Efforts to keep the West Nile virus out of the U.S. blood supply appear to be working.

In 2003 and 2004, tests flagged 519 blood donors who were positive for the West Nile virus. That kept more than 1,000 potentially contagious blood products out of the American Red Cross' blood supply.

That's according to the American Red Cross' Susan Stramer, PhD, and colleagues. Their study appears in The New England Journal of Medicine.

Stramer's team noted 540 blood donations that were positive for the West Nile virus in routine tests in 2003 and 2004. About two-thirds of those donations were probably infectious, writes Stramer.

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Safeguarding Blood Donations

The West Nile virus is mainly spread by mosquitoes. In 2002, health experts learned that the virus could also be spread through blood transfusions.

In late 2002, the FDA ordered the development of tests to keep the West Nile virus out of the U.S. blood supply.

In the test's early days in 2003, there were 183 out of more than 677,000 blood donations containing the virus.

That's 0.027 percent, or one in 3,703 donations. Follow-up tests showed low levels of the virus in about 30 other units, write Michael Busch, MD, PhD, and colleagues in The New England Journal of Medicine.

Blood Test Prevented Many West Nile Infections

In 2003, the test prevented "hundreds of cases of West Nile infection," writes Busch, who works at the Blood Systems Research Institute in San Francisco.

However, the test wasn't perfect. "It failed to detect units with a low level of [the virus], some of which ... were infectious," Busch writes.

He studied tests done in "minipools" of blood donors. Testing individual donations might work better in areas where West Nile is common, write Busch and colleagues.

In Stramer's study, most virus-positive donations that were only spotted through individual testing weren't infectious.

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'Model' Program

The West Nile test is a "model of cooperation among public health agencies, regulators, manufacturers, and the blood-supply system," writes Stramer.

It's hard to predict what infectious disease will next threaten the blood supply, states an editorial in The New England Journal of Medicine.

"Nevertheless, with the right collaboration of scientific disciplines and the adaptation of newer forms of technology, our ability to respond has never been better," write the CDC's Lyle Petersen, MD, MPH, and colleagues.

West Nile Virus: First Human Case of 2005

By Miranda Hitti, reviewed by Michael W. Smith, MD

SOURCES: Stramer, S. The New England Journal of Medicine, Aug. 4, 2005; vol 353: pp 451-459. Busch, M. The New England Journal of Medicine, Aug. 4, 2005; vol 353: pp 460-467. Petersen, L. The New England Journal of Medicine, Aug. 4, 2005; vol 353: pp 516-517.