When we come down with the flu, we might think the worst is over after a week of a sore throat and body aches. But such viral infections may have lasting, unseen effects on the brain, emerging research suggests.
Viruses such as influenza and herpes may leave brain cells vulnerable to degeneration later in life, and increase the risk of developing diseases such as Alzheimer's and Parkinson's, research suggests. That's because these the viruses can enter the brain and trigger an immune response — inflammation — which can damage brain cells.
Viruses and other sources of inflammation "may be initiating factors in some of the most common neurological diseases," said Dr. Ole Isacson, professor of neurology at Harvard Medical School, who discussed the topic in an article published on Feb. 15 in the journal Science Translational Medicine.
It's unlikely one bout of the flu will cause significant damage. But over a lifetime, injuries to cells accumulate, Isacson said, and along with environmental stresses, this can kill cells and the development of brain diseases. Variations in the number of infections we get may be the difference between a person developing Parkinson's disease at age 65 or at age 95, Isacson said.
It's possible that toning down the inflammation that occurs shortly after viral infection could reduce cell damage and the risk of subsequent brain disease, Isacson said. Isacson pointed to a 2011 study of 135,000 men and women found that those who took ibuprofen (a medication that can reduce inflammation) were 30 percent less likely to develop Parkinson's over a six year period compared to those who did not take the medication.
One of the earliest pieces of evidence for the virus-brain disease link comes from the 1918 influenza pandemic, according to Isacson's article. After that outbreak, there was a dramatic increase in cases of a disease called postencephalitic parkinsonism, which has many of the same symptoms as Parkinson's.
In a more rigorous test of the link, a 2009 study showed that mice injected with the H5N1 flu virus developed infections in cells in a brain region known to be significantly impacted by Parkinson's disease, Isacson said.
Research has also shown that infection with certain herpes viruses can increase the risk of Alzheimer's disease. And very rarely, encephalitis, or brain inflammation caused by viruses, can lead directly to an acute, but transient, form of Parkinson's disease.
But more often, viral infections in our brain are silent, Isacson said. We don't see the full impact of these infections until brain degeneration is substantial, he said.
Several weeks after infection, inflammatory molecules known as cytokines reach a peak concentration, Isacson said. It's this "cytokine storm" that Isacson and his colleagues suspect is responsible for the brain cell damage associated with viral infections.
If researchers could find a way to block this peak from occurring, they might reduce the risk of certain neurological diseases, Isacson said.
In addition, researchers could also try to identify viruses that cause particularly severe cytokine storms, to better understand which infections pose the greatest risk to the brain, Isacson said.
The idea that immune system inflammation may influence the development of Alzheimer's disease and other neurological disorders is just one hypothesis out of many that are being investigated today, said Heather Snyder, senior associate director of medical and scientific relations at the Alzheimer's Association. More research is needed to understand what, if any, effect the immune system has on brain diseases, Snyder said.
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