West Nile Virus returns to United States for another season

The early bird may get the worm, but in several parts of the United States this spring, those birds are getting West Nile Virus.

The winter cold and flu season is barely over, and the new mosquito season won't really get rolling for a few more weeks, but the Centers for Disease Control and Prevention (CDC) already is reporting the presence of West Nile Virus in mosquitoes, birds and other animals in nine states.

There even is confirmation of a human case in Ohio. Last year's first human case wasn't confirmed until July.


Neurons that have been infected with West Nile Virus.

Neurons that have been infected with West Nile Virus.

"It certainly is early for the virus to be reported in humans, but with a mild winter in many parts of the country, it's possible significant numbers of infected mosquitoes survived," says Michael Diamond, M.D., Ph.D., assistant professor of medicine, of molecular microbiology and of pathology and immunology at Washington University School of Medicine in St. Louis. "It's too soon to tell how bad this summer might get, but it's always important to take precautions."

The virus poses a threat mainly to certain wild birds — more than 100 species have been killed, with crows, blue jays and some sparrows most commonly infected by the virus — and some livestock, especially horses. But it can infect people, too.

Michael S. Diamond, M.D., Ph.D.
Michael S. Diamond, M.D., Ph.D.

Symptoms of West Nile fever — usually a mild illness that lasts a few days — include fever, headache, body aches and sometimes a skin rash and swollen lymph glands.

People can become infected when a mosquito bites a virus-laden bird and then bites a human. An estimated 20 to 25 percent of people who become infected develop the flu-like symptoms of West Nile fever, and a much smaller number go on to develop meningitis or West Nile encephalitis, an inflammation of the brain that can be fatal.

Signs and symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis.

West Nile Virus first appeared in the United States in 1999. In that initial outbreak, there were 62 human cases of the virus in the United States, but by last summer that number had grown to 9,858 cases in 45 states and the District of Columbia. More than 2,800 people went on to develop severe disease — such as West Nile meningitis and West Nile encephalitis — and 262 died. Last year in Missouri, WNV caused illness in 64 people, and eight of them died.

The people at greatest risk are those 50 and older and those with weakened immune systems. Younger people also can acquire the infection, but their risk is significantly lower.

Heading into the summer of 2004, the question is: How great a threat will the virus pose this year, and how soon will the peak season begin?

"That's difficult to say," Diamond says, "but the first clue will come from the number of last year's infected mosquitoes that survived the winter or managed to pass the virus to their offspring before dying. We have to assume that since we already have a confirmed human case and the virus has been found in nine states, that a significant number of mosquitoes did survive, especially in warm weather states such as Florida and Louisiana."

Public health officials have set mosquito traps to learn how many of the insects carry the virus. Another clue will come from the numbers of susceptible birds that begin dying. In past years, Diamond says, birds began dying, and then cases of human illness followed a few weeks later.

The time lag between bird deaths and the onset of human illness occurs because West Nile infection first must reach a high level in the bird population before it spreads efficiently to humans. Then the virus must spread from mosquitoes that primarily bite birds to mosquitoes that primarily bite humans.

That takes time. The bird deaths also provide a useful warning. Like the canary in the coal mine, they indicate that human infections soon will follow and that people should take precautions to reduce their risk of infection.

Diamond recommends using skin treatments that deter mosquitoes, especially those containing the chemical DEET, minimizing time spent outdoors at dawn and dusk when mosquitoes are most apt to bite, and wearing long pants and long-sleeved shirts if one must go outside during those hours. He also recommends eliminating standing water where mosquitoes tend to breed, especially in gutters, backyards and birdbaths.

A vaccine for horses is available, and other vaccines to prevent West Nile fever and encephalitis are being tested, Diamond says, but they won't be available for at least another year or two. An investigational therapeutic trial with antibodies against West Nile virus will begin this year as part of a joint study by the University of Alabama at Birmingham and the National Institute of Health.

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