Atlanta ? A heart transplant patient’s diagnosis of West Nile virus is raising questions about how the virus spreads and whether blood donors could unknowingly transmit it.

Federal experts and the American Red Cross met Sunday to determine how to deal with the potential threat.

Dr. Mary Jo Trepka, left, epidemiology director for the Miami-Dade County Health Department, and Nancy Humbert, deputy director of the health department, speak to the press at a news conference in Miami. Health officials revealed Saturday they were investigating the possibility that a 63-year-old man contracted West Nile virus from a heart transplant or a blood transfusion.

“We’ve known for some time that there is a theoretical possibility that people can get this through blood or organ transplants,” said Tom Skinner, a spokesman with Atlanta-based Centers for Disease Control and Prevention. “It’s highly unusual, but it’s certainly possible.”

Officials with the Food and Drug Administration said they issued an alert to blood banks two weeks ago to exercise extra caution when screening donors, said Dr. Jesse Goodman with the FDA.

“We have been very active and tried to anticipate the possibility of something like this,” Goodman said.

Four people, who were not identified, might have been infected with West Nile virus after receiving the kidneys, heart and liver of a woman who died in Georgia in early August after a car accident, the CDC said.

Three of the four patients developed symptoms of encephalitis, the inflammation of the brain and central nervous system, which is the most serious consequence of West Nile virus.

One of the four recipients, who was in Atlanta, has died, said Dr. James Hughes, director of the CDC’s National Center for Infectious Disease.

Standard pathology tests from an autopsy confirmed the recipient had encephalitis. Tests are ongoing to see if the recipient was infected with West Nile, which causes encephalitis.

Another recipient from Jacksonville, Fla., showed symptoms of encephalitis Sunday, said Dr. John Agwunobi, the Florida Secretary of Health.

The heart recipient, a 63-year-old man hospitalized in Miami, was diagnosed with West Nile last week. He was upgraded from critical to serious condition Sunday, said Evelyn Lichterman, an administrator at Jackson Memorial Hospital.

Officials were sure the man didn’t contract the disease the disease from a mosquito in Miami-Dade County, said Mary Jo Trepka, epidemiology director with the county health department.

Officials say it is unknown whether the Georgia organ donor was already infected or got West Nile through blood transfusions in the emergency room. The CDC is backtracking to trace donors who contributed the transfused blood, the blood products made from those donations and any other patients who may have received blood or blood products from those donations, Hughes said.

Samples from the four transplant recipients were sent to the CDC’s lab in Fort Collins, Colo., Hughes said. Tests results are expected within the week.

There is no test yet that can quickly or accurately identify the presence of the West Nile virus. Patients are diagnosed on the basis of their immune response to the virus.

However, researchers at the CDC are trying to find a way that will cut down the time from when infection occurs and when a response to the virus can be measured, the CDC said. Presently, it takes about 15 days between the time infection occurs and when the response can be measured.

Officials say they remain optimistic that the chances West Nile can spread through blood is low because there have been no confirmed cases to date.

There are also no known cases either of person-to-person transmission of other diseases in the same family as West Nile called the arboviral encephalitides: St. Louis encephalitis, La Crosse encephalitis and Eastern and Western equine encephalitis.

While there has been no confirmation that West Nile was passed by blood donation, there are cases on the record in which mosquito-borne diseases have been passed by blood transfusion or transplant.

Last year, a CDC review of blood-donation problems turned up 93 patients who contracted mosquito-borne malaria after blood transfusions.

Cases have also been reported in which dengue, another mosquito-borne disease, was transmitted to a health care worker by a needle-stick and between siblings after a bone-marrow transplant.

Still, officials at the American Red Cross say it is important for the public not to fear donating or accepting blood transfusions.

“The many safety layers that are used to protect our nation’s blood supply include carefully screening blood donors who may be experiencing symptoms of West Nile virus, such as fevers and chills, and they would be disqualified through our rigorous screening process,” said Trudy Sullivan, a spokeswoman with the American Red Cross in Washington, D.C.

Sullivan said the blood supply is safer than it’s ever been, and that her organization is “focused on allaying any fears or concerns about the safety of the blood supply for both donors and recipients.”

Every unit of blood donated goes through up to 12 tests to ensure patient safety, including tests for HIV and hepatitis C.