WASHINGTON (AP) -- Blood banks are quarantining all plasma frozen during the West Nile virus epidemic -- an estimated 30,000 pints -- to reduce further the low risk of spreading the disease through blood products.

West Nile virus is spread mostly by infected mosquitoes, but federal health officials discovered in September that it also occasionally is spread through donated blood or organs. Of the more than 3,800 West Nile cases this year, about 13 are believed to have been caused by a blood transfusion.

Most donated blood -- the red blood cells and platelets -- is used right away. The government had tried to limit West Nile risk by warning blood banks earlier this fall not to accept donations from people with West Nile-like symptoms.

But plasma, the liquid part of blood, is routinely frozen and thus can be used up to a year later. It's needed mostly by people with advanced liver disease and to treat serious trauma.

So large hospitals in states that were hard hit by the West Nile epidemic may face temporary tight supplies, as blood banks race to replace quarantined plasma with plasma donated in West Nile-free states or after the epidemic ended.

The American Association of Blood Banks and the nation's two major blood suppliers -- the American Red Cross and America's Blood Centers -- announced the move Thursday. Included in the quarantine are frozen blood products that were collected anywhere from a week before each state's first case of West Nile virus, through a week after the last case.

That means that how much frozen plasma is quarantined will differ greatly from state to state, depending on how many West Nile cases each had.

Will there be serious shortages?

"Nobody really knows," said Dr. Steven Kleinman of the AABB, who said next week will be the crucial period.

But he said if hospitals have emergencies to treat and only quarantined plasma on the shelf, they should use it - but try to use bags dated near the beginning or end of their region's West Nile outbreak, when the risk of donations by sick people would have been lower.

During the holidays, blood banks always need more donations, particularly of red blood cells that make up most lifesaving transfusions.

But officials are hopeful of replacing the quarantined plasma fairly quickly. The quarantined amount makes up only about 15 percent of the Red Cross' frozen inventory, said medical officer Dr. Peter Page.

"The problem is worst today. It'll only get better in coming weeks," as shipments of pre-West Nile frozen bags go out and new blood donations come in, he said.

The quarantine was a voluntary decision by the blood industry. But the Food and Drug Administration, which regulates blood safety, called it a logical move considering hospital freezers still harbored plasma that might contain some West Nile virus.

Yet the quarantine couldn't have come earlier, because until winter ended the epidemic in the South, there was no way to replace the supply, noted FDA blood chief Dr. Jay Epstein.

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