Two West Nile cases tied to tranfused blood: watch for fever, headache, encephalitis.(News)

Nucleic acid screening of the nation's blood supply successfully intercepted more than 600 units of West Nile virus-contaminated blood during the past summer, but the tests are not perfect.

At least two patients were newly diagnosed with West Nile encephalitis that was tracked to transfusions, Dr. Julie Gerberding announced at a teleconference Sept. 18.

"The take-home message here is that our transfusion supply is far safer than it was a year ago, thanks to the incredible work of the FDA [Food and Drug Administration] and all of our colleagues in the blood-banking industry, but we are still on the lookout for an occasional case that could slip through," according to Dr. Gerberding, who is director of the Centers for Disease Control and Prevention in Atlanta.

Patients who received contaminated blood in Nebraska and Texas are recovering.

In 2002, 23 cases of West Nile virus were traced to the blood supply, although officials have said many more probably went undetected.

An unprecedented effort by industry and the federal government rushed two nucleic acid screening assays into production in time for the onset of West Nile virus season in late June and early July 2003. The nation's blood banks immediately began testing blood donations under an Investigational New Drug Application.

The testing protocol calls for pooling samples from 6-16 blood donors for each screening test. Researchers recently determined that "'on very rare occasions, the virus concentration [was] too low to be picked up when the samples were mixed, but when tested individually, they could find some virus," Dr. Gerberding said.