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West Nile Laying Low, So Far

Officials predict more infections as summer heats up

FRIDAY, July 14, 2006 (HealthDay News) -- The West Nile virus season is off to a slow start this year, U.S. health officials say, but that doesn't portend a worry-free summer.

"Just because we're having a late start doesn't mean you can infer it's going to be an easy season," said David Daigle, a spokesman for the U.S. Centers for Disease Control and Prevention. "Mosquito-borne arboviral diseases are very difficult to predict."

"We really don't know what the year will be like, but we know it's about to start," added Dawn Wesson, an associate professor of tropical medicine at the Tulane School of Public Health and Tropical Medicine in New Orleans.

So far, the virus has been detected in birds, animals or mosquitoes in 28 states. At the same time last year, 48 states plus the District of Columbia reported activity. The year before that, it was 47 states, the District of Columbia and Puerto Rico, Daigle said.

Ten people have tested positive for the virus so far this year, one each in California, Colorado, Iowa, Mississippi, Missouri and South Dakota, and two each in Nebraska and Texas. Seven of the cases involved meningitis or encephalitis, which can result from the bite of an infected mosquito.

Seven blood donors have also tested positive for the virus -- two in Colorado and one each in Iowa, Kentucky, Oklahoma, South Dakota and Texas. Blood donations are now routinely screened for the pathogen, and infected blood is discarded.

July, August and September are considered peak mosquito-bite months, and health officials expect to see more activity during this period.

Since it's introduction to North America in the New York City area in 1999, West Nile virus has spread across the continent and is now found in every contiguous state. In 2005 alone, there were 3,000 human cases and 119 deaths linked to infection, up from 2,539 cases and 100 deaths in 2004, according to the CDC.

West Nile virus is normally passed from an infected mosquito to a bird, and then from the bird to other mosquitoes. The mosquitoes then pass the virus on to humans.

The majority of people -- about 80 percent -- who become infected with West Nile virus have no symptoms at all. Up to 20 percent may experience flu-like symptoms such as fever, headache and body aches or even nausea and vomiting. Only about one in 150 people will suffer severe illness, resulting in meningitis or encephalitis. People aged 50 and older are at higher risk for developing severe complications if infected.

While it's not impossible to predict the geographical course of the disease this year, officials say that once a state has reported infections, it's likely they will occur again.

In past years, Daigle said, "areas that had unusually hot or dry summers tended to see more activity but there are exceptions."

"A lot of people are watching the Gulf Coast after the storms last year," he added. "There may be more debris that will give mosquitoes more sites to breed in."

"We're poised to see human case here in Louisiana," Wesson said. "Upwards of 40 percent of mosquitoes tested have been positive, and the time of year is about right."

While human infections have been limited so far, it's never too soon to take precautions.

Health officials recommend removing any standing water where mosquitoes are likely to breed. People should also wear insect repellant and long-sleeved clothing whenever outside.

"It just takes one infected mosquito bite to transmit WNV," Daigle said. "Even if you aren't noticing a large number of mosquitoes, it's still worth it to use repellent. It only takes a few seconds to apply repellent."

Last year, in addition to the standard DEET-containing products, the CDC also endorsed mosquito-repellent products containing oil of lemon eucalyptus and Picaridin. Products containing permethrin can be used on fabrics such as clothes and tents, officials said.

More information

The CDC has more on West Nile Virus.

SOURCES: David Daigle, spokesman, U.S. Centers for Disease Control and Prevention, Atlanta; Dawn Wesson, Ph.D., associate professor, tropical medicine, Tulane School of Public Health and Tropical Medicine, New Orleans; July 11, 2006, CDC West Nile Update
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