Having confirmed that West Nile virus can be spread through organ transplants, federal health officials have found new evidence that it may be transmitted through blood transfusions as well.

The Centers for Disease Control and Prevention said Thursday that it is investigating five cases of people infected with West Nile who also received blood transfusions. That includes a Georgia woman who died and became an organ donor.

In each case, health officials are tracing donated blood to see if the donors had the virus, too. They said Thursday that one of these patients, a woman in Mississippi, did in fact receive blood from three infected donors.

Still, they said it will be very hard to determine whether this patient -- or any patient -- got West Nile through the blood, or whether it was transmitted by a mosquito bite.

"There is no single laboratory test that's going to tell you the person got it from a mosquito bite or from a transfusion," said Dr. Lyle Petersen, deputy director of the CDC's vector-borne disease division. "So unless something highly unusual happens, like a transfusion recipient in an area where West Nile virus transmission is not occurring ... it's going to be very difficult to sort out natural mosquito infection from transfusion infection."

Officials added that they were testing the blood after it had been processed for transfusion, and it was unclear what effect that may have on the West Nile testing.

Overall, Petersen said, earlier research suggests the chances of a blood transfusion containing the virus is about one in 10,000.

All of five patients who received blood transfusions lived in areas where West Nile is prevalent, he noted. Two are in Mississippi, one in North Dakota and one in Louisiana, plus the Georgia organ donor. Investigations are under way to see if they can prove that blood was the cause of their infection.

Testing is complete on blood donated to one of the patients, and neither of those donors tested positive. Testing is still under way on the three others.

The CDC had already said that four transplant patients who received organs from the infected Georgia donor and then developed West Nile were almost certainly infected by their transplants. On Thursday, they confirmed that this was the route of transmission.

One of these four patients died of West Nile encephalitis, a brain inflammation caused by the virus. The three others are recovering, two after developing encephalitis.

Nationally, the West Nile toll continues to climb, with 1,295 people confirmed with the disease, and 54 dead. Those numbers will continue to climb because the disease is now in its peak transmission period, Petersen said.

Even if officials conclude that West Nile can be transmitted through blood, there is no blood screening test for the disease available. And health authorities have repeatedly said that the benefits of transfusions for sick people far outweigh the risk of catching West Nile.

Most people who get the virus show no symptoms at all, but infection can be deadly in the weak and elderly.

At the Food and Drug Administration, which regulates the nation's blood supply, Dr. Jesse Goodman said: "In none of these cases is it considered proven that transfusion transmitted the virus, but let me turn that around and say that at FDA we are very concerned about this. So is CDC."

"We regard some of these findings as suspicious but in need of further study," he added.