Screening Blood Supply Stems Spread of West Nile

Studies found two-tiered testing system works

WEDNESDAY, Aug. 3, 2005 (HealthDay News) -- The nation's system for screening blood donations for West Nile virus appears to be working.

According to reports in the Aug. 4 issue of the New England Journal of Medicine, no one contracted the virus through transfusions in either 2003 or 2004.

Christine Pearson, spokeswoman for the U.S. Centers for Disease Control and Prevention, said figures released Tuesday indicate that 34 potential blood donors have tested positive so far this year.

U.S. blood experts say there have been no reports of West Nile transmission through blood donations this year.

The journal reports constitute the first large-scale evaluation of the blood-screening program, which has tested 7.1 million donations for West Nile virus, said an accompanying editorial.

Since it first appeared in North America in 1999, West Nile virus has made its way across the continent and is now found in animals or humans in every contiguous state. Since that time, according to the CDC, about 15,000 humans have been infected with the virus, and more than 650 people have died. Last year, there were 2,539 human cases and 100 deaths.

Experts realized the virus could be transmitted via blood transfusion in 2002; close to 30 cases transmitted that way were confirmed that year.

In a quick public health response, by 2003 all blood donations were being screened for West Nile with investigational nucleic acid amplification tests.

"The time period was very rapid," said Susan Stramer, lead author of one study appearing in the journal and executive scientific officer of the American Red Cross. "We acted on an infectious disease agent that's transmitted by blood in an unparalleled rapid action that was implemented very quickly."

Stramer and her co-authors described the Red Cross's program of laboratory testing in 2003 and 2004.

During those two years, routine testing of blood identified 540 donations that were positive for West Nile virus.

According to the research, 1.49 out of every 10,000 donations in 2003 tested positive for West Nile and 0.44 per 10,000 tested positive in 2004. In 2004, more than half (52 percent) of these positive donations were from four counties in southern California.

Testing blood donations for West Nile is a two-tiered system. First, nucleic acid amplification testing is done on "minipools" of 16 samples.

"The concern was that minipool testing may not be sensitive enough during an epidemic period," Stramer said. "It's a good surveillance tool, but once West Nile virus is in an area, we have to do more sensitive testing."

The second step is targeted testing of individual donations in regions where the virus is highly prevalent.

It appears that individual testing is best done early in a season, when new infections are on the rise.

"In individual donation testing, the units identified that were most dangerous were those that were earliest in the epidemic period in any given area," Stramer explained. This was probably because people had few antibodies to the virus.

According to the study, the cost of minipool testing was $120,000 per unit intercepted in 2003 and $232,000 per unit intercepted in 2004.

The targeted individual screening of about 60,000 donations in 2004 cost $32,000 per incremental case detected. If all donations had been individually tested, the cost would have been $281,000 per intercepted donation, the study authors said.

The two-step system appears to be both effective and cost-effective, the authors concluded.

More information

For more on how you can protect yourself from the West Nile virus, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Susan Stramer, Ph.D., executive scientific officer, American Red Cross, Gaithersburg, Md.; Christine Pearson, spokeswoman, U.S. Centers for Disease Control and Prevention, Atlanta; Aug. 4, 2005, New England Journal of Medicine
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