Alarming West Nile fever risks emerge. (Another polio?).

Having struck nearly 2,100 people and killed 98 in the United States so far this year, West Nile virus infection amounts to an epidemic. Now, medical workers have found poliolike symptoms in a few victims, and last week, federal officials said that blood transfusions appear to have infected some people.

The first U.S. cases of West Nile fever were reported in 1999 in New York. Since then, it's hit 35 other states. The disease is generally transmitted by mosquitoes.

Though flulike symptoms are most common, some severe cases progress to encephalitis, a potentially fatal swelling of the brain, occurs. Two reports to be published in the Oct. 17 New England Journal of Medicine link the virus to rare instances of long-term paralysis, a symptom that had shown up in birds, horses, and monkeys.

Discovery of West Nile patients with polio symptoms is a surprise, says Jonathan D. Glass of the Emory University School of Medicine in Atlanta, a coauthor of one of the reports. Poliolike paralysis is something "you've read about" but never see in the United States anymore, he says.

Poliomyelitis paralysis--caused by spinal cord damage--is historically associated with the poliovirus, though other viruses can also cause it, says Glass. Before polio vaccinations began in 1955, the disease was common in the United States.

Just as Glass' team realized this past summer that West Nile infection could yield paralysis, researchers and physicians at the Methodist Rehabilitation Center in Jackson, Miss., discovered the same symptom in patients there. Between them, the groups have diagnosed seven people with paralysis from West Nile fever. Some previous cases involved muscle weakness, says Dobrivoje S. Stokic from the Jackson team.

Polio symptoms probably aren't caused by any particular West Nile strain, says Glass. "The more cases [of West Nile fever] you get, the more likely you are to see its rare manifestations," he says.

Most of the paralysis cases were revealed by the Centers for Disease Control and Prevention in Atlanta on Sept. 19. CDC epidemiologist James J. Sejvar noted then that several patients with West Nile paralysis had been misdiagnosed with another paralytic malady, Guillain-Barre syndrome. However, Sejvar noted, unlike that syndrome, West Nile paralysis affects a patient's body asymmetrically and doesn't impair the sense of touch. The distinction is important, Sejvar notes, because side effects from unnecessary treatment for Guillain-Barre syndrome could be harmful.

Lyle R. Petersen of the CDC in Fort Collins, Colo., notes that live West Nile virus has been found in a sample of a banked blood product. Jesse L. Goodman of the Food and Drug Administration in Rockville, Md., adds that "at least a few of the reported [West Nile fever] cases may be related to transmission by blood and, certainly, in ... one case, to transmission through organ donation."