Patients infected with the West Nile virus may be eligible to receive a drug previously used to treat hepatitis C as part of a government-approved national trial to combat the mosquito-borne virus.

The testing of alpha-interferon will begin immediately in New York City, where the virus was first detected in the United States three years ago. Since then, it has killed at least 31 people across the nation.

Dr. James Rahal, the study's chief investigator, said 40 people 50 and older who have been hospitalized with the virus will be enrolled in the study. Patients across the country can enroll, but the trial most likely will focus on Louisiana and Mississippi, where 10 West Nile deaths have been confirmed this year.

Alpha-interferon is sold as Intron A for treatment of hepatitis C by Schering-Plough, which is paying for the study.

Tests have shown interferon to be effective in lessening the symptoms and length of hepatitis. It has also proven effective against St. Louis encephalitis, a virus similar to West Nile. There is no known treatment for West Nile virus.

Rahal said Wednesday that he treated 15 Louisiana patients with the drug to make sure it was safe for those with West Nile. He said the results were promising enough to get the Food and Drug Administration to approve the new trial.

"Encouraging," he said. "Not convincing, but encouraging."

The West Nile virus is passed on to humans by mosquitoes that have bitten infected birds. Government researchers say less than 1 percent of people who are bitten will become severely ill.

Those who do suffer flu-like symptoms and, in the worst cases, encephalitis, a potentially fatal inflammation of the brain.

The study will target patients whose virus is still in the blood, where it circulates before entering the brain, Rahal said. Therefore, patients must begin treatment within the first four days of being admitted.

"Once damage has occurred in the brain, it's not likely to be reversible, at least not by a drug," Rahal said. "What we want to do is increase the body's defense against the virus, and decrease the amount of virus that ultimately enters the brain or the nervous system."

The protocol calls for two weeks of interferon, whose side effects include a decrease in the white blood count and inflammation of the liver — both reversible once the drug is stopped. Rahal said long-term use could cause depression, confusion and fatigue.

The National Institute of Allergy and Infectious Diseases at the National Institutes of Health is also funding research on three West Nile virus vaccines and said this week that one may be ready for human trials next year.

West Nile virus has been found in other countries for decades. From 1999 through 2001, government researchers confirmed 161 U.S. cases, including 18 deaths.

As of Wednesday, the Centers for Disease Control and Prevention reported 269 suspected cases this year. The CDC also said the virus has been blamed for 13 deaths in five states: Illinois (1), Kentucky (1), Louisiana (8), Mississippi (2) and Texas (1).

Health and Human Services Secretary Tommy G. Thompson said the CDC will provide an additional $4 million to states hardest hit by the virus, raising the total provided this year to more than $31 million.