Case counts are expected to rise through October, says Lyle Petersen, MD, MPH, director of the CDC's division of vector-borne diseases. Every state except Alaska and Hawaii has reported pools of mosquitoes carrying the virus. There have been human cases in 44 states.
West Nile Virus 2012
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"This week's numbers show about a 25% increase over last week," Petersen said. "The cases reported so far are the highest for the first week of September since the virus entered the U.S. in 1999."
About half the case reports have come from Texas, notes David Lakey, MD, Texas state health commissioner.
"Unfortunately, those numbers will continue to go up," he said. "We know there will be additional deaths and, sadly, more folks diagnosed with West Nile neuroinvasive disease."
A Look at the Numbers
Many states had not yet reported their latest case counts when the CDC compiled its figures earlier this week. When adding in more recent reports from the six states with the most cases -- Texas, Louisiana, South Dakota, Mississippi, Oklahoma, and Michigan -- the case count now stands at 2,044 reported cases of illness, with 85 reported deaths.
The most severe cases of West Nile disease -- "neuroinvasive" cases of encephalitis, meningitis, and polio-like paralysis -- usually are reported. Nearly everyone with severe disease needs to be hospitalized. The long-term effects, including thinking problems, numbness, and limb paralysis, can be permanent.
Fortunately, only one in 150 people infected with West Nile virus gets severe disease. Since there have been 1,058 reports of such cases, an estimated 160,000 Americans have been infected so far this season.
CDC figures show that 40% of people sickened by West Nile virus are 60 or older. Another 22% are 50 to 59. Only 4% of cases are in people under 20.
People who spend the most time outdoors are most at risk of West Nile infection. That's especially true at dawn and dusk, when the mosquito species that carries West Nile virus is most active.
People typically get West Nile virus from the bite of a mosquito that has bitten an infected bird.