MIAMI -- Health officials said Saturday they are investigating the possibility that a 63-year-old man contracted West Nile virus from a heart transplant or a blood transfusion.

It would be the first time such a method of transmission was recorded, according to the national Centers for Disease Control and Prevention. All cases of West Nile virus in the United States have come from contact with a mosquito, according to the CDC.

However, Florida health officials do not believe he could have been bitten by a mosquito because he has been hospitalized for some time.

"In the course of his daily activities, he wouldn't have come in contact with mosquitoes," said Mary Jo Trepka, epidemiology director of the Miami-Dade health department. "It's extremely unlikely he was bitten by a mosquito in Miami-Dade County. ... We're examining other possibilities."

The man has not left Miami-Dade County "for quite some time," she added. She wouldn't be more specific.

There have been no traces of West Nile in Miami-Dade this year. A dead horse with the virus was found in the county in March 2001.

The patient, who was in critical condition Saturday, received a transplanted heart from a Georgia donor, said Jackson Memorial Hospital administrator Carlos Cruz. He wouldn't say when or where the operation occurred.

Outside Miami-Dade, state officials have placed 11 Florida counties on alert for West Nile, urging residents to take added precautions against mosquito bites.Dangerous weather hunting as a way of life