West Nile both flares and fizzles

Movement: The virus has moved west like a wildfire with an initial burst of intensity, but cooling down its wake.

Medicine & Science

August 30, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Just five years after its arrival, the West Nile virus has completed its east-to-west invasion of the United States and Canada. At the same time, the mosquito-borne virus may be having a diminished impact on Maryland and other states where it has resided for several years.

Oregon reported its first infected birds and horses this summer -- the last of the lower 48 states and contiguous Canadian provinces to discover West Nile within their borders.

California and Arizona are this year's hot spots. More than 530 people there have been diagnosed with West Nile encephalitis or meningitis -- nearly two-thirds of the nation's total. Ten have died.

Arizona health officials note their large and vulnerable elderly population, and blame irrigated lawns and neglected swimming pools in Phoenix for breeding the mosquitoes that have rattled the desert city. Newspapers in California are full of heartbreaking stories -- an 81-year-old survivor of the Bataan Death March killed by a mosquito; a 47-year-old woman, once a champion water skier, paralyzed.

Health officials in the West are urging the usual precautions against mosquito bites. But they're also assuring residents that the unsettling number of illnesses they're seeing this year will subside in a year or two.

It's a pattern seen in many states as the virus has spread south and west from New York like an invisible wildfire, and it may finally be happening in Maryland.

After three years of rising case counts, West Nile numbers in Maryland seem to be off sharply this year. Public health authorities have reported only two cases of human West Nile fever this summer. And not a single bird or horse has been diagnosed with the disease.

And something --it's unclear what -- seems to have reduced the number of Maryland mosquitoes found infected with the West Nile virus.

"I can't pinpoint the factor that accounts for this. It's probably multiple things," said Kimberly Mitchell, coordinator for West Nile surveillance in nonhuman species at the state's Department of Health and Mental Hygiene. "Especially in light of the fact we've had a lot of rainfall, I actually would have expected a lot more [infected] mosquitoes turning up."

West Nile, carried by birds and transmitted by mosquitoes to humans, first appeared in North America in a 1999 outbreak in New York City. It's received heavy news coverage, but West Nile is still regarded as a minor threat to public health compared with hazards such as the flu, which kills 36,000 Americans each year.

Eighty percent of people infected with West Nile through a mosquito bite will have no symptoms. Nearly 20 percent will experience flu-like symptoms and recover.

Still, roughly one out of 150 -- especially the elderly and others with weakened immune systems -- will develop severe illness, including encephalitis or meningitis. The neurological effects can be lasting or fatal.

In all, West Nile has killed 586 people in the United States and sickened more than 15,000 since its arrival. Seventeen Marylanders have died, with 117 made seriously ill.

This year, the numbers in Maryland are down, at least so far. Only 16 mosquito samples in Maryland have tested positive for West Nile. That compares with 82 at this time last year, Mitchell said.

There has been just one confirmed human case, in Prince George's County, and one "probable" case, in Baltimore. By the same time last year, five human cases had been reported.

"It's hard to know whether we're going to have a better year based on those numbers," said Dr. David Blythe, a state medical epidemiologist. But "we can be hopeful."

It's still early. Most cases appear in September and October. Last year, the first human case of West Nile fever was reported Aug. 19, and the count eventually reached 73, with eight fatalities.

Not a single horse has been stricken so far this year. Last year there were 234 equine West Nile cases reported, up from just 30 in 2002. Seventy-five horses died.

Widespread vaccinations may explain the horses' good health. But the vaccines are only effective for 90 days, and agriculture officials urged that animals vaccinated in the spring receive booster shots.

No human vaccine is available.

The reasons for the lower West Nile numbers aren't clear. It may be that there are simply fewer mosquitoes. "We're coming off two years of very high mosquito populations," said Cy Lesser, chief of mosquito control at the state Department of Agriculture.

Mosquitos breed in standing water, and it's been rainy. But except for some localized outbreaks after thunderstorms, Lesser said, "it's been a relatively mild mosquito year so far."

On the other hand, he said, surveillance in Virginia -- with similar weather -- has turned up higher numbers of infected mosquitoes. And there may be buggier days to come.

"It might just be that it's a little bit late this year," Lesser said. "We are starting to see quite a good increase in mosquito activity over a wide geographic area."

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