(10-04) 12:27 PDT BERKELEY, Calif. (AP) --
Federal health officials said this year's West Nile season has revealed weaknesses in a test used to screen the nation's blood supply for the disease, and that the illness has caused nearly two dozen cases of paralysis among previously healthy adults.
This year's outbreak of the mosquito-borne illness, which has been particularly bad in the Great Plains and some Western states, has infected 5,861 people and killed 115.
Officials of the Centers for Disease Control and Prevention said Thursday that a screening test used since July cannot detect all donated blood units infected with the virus.
The government said the test has potentially protected thousands of people, flagging 617 infected blood donors. But the test, which screens samples in a pool of multiple blood units, loses its sensitivity in the diluted mix.
"We catch most of the (infected) people, but not all of them," said Dr. Lyle Petersen, acting director of the CDC's division of vector-borne diseases. The CDC reported last month that two people had become infected through blood transfusions.
Testing individual blood units catches more infections but may be too costly, with an annual cost of $300 million, Petersen said. False positives from screening individual units of blood may mean the blood industry would have to throw out "tens of thousands of units of blood that are probably OK," he added.
Meanwhile, nearly two dozen patients in northern Colorado have developed polio-like paralysis from the virus. Paralysis was first noted as a West Nile symptom last year, but only a few cases were reported.
"We're recognizing a lot more of these cases -- we're kind of surprised to find this many," Petersen said at the University of California's Berkeley campus.
The paralysis, Petersen said, "looks exactly the same as polio." The CDC is working to investigate how many more cases of paralysis, which has struck otherwise healthy adults in their 40s and 50s, have emerged in this year's West Nile epidemic. Many of those afflicted had to be placed on respirators. Petersen did not say whether any died.
CDC officials also noted that people with weakened immune systems, such as those who are taking drugs to suppress the immune system after a kidney transplant, are up to 10 times more likely to develop severe West Nile symptoms than a healthy person, Petersen said.
The virus is passed to people by bites of mosquitoes that have fed on infected birds. Most of the time, the virus causes mild, flu-like symptoms, but in serious cases, the virus can cause a deadly inflammation of the brain.
CDC officials estimate up to 100,000 people are infected with mild symptoms, most of which do not result in a doctor's visit. Another 400,000 people are infected but never show any symptoms.
On the Net:
www.cdc.gov
