Oct. 20 — Experts are unsure if America has seen the worst of the West Nile virus, even after a summer when, for the first time, thousands of human cases stretched from coast to coast, and more than 100 people died.

"I don't have a crystal ball," said Dr. Lyle Petersen, an infectious disease specialist at the Centers for Disease Control and Prevention. "If I had to predict, I would say there will be continuing outbreaks of varying size throughout the country, so the incidence of disease could vary tremendously from year to year."

Thousands of Cases

This year, as of Friday, 3,174 people in the United States tested positive for West Nile and 173 of them died, the CDC reported. With West Nile established in warm-weather states where mosquitoes bite year round, Petersen expects the number of cases to keep rising through the winter months.

The deaths, illnesses and geographical range of the disease — with human or animal cases in 42 states and the District of Columbia — are up steeply from prior years, when West Nile was confined to Eastern states and the central part of the country.

There were only 66 human West Nile cases reported in 2001, when the virus was detected in 27 states and the nation's capital, Dr. James M. Hughes, director of the CDC's National Center for Infectious Diseases, told a congressional subcommittee recently.

West Nile's spread to new areas may have aided its explosion this year, some believe.

"The ability to respond to the virus is absent or very low in most animals, especially birds, right now, so those animals, especially birds, are dying," said Dr. Dawn Wesson, associate professor of tropical medicine at Tulane University Health Sciences Center in New Orleans, and president of the Louisiana Mosquito Control Association.

There is hope the disease could fade as immunity gets built up among Americans and U.S. birds — key carriers of West Nile.

"It's going to settle down," said Dr. Susan L. F. McLellan, an associate professor of infectious diseases at Tulane. "Whether it's happened this year, I don't know, but most of the susceptible population will have been exposed and so will be immune."

"[West Nile] may turn out to be no more of a big deal than any of the other arboviruses [illnesses spread by insects] whose names nobody can remember," she added.

West Nile Model?

Some see St. Louis encephalitis, a mosquito-borne virus related to West Nile, as a model of what the future may hold for West Nile in America. The disease seems to disappear for stretches of time, then suddenly appear in localized outbreaks.

But St. Louis encephalitis still presents a danger, Petersen said. He noted that there have been occasional years when, with little explanation, nationally reported St. Louis encephalitis cases jumped from a few dozen into the thousands.

In addition, some health experts are worried newly observed complications — including polio-like symptoms and paralysis, and apparent transmission through transplanted organs, blood and breast milk — may send West Nile down a more complicated path than some other viruses, and could muddy the U.S. prognosis.

Petersen is not among them.

"All these other modes of transmission that have been uncovered have nothing to do with the maintenance of the virus in nature," Petersen said. "Ninety-nine percent of the transmissions that occur this year or in any future year are going to be because of mosquitoes."

Birds Are Key

The West Nile virus mainly spreads via mosquitoes who bite infected birds, and then bite other birds, people or animals, experts say.

Only birds are believed to start the mosquito-borne transmission cycle because of higher concentrations of West Nile in their blood, McLellan said, so people and mammals with West Nile are not significant public health risks.

That includes common household pets, which rarely become ill from the disease, several West Nile experts said.

However, four dogs with encephalitis-like symptoms in Louisiana — including three who died — had the West Nile virus, the Associated Press reported Saturday. The state veterinarian said there is no proof that West Nile caused the illnesses, but suggested veterinarians should watch more closely for the disease in dogs.

Infected animals are not believed to represent a threat to the U.S. food supply because cooking kills the West Nile virus.

Other than birds, humans and horses appear most frequently to get severe reactions to the virus. For unknown reasons, though, a majority of people only get flu-like symptoms or no symptoms at all, perhaps not even knowing they have the virus.

Less than 1 percent of West Nile-infected humans become severely ill with West Nile encephalitis or meningitis, which involve inflammation of the brain or nervous system membranes. About 10 percent of those with severe illness die, with most deaths involving the elderly or victims with compromised immune systems.

Protect Yourself

Despite the low fatality rates, health experts agree, people in affected areas should aggressively protect themselves from mosquitoes with long sleeves, insect repellant containing DEET and other precautions (see CDC Web link, in the right column of this page, for a detailed list) .

"As a clinical allergist, I'm not looking to throw chemicals onto people … but this year West Nile is here and you want to use something," says Dr. Clifford W. Bassett, an allergist and immunologist who teaches at the New York University School of Medicine. "When you use insecticide, you use the smallest amount [as measured by chemical concentration] to get the job done."

Higher concentrations of DEET do not give added protection, but stay effective longer, he adds.

West Nile is not a newly discovered disease. It was isolated in Uganda, in 1937, and there have been cases elsewhere in Africa, the Middle East and Eastern Europe.

The American strain of West Nile that first appeared in New York City in 1999 was "essentially identical to a West Nile virus strain which had been isolated from geese in Israel in 1998," the CDC's Hughes said.