Federal health officials confirmed that at least three of the four people who received transplants from a car crash victim became infected with West Nile virus, likely through the donated organs.
Officials still haven't determined how the organ donor became infected, but as they investigate, they are trying to track down about 60 people who donated blood that was used in transfusions as doctors tried to save the crash victim.
The cases are the first in which humans are believed to have contracted the disease through something other than a bite from an infected mosquito.
Doctors say the possibility of getting West Nile virus through medical procedures is remote, and there are no plans to screen donated blood and organs for the virus.
"We don't even know for sure whether it's possible to transmit West Nile through transfusion or organ donation," said Dr. Jay Epstein, the U.S. Food and Drug Administration's top official for regulating blood products.
Officials at the Centers for Disease Control and Prevention said Tuesday that one of the four organ recipients died Aug. 29 in Atlanta. Two others have the virus and have developed encephalitis, an inflammation of the brain. West Nile has not been confirmed in the third person, who is recovering from a milder infection.
The organ donor, a woman from Georgia, could have been infected before the crash, or she may have gotten West Nile through blood transfusions in the emergency room afterward, the CDC said. Health officials have stopped using the donors' blood and said they were searching for about a dozen people who had already received blood from those donors.
Epidemiologists also are trying to detect the virus in the small portion of blood that is routinely kept after each blood transfusion.
So far this year, 32 people have died of West Nile virus, and more than 670 have been infected. Six potential West Nile deaths were reported Tuesday in three states: Tennessee, Illinois and Kentucky.
The CDC said a test to screen all blood and organ donations for the virus is not currently in the works.
"There hopefully will be a test someday, but it's not what we're working on now," CDC spokesman Tom Skinner said.
Patients now can be quickly diagnosed based on their immune response to the virus. Confirming the presence of the virus can take longer, up to about 15 days.
The risk of catching West Nile through a blood transfusion is so remote that some doctors wonder if it's worth screening for. If a test is developed, the FDA will consider it then, Epstein said.
Doctors outside the CDC said people in need of transfusions or donated organs shouldn't be alarmed by the recent infections.
Viruses similar to West Nile have been present in the United States for centuries, but doctors know of no cases where those diseases, such as St. Louis encephalitis, were spread through medical procedures.
The CDC said last year it had documented cases where malaria, which is not a virus, was spread through blood donations, as many as 93 cases in one year. Blood donors, however, already are turned away if they've recently had malaria or visited an area of the world where it's common.
The real danger in blood transfusions often comes from simple bacterial contamination or mishandling, so that's where the resources go in keeping the nation's blood supply safe, said Dr. Louis Katz, president-elect of America's Blood Centers, an association of independent community blood centers.
"On the list of things I'd be worried about going wrong in a blood transfusion, West Nile's way down at the bottom of the list," Katz said.
Donated blood is tested at least 11 times for various diseases, including HIV and two kinds of hepatitis. Katz said doctors have debated whether screeners should ask donors whether they've been bitten by mosquitoes lately, but decided against it.
"We can't go there. That's been proposed, but we wouldn't have any blood left if we took out anyone had been bitten by a mosquito. That's crazy," he said.