ATLANTA (AP) - Government health officials are warning doctors that the West Nile virus can cause acute paralysis after the mosquito-borne virus apparently caused six people to become paralyzed.

The Centers for Disease Control and Prevention urged doctors to test patients for West Nile if they report sudden, painless paralysis but do not appear to have had a stroke.

The warning came as health officials confirmed that the West Nile virus can apparently be transmitted through blood transfusions and said that all blood donations will probably be screened for the virus as soon as a test can be developed.

The most serious effect of infection - life-threatening brain inflammation - occurs in only a small percentage of cases, mostly among the sick or elderly. But most of the reported paralyses, in Mississippi and Louisiana, have occurred among middle-aged people who were previously healthy, CDC medical epidemiologist Jim Sejvar said.

Some of these victims lost the use of an arm or leg; others needed ventilators to help them breathe.

A CDC report said the symptoms can be confused with a condition known as Guillain-Barre syndrome. There is no known way to prevent West Nile paralysis, but Sejvar said treatments for Guillain-Barre syndrome may be harmful to patients who actually have West Nile.

Health officials also suggested Thursday that the virus can survive in donated blood for days.

A woman in Mississippi caught West Nile after receiving transfusions from three infected donors. CDC doctors called that case ``highly suspicious'' and said that West Nile ``probably can be spread by transfusion.''

Previously, doctors were not sure whether it was possible to spread West Nile through blood, although a Georgia case in which donated organs spread the virus to four recipients in August raised that possibility.

Dr. Jesse Goodman of the Food and Drug Administration said that all blood donations will probably be screened for the virus as soon as a test can be developed. He could not predict how long it would take to develop such a test or how much it would cost.

``What we're trying to do here is jump-start this process,'' he said.

But screening for the virus could prove difficult. West Nile is much harder to detect than a virus such as HIV because there are relatively small amounts of West Nile in tainted blood.

Furthermore, a screening would have to detect the virus itself - not just antibodies the body produces in response - because the virus spends several days in the blood stream before symptoms show up.

Blood banks said they expected the FDA's announcement. Dr. Louis Katz, president-elect of industry group America's Blood Centers, said the important question is not whether West Nile can be transmitted through blood, because it probably can, but whether it's a threat big enough to warrant testing all donations for it.

``Does it rise to the level of appropriate screening? That's what we have to figure out,'' Katz said, adding that some diseases known to be spread through transfusion aren't screened because they're so rare.

West Nile was first detected in the United States just three years ago. The CDC has reported more than 1,700 human cases of West Nile virus so far this year, including 84 deaths.

Most people who become infected with the virus suffer no ill effects at all; others develop only flu-like symptoms.

Researchers also point out that not all patients who receive tainted blood will become infected with West Nile. The CDC reported a July case in which a 55-year-old woman received contaminated blood after an orthopedic procedure but never tested positive for the virus.

While repeating that the benefits of blood transfusions far outweigh the risks of catching West Nile, Goodman said patients seeking elective surgery may want to talk with their doctors about delaying the procedure or donating their own blood to be used later on themselves.

Source: CDC