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West Nile virus is on the move in the USA, so holiday merrymakers should be on guard: Experts warn that the virus could cause as much harm this year as it did last.
The size and severity of last summer's epidemic, when 4,156 people became ill and 284 died, caught health officials by surprise, says Dan O'Leary, a medical epidemiologist with a division of the Centers for Disease Control and Prevention in Fort Collins, Colo.
''For severe disease, it was unprecedented in the history of recorded medical literature,'' he says. Nearly 3,000 people suffered central nervous system infection such as encephalitis, a brain inflammation. That's far more than in earlier outbreaks in Africa, Asia or the Middle East, in which West Nile virus usually caused only mild fever and headache.
The strain of West Nile virus that found its way to the USA in the summer of 1999, the first year it was detected on this continent, is an especially nasty one.
''It seems to be a more virulent strain than strains found in past decades,'' O'Leary says, and because few Americans have been exposed to the virus, almost everyone is susceptible to it.
''It's hard to say the exact proportion of the population carrying immunity to West Nile virus, but it's safe to say it is probably a low level at this point, since the virus has only been here four years now,'' he says.
The virus so far has been detect- ed in birds, animals and mosquitoes in 33 states, starting as early as January in some Southern regions. State health departments began mosquito-control programs in early spring and have been posting warnings to residents for weeks. No cases have been detected in humans this year.
The upcoming Fourth of July holiday and the outdoor activities that go along with it worry West Nile experts such as O'Leary.
''People will be watching fireworks into the evening, the hours at which the mosquitoes that carry this virus will be biting,'' and they should take precautions, including covering exposed skin and wearing insect repellant that contains DEET.
The CDC says only about 20% of people who are infected with the virus become ill. Symptoms range from mild -- such as headache, fever and body aches -- to severe, including high fever, muscle weakness, inflammation of the brain or surrounding membrane and coma. People age 50 and older are at highest risk.
CDC Director Julie Gerberding, who revealed in a press briefing last week that her husband contracted the virus last summer, says people should make sure to eliminate mosquito-breeding sites around their homes. ''We can't emphasize enough how important it is to remove the standing water from your property,'' she says.
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