In their study report, the researchers write that mice that survived infection with an H5N1 flu strain were found to be more likely than uninfected mice to develop brain changes associated with neurological disorders like Parkinson's and Alzheimer's diseases.
Parkinson's and Alzheimer's involve loss of brain cells crucial to a variety of tasks, including movement, memory and intellectual functioning.
The researchers say that their study has shown that the H5N1 flu strain causes a 17 percent loss of the same neurons lost in Parkinson's as well as accumulation in certain brain cells of a protein implicated in both diseases.
"This avian flu strain does not directly cause Parkinson's disease, but it does make you more susceptible," said Dr. Richard Smeyne, associate member in St. Jude Developmental Neurobiology.
"Around age 40, people start to get a decline in brain cells. Most people die before they lose enough neurons to get Parkinson's. But we believe this H5N1 infection changes the curve. It makes the brain more sensitive to another hit, possibly involving other environmental toxins," Smeyne added.
Smeyne revealed that the study focused on a single strain of the H5N1 flu virus, the A/Vietnam/1203/04 strain, and that the threat posed by other viruses, including the current H1N1 pandemic flu virus, was still being studied.
During the study, the researchers infected some mice with an H5N1 flu strain isolated in 2004 from a patient in Vietnam, which is still considered to be the most virulent of the avian flu viruses.
About two-thirds of the mice developed flu symptoms, primarily weight loss. After three weeks, there was no evidence of H5N1 in the nervous systems of the mice that survived.
However, the inflammation triggered by the infection within the brain continued for months, and it was found to be quite similar to inflammation associated with inherited forms of Parkinson's.
Although the tremor and movement problems disappeared as flu symptoms eased, the researchers reported that 60 days later, mice had lost roughly 17 percent of dopamine-producing cells in SNpc, a structure found in the midbrain.
They also found evidence that the avian flu infection led to over-production of a protein found in the brain cells of individuals with both Alzheimer's and Parkinson's diseases.
"The virus activates this protein," Smeyne said.
The study has been reported in the online early edition of the Proceedings of the National Academy of Sciences. (ANI)