HONOLULU (Reuters Health) - An analysis of West Nile cases from 2002 suggests that only a small percentage of people will develop polio-like symptoms, researchers reported here this week at the American Academy of Neurology meeting.

In September of last year the U.S. Centers for Disease Control and Prevention reported that at least six people infected with the West Nile virus in Mississippi and Louisiana developed a polio-like syndrome with muscle weakness or paralysis in their arms or legs.

"Only one of every five people who are bitten by an infected mosquito develop any kind of symptoms, such as mild fever," Dr. Russell E. Bartt told Reuters Health. "One of 200 will develop one of the severe neurological symptoms we found to be associated with the disease."

Overall, 80 percent of people who are bitten by a mosquito carrying the virus will have no symptoms at all and simply develop an immunity, said Bartt, an assistant professor of neurological sciences at Rush-Presbyterian-St. Luke's Hospital in Chicago and a senior attending physician at Cook County Hospital.

Elderly people and those with compromised immune systems are most vulnerable to severe disease, he said.

In the study, lead investigator Dr. Nidhi K. Watson, Bartt and their colleagues reviewed the records of several Chicago hospitals to identify people who had West Nile virus and had been seen for health problems. They looked at 43 cases, about six percent of the total 738 documented cases of West Nile virus infections seen in Illinois as of early November 2002.

Among the 28 patients for whom detailed data were available, 15 had "focal neurologic deficits" that surfaced an average 10 days after the development of a fever.

A focal deficit is damaged nerve function in only one part of the body, such as weakness in one limb, or disturbed vision in one eye.

In seven of these cases, the nerve deficit developed with no signs and symptoms of viral meningitis or encephalitis, symptoms that are associated with severe West Nile virus infection. The remaining eight had either meningitis, an inflammation of the membranes covering the brain, or encephalitis, an inflammation of the brain itself.

Two of the patients died; the surviving patients have recovered completely, Watson said.

In other research, Dr. Lara E. Jeha of the Cleveland Clinic in Ohio, reviewed the records of all patients who had been admitted to the Clinic last summer with West Nile virus. Among the 23 patients, most of whom were elderly men, half developed weakness on one side of the body.

As summer approaches and the risk of West Nile virus infections increase, Bartt said that people should make sure that standing water -- a breeding ground for mosquitoes -- is not allowed to collect near their homes. Typical culprits would be old tires, garbage containers without lids, and gutters.

"If you're planning a barbecue at sunset, or a picnic in a shady area, wear long sleeves and insect repellant," he told Reuters Health.

There were 4,161 human cases of West Nile in the United States last year, including 277 deaths, according to the CDC's Web site. The outbreak was the largest since the virus struck the United States more than three years ago.

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