Health officials were scrambling to determine if the virus can be spread through blood transfusions, even as they emphasized that the blood supply is very safe. The risk of contracting West Nile from blood is significantly lower than the risk of forgoing any procedure that would call for a blood transfusion, they said.
Ultimately, a screening test is probably needed, said Dr. Lester Crawford, acting commissioner of the Food and Drug Administration. He said government would work with industry to stimulate faster development of a test.
Even so, testing for the virus is complicated. Some of the tests that are used to diagnose West Nile in sick patients won't pick up the virus in donated blood. Other tests are more promising, but they would require significant improvements to be practical on a mass scale, enabling blood banks to screen millions of pints each year.
"I'm reasonably optimistic that if needed, it could be done," said Dr. Jesse Goodman, deputy director of the Center for Biologics, Evaluation and Research at the FDA.
Others are less confident.
"It's going to take several years to have a test suitable for blood donors," said Dr. Harvey G. Klein, chief of the Department of Transfusion Medicine at the National Institutes of Health and past president of the American Association of Blood Banks.
West Nile, which emerged in the United States just three years ago, has exploded across much of the country this summer. A total of 737 human cases — including 257 in the last week alone — and 35 deaths have been reported, the Centers for Disease Control and Prevention reported Thursday. The median age of patients was 52, with 57 percent of the victims male.
Concern hit new heights Tuesday when officials confirmed that at least three of four people who had received organs from a Georgia woman had contracted the disease. One died.
Officials said they are convinced that these patients contracted the disease through their transplants, though they don't yet know whether the virus can be spread through blood as well. The CDC was advising transplant doctors to be alert for West Nile in their patients, but say current information does not warrant changes to national blood or transplant policies.
Dozens of epidemiologists at the CDC headquarters in Atlanta and the CDC lab in Fort Collins, Colo., were trying to figure out how the organ donor, a Georgia woman who died in a car crash, got West Nile. She had received blood from more than 60 donors before she died, and they were tracing those blood donors to see if any of them have the virus. They are also tracking down about a dozen other people who had received transfusions from the same donors.
It's possible that the organ donor may have contracted West Nile from a mosquito bite, like others have. And it's possible that the virus can be spread through organ transplants but not through blood. Still, health officials suspect that blood can carry the virus, at least in some cases.
For now, they are reminding blood banks to be sure that no one who has a fever or appears ill donates blood, which could eliminate those with mild West Nile symptoms. They are also urging organ procurement organizations to be aware of the issue.
While scientists are developing a test, Goodman suggested, early versions could be used to screen blood going to patients who are particularly susceptible to West Nile. The FDA could allow use of the test as an experimental product before it is officially proven effective and licensed. That, he said, could give the blood supply some quick added protection.
Still, Klein cautioned, the tests available today would produce a host of false positives, where healthy blood appears to be infected. Using these tests could mean throwing away many pints of good blood at a time when the blood supply barely keeps up with demand.
West Nile is currently diagnosed in patients using an antibody test, which looks for signs in the blood that the body is fighting off the disease.
But that test cannot be used to screen donated blood because the virus lingers in the blood for at least a few days — maybe as long as two weeks — before a patient develops symptoms and detectable antibodies, which the body produces to fight off disease. Officials are worried about people who donate blood before they know they are sick.
Furthermore, people with West Nile don't always get sick. Just one in 150 people with the virus gets severely ill with a potentially fatal brain inflammation. An estimated one in five people will get minor symptoms.
More promising, experts say, is a test that looks for the virus itself in the blood. Nucleic acid tests look for genetic material that is present in the blood and have already been successfully licensed to screen blood for HIV and hepatitis C.
But it will be difficult to transform this sort of test into reliable mass production, where many different people will use it in many places.
"There are many challenges in taking something that works in a lab and moving it into the field," Goodman said. "Those are challenges that can be met if needed, but they are substantial."