TALLAHASSEE, Fla. (Reuters) - A Florida 7-year-old may have contracted West Nile disease from a blood transfusion, state health officials said on Friday, heightening fears the potentially deadly virus may have entered the national blood supply.

The virus, which can cause severe brain inflammation, is normally spread by mosquitoes. But amid an outbreak this year in the United States there have been 29 cases nationwide since Aug. 28 in which patients became ill with West Nile after receiving blood or blood products, the Florida Department of Health said.

The US Centers for Disease Control and Prevention has previously said there was no evidence linking the infections to the nation's blood supply, because most have been in areas where West Nile virus is present in the mosquito population.

The Florida child had recently received "multiple blood products," but investigators had not determined whether that was the source of the virus. "We're in the process of looking at natural as well as blood-related transmission," Florida Department of Health spokesman Bill Parizek said.

Epidemiologists were trying to trace the blood products to determine whether the donors carried West Nile.

The spokesman declined to comment on the child's condition, citing patient confidentiality.

The child, who was not identified, lives in Alachua County in north central Florida, one of 34 Florida counties under medical alert for West Nile and other mosquito-borne viruses.

Fears that West Nile could be transmitted through blood surfaced in September after four organ recipients were infected from a single donor in Georgia.

Health officials have conceded they do not have the capability to test the nation's blood supplies for the virus. They said the potential risk of contracting West Nile from blood was likely very small.

There have been 3,104 confirmed cases of West Nile in the United States this year, 172 of them fatal. The outbreak is the largest since West Nile, which is common in Africa and the Middle East, surfaced in the Americas three years ago striking mainly the elderly and people with weak immune systems.

Florida has confirmed 13 human cases of the disease, none of them fatal.