Environmental factors, including infection with seasonal influenza, may increase the risk of developing Parkinson's disease, a new study suggests.
Most cases of Parkinson's have no known cause and researchers continue to debate and study possible factors that may contribute to the disease.
Now, researchers at Thomas Jefferson University in the US suggest that a certain strain of influenza virus predisposes mice to developing pathologies that mimic those seen in Parkinson's disease.
"This study has provided more evidence to support the idea that environmental factors, including influenza may be involved in Parkinson's disease," said Richard J Smeyne, Professor at Thomas Jefferson University.
"Here we demonstrate that even mice who fully recover from the H1N1 influenza virus
responsible for the previous pandemic (also called 'swine flu') are later more susceptible to chemical toxins known to trigger Parkinson's in the lab," said Smeyne.
Previously, researchers showed that a deadly H5N1 strain of influenza (so-called bird flu) that has a high mortality rate (60 per cent of those infected died from the disease) was able to infect nerve cells, travel to the brain and cause inflammation that would later result in Parkinson's-like symptoms in mice.
Inflammation in the brain that does not resolve appropriately, such as after traumatic injury to head, has also been linked to Parkinson's.
The researchers looked at a less lethal strain, the H1N1 "swine flu," that does not infect neurons, but which still caused inflammation in the brain via inflammatory chemicals or cytokines released by immune cells involved in fighting the infection.
Using a model of Parkinson's disease in which the toxin MPTP induces Parkinson's-like symptoms in humans and mice, Smeyne showed that mice infected with H1N1, even long after the initial infection, had more severe Parkinson's symptoms than those who had not been infected with the flu.
When mice were vaccinated against the H1N1, or were given antiviral medications such as Tamiflu at the time of flu infection, the increased sensitivity to MPTP was eliminated.
"The H1N1 virus that we studied belongs to the family of Type A influenzas, which we are exposed to on a yearly basis," said Smeyne.
The research was published in the journal npj Parkinson's Disease.