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.
West Nile Virus 2012
Read the latest news and information about the outbreak.
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.