreds if not thousands of people, and their lives will be changed by this outbreak. Our hearts go out to them,” Lakey said.

Since 2003, when West Nile virus spread across the nation, most West Nile seasons have been relatively mild. Why is this year suddenly so bad?

“We really don’t know,” Petersen said. “Many major outbreaks in Europe and Africa and now in the U.S. have appeared during abnormally hot weather. Hot weather, in lab tests, does increase transmissibility of the virus from mosquitoes, and that may be one factor.”

The CDC also is investigating whether the virus might have mutated into a more dangerous form.

West Nile Symptoms

The good news is that only one in five people infected with West Nile virus gets West Nile fever. Symptoms appear three days to two weeks after the bite of an infected mosquito.

So far this year, there have been 489 reported cases of West Nile fever. Many cases go unreported.

Illness appears suddenly, says Petersen, who was infected in 2003.

“I was out for a jog, and in one mile I went from perfectly normal to the point where I could barely walk,” he reports. “That is probably the norm.”

And it’s not usually a mild or brief illness.

“Those who get more ill with West Nile fever will be laid up in bed for days or a week, followed by a period of just feeling awful. And there can be a fatigue syndrome where people remain fatigued for weeks or months. It lasts longer than we used to think,” Petersen said.

And these patients are lucky compared to those who get what the CDC calls “neuroinvasive disease.” In about one in 150 people, West Nile virus infects the brain (encephalitis) or the spinal cord and connecting nerves (meningitis). So far this year, there have been 629 reported cases, with 58 cases of paralysis.

“The meningitis or encephalitis can cause paralysis that affects one or more limbs. It can also affect breathing. It is one of the more severe and dreaded complications,” Petersen said. “With meningitis, symptoms include headache, stiff neck, eye pain, and fever. Encephalitis, infection of the brain itself, causes cognitive problems, where people can’t think properly. It can also cause coma, along with all the symptoms of meningitis as well.”

And there’s another risk. Last month, Baylor University researchers reported that West Nile virus doesn’t go away in some people. The virus hides in the kidneys. Over the course of years, it causes kidney disease that worsens over time.

People with neuroinvasive West Nile disease were most likely to have long-lasting infection and kidney damage. But this also happened to about 9% of those with mild or no symptoms.

Other laboratories have yet to confirm these findings. “But if they are true, they are of importance,” Petersen said.

West Nile Virus: Who’s at Risk?

Most serious cases of West Nile virus occur in people over age 50. This year, 61% of cases have been over 50, and 39% have been over 60. The elderly are at particularly high risk.

Infants are not at high risk, as very few infant infections have been reported. So far this year, there’s been only one reported West Nile infection of an infant.

There have been isolated reports of pregnant women passing the infection to their unborn children, but so far most women known to be infected with West Nile virus have given birth to healthy, uninfected babies.

It’s not yet clear whether people with weakened immune systems are at higher risk.

Because West Nile virus is spread by mosquitoes, the CDC notes that people who spend a lot of time outdoors — especially during dawn and dusk — are at higher risk.

West Nile virus can be transmitted by blood transfusion. In the U.S., blood is tested for West Nile virus. So far this year, West Nile virus has been detected in 242 samples of donated blood. There have been no known infections via transfusion, and the U.S. blood supply is considered safe.

WebMD Chief Medical Editor Michael Smith, MD, will be taking questions about the West Nile virus outbreak on Twitter today at 4 p.m. ET. Follow #webmdwestnile, to join the conversation and ask questions.
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