CDC predicts West Nile upswing

Epidemic hit as mosquito-control efforts were cut back

The West Nile virus is transmitted by mosquitoes.

ATLANTA, Georgia (CNN) -- About 1,000 people could become ill with the mosquito-borne West Nile virus before this year's epidemic is halted by the arrival of cool fall weather, a government scientist said Thursday.

"We're still on the upslope of the epidemic curve," said Dr. Lyle Petersen, an infectious disease expert at the Centers for Disease Control and Prevention here.

The CDC reported 156 cases nationwide as of Wednesday. Typically, cases don't peak until the last week of August or the first week of September, he said.

Illinois health authorities Thursday reported an additional three cases that are not yet included in the federal tally

If the epidemic continues as in prior years, about 1,000 cases could be expected by the end of the season, he said. "That would be a ballpark estimate."

But mosquito-control efforts may succeed at blunting the spread of the disease, he said. "If, in fact, those efforts are highly successful, we may see a lot less than 1,000 cases."

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From 1999 through 2001, the CDC confirmed just 149 cases of human illness caused by the virus in the United States, including 18 deaths.

Petersen acknowledged that any predictions are imprecise, because the factors that determine the spread of the disease are complex. Nevertheless, he said, "I think it is safe to assume that we can expect more cases, and potentially a lot more cases."

Some counties quit testing dead birds

The cases, which include nine deaths, have been reported in eight states and the District of Columbia, according to the CDC's Morbidity and Mortality Weekly Report, which was issued Thursday.

In addition, the virus has been found in birds, mosquitoes and horses in 37 states and the District of Columbia, the agency said.

About 20 CDC scientists are working in the epicenter of the outbreak -- in Louisiana, Mississippi and Arkansas -- searching for more cases, trapping birds, studying mosquito populations and investigating the clinical aspects of the disease, Petersen said.

The virus is so prevalent that health authorities in some counties have stopped testing the carcasses of birds, such as crows, known to be susceptible. Mosquitoes bite infected birds and then transfer the virus to people by biting them.

"It doesn't make any sense to keep testing dead birds when you know that all of them are positive," Petersen said.

Instead, it suffices simply to count the number of dead birds -- particularly crows, he said. Such counts have "correlated well to the eventual risk of human infection."

From mild to fatal symptoms

Most people infected with the virus display no symptoms. About one in five develops mild fever or other flu-like symptoms that typically go away on their own in a few days, Petersen said.

About one in 150 infected people develops encephalitis or meningitis, potentially fatal inflammation of the brain or its membrane, he said.

The death rate among people who get either of those symptoms has ranged in past years from 11 percent to 14 percent, Petersen said. "It may be lower this year, because the age distribution of the cases is lower, and age is related to mortality."

The average age of this year's cases is 54, down more than a decade from the average age last year, and more than half (55 percent) of this year's victims are men, he said.

The significance of the younger age is unclear. "One possibility is our surveillance systems in these cases are picking up milder and younger cases," he said. Another possibility: Different mosquitoes are transmitting the disease.

Studies done since the virus was first detected in the United States -- in New York City three years ago -- indicate its effect can be long-lasting, Petersen said.

More than a year after the New York City outbreak, "more than half of patients still had persistent and potentially severe neurological syndromes," such as the inability to walk, he said.

The emergence in the United States of the disease -- deemed an epidemic by the director of the CDC -- comes at a time when many counties -- particularly in Mississippi, where there have been 48 cases, including two deaths -- had de-emphasized mosquito-control efforts, Petersen said.

"This is a classic case of an ignored problem that has now resurfaced," Petersen said. "Over the last several decades, mosquito-borne diseases were thought not to be much of a problem any more. A lot of mosquito-abatement programs dried up or disappeared.

"The ability to deal with this virus is generally less than it would have been two or three decades ago."

Although a vaccine for horses is available, "a human vaccine is a number of years off," he said.

People in areas where the virus is present are urged to use repellents containing DEET, wear long-sleeved shirts and long pants and stay inside at dusk and dawn, when mosquitoes are most active.

A CDC spokeswoman said the agency has received more than 3,500 calls to its hotlines, which were set up last week.