Officials investigate link between transplants, West Nile virus.(Knight Ridder Newspapers)

Federal health officials launched an investigation Sunday into whether blood transfusions and organ transplants are indeed linked to West Nile virus infections.

The announcement came on the same day that the condition of a Miami-Dade man who contracted the virus was upgraded from critical to serious.

At issue is whether four Georgia and Florida transplant recipients contracted the virus from a Georgia organ donor killed last month in a car accident. The virus has been confirmed only in a 63-year-old Miami-Dade man who was diagnosed last week and who received a heart from the Georgia donor. The Dade patient is hospitalized at Jackson Memorial Hospital.

But three other organ-transplant recipients, one of whom has died, displayed similar virus symptoms, according to the Centers for Disease Control and Prevention. Autopsy results showed that encephalitis in the brain tissue led to Thursday's death of an Atlanta-area resident who received a kidney, also from the same dead Georgia donor, a woman, said Dr. James Hughes, director of the CDC's National Center for Infectious Diseases.

The two organ-transplant patients are being tested and have shown fever-like symptoms. One, also a resident in the Atlanta area, is hospitalized with encephalitis-like symptoms _ which consist of brain and central nervous system swelling _ but is improving, said Susan Lance-Parker of the Georgia Health Department.

Test results in the next few weeks could confirm whether the organ transplants led to the first-ever human-to-human transmission of the West Nile disease, usually spread by mosquitoes. Until now, research has shown mosquitoes have transmitted the virus from infected animals to humans.

Officials have placed blood banks and organizations on alert, but said it was too soon to press any panic buttons in regard to transfusions.