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The mosquitoes this year are barely astir, but health officials already are bracing for the next onslaught of West Nile virus.
''We could get transmission as soon as the mosquitoes are out,'' says epidemiologist Ned Hayes of the Centers for Disease Control and Prevention. ''In the southern part of the U.S., that could happen any time now.'' The CDC says the first human case of West Nile infection last year occurred on March 28 in Pennsylvania. This year, states have reported West Nile-infected birds in Louisiana, Florida, Texas and New York and a horse in Alabama.
Since its discovery in New York in 1999, the mosquito-borne virus has spread across the continental USA, but states in the far West haven't been hit very hard. Only three people in California, one each in Idaho and Utah, two in Nevada and 13 in Arizona have been infected. Oregon and Washington have been untouched.
That could change this year.
''We can't predict with certainty what will happen,'' Hayes says. ''But the virus has been detected in enough areas to suspect there will be activity in most states.''
Birds and mosquitoes infected with West Nile were found in southeastern California last summer, says entomologist William Reisen of the University of California-Davis. He says the state's mosquito-control program has kept mosquito-borne viruses that cause Western equine encephalitis and St. Louis encephalitis in check, and it could do the same for West Nile.
John Edman, director of the Center for Vector-borne Disease at UC-Davis, is skeptical. Though there are similarities between West Nile and the other viruses, Edman says, the level of West Nile virus that builds up in birds is higher. That makes it more likely that mosquitoes that bite birds, and then bite humans, will pass on the virus.
It's possible California will have a lot more West Nile cases than other mosquito-borne diseases, despite the preparation, he says. The state has a lot of people, birds and irrigated farmland -- prime breeding sites for the species of mosquitoes that are the most effective West Nile spreaders, he says.
''We certainly have the ingredients here to have a pretty significant outbreak,'' he says.
In Colorado, health officials hoping to avoid a repeat of last year's epidemic -- when there were 2,945 West Nile cases -- are meeting with local health departments to plan mosquito-control strategies and update public health messages. State epidemiologist John Pape says it's expected that counties within the state that were only lightly affected last year will see more activity this year as the westward expansion continues.
Meanwhile, Colorado experts are sharing their hard-earned expertise with colleagues in Arizona, California and Utah.
Among lessons learned, Pape says, is the importance of planning for a major outbreak. ''Now is the time to do the preparation, because when these things start to develop, they move pretty quickly,'' he says. ''It's explosive.''
In Colorado, 79% of the cases last year were the milder form of the disease called West Nile fever, which is marked by fever and fatigue. More severe, life-threatening forms of the illness affect the central nervous system, causing encephalitis, meningitis or acute flaccid paralysis, a polio-like illness.
But even West Nile fever can be debilitating and should not be taken lightly, Pape says. ''For most people, it's not mild or flu-like. It's very severe and can have significant complications and last several weeks. Most people think fever, achy and in bed two or three days then back to work. Some may go through that, but we had some people flat on their back for two or three weeks before being able to resume normal activity.''
Experts say only 20% of people who are infected with West Nile develop symptoms. Though thousands of Americans have been exposed to the virus and are thought to be immune to reinfection, studies suggest that only 5% to 10% of Americans have been exposed, even in areas where the epidemic has been severe, Hayes says.
''Just because an area had epidemic activity in one year, it certainly could have another epidemic the following year, because not enough people in the population would be immune,'' he says.
In a study of blood donations screened last summer to prevent the spread of the virus through transfusions, blood centers found just over 1,000 of 6.2 million pints infected with West Nile, suggesting that most Americans have not been exposed to the virus.
That means one thing to Pape: ''I hope after seeing the activity of the past couple of years that citizens, when they hear their local health officials say you should take precautions, do.''
The presence of the virus in a community, he says, is ''not a reason to panic and lock yourself in the house, but if they say to put on repellant and take other precautions to protect yourself from mosquitoes, pay attention.''
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