A Northwestern Medicine research lab has found a usually harmless virus in brain samples from Parkinson’s patients. The idea that Parkinson’s could be linked to a virus had been theorized for years, but this is the first study to pinpoint a specific virus as more common in Parkinson’s patients.

“The message that we want to give to the general public is, it opens a new field of investigation, something that we didn’t know about,” said Dr. Igor Koralnik, Northwestern’s chief of neuroinfectious diseases and global neurology and lead author of the study.

Parkinson’s disease is a movement disorder caused by the loss of neurons that produce dopamine, a chemical messenger in the brain. Why these neurons break down is unknown, but it’s thought that this breakdown is caused by many factors, both genetic and environmental.

The team used a tool called ViroFind, which is able to test samples for all known viruses that infect humans much quicker than the usual one-at-a-time “brute force” method. The lab found human pegivirus, or HPgV, in 5 of the 10 Parkinson’s-affected brains they tested, and none of the brains without Parkinson’s.

“Fifty percent of any population having this virus would be very, very high,” Barbara Hanson, the lab’s post-doctoral fellow, said. Estimates calculate HPgV as being present in about 5% of blood donors in North America, and people with healthy immune systems generally lose the virus within two years of exposure. It is not known to cause disease in humans, so most who catch the virus will never know they had it.

Before this study, Hanson said, HPgV had not been found in human brain tissue. It was understood to primarily “live” in blood.

“For this virus to be present in the brain, there must be a reason for that,” Koralnik said.

This doesn’t mean that the virus itself is a trigger for Parkinson’s, though. It could be that an as-yet unidentified genetic mutation that makes people susceptible to Parkinson’s, also allows for the virus to spread throughout the body differently than in people without the mutation.

And even if further research shows HPgV as a direct cause of Parkinson’s, both Koralnik and Dr. Danny Bega, medical director of Northwestern’s Parkinson’s Disease and Movement Disorders Center, said people shouldn’t necessarily worry about getting HPgV.

A patient likely has to encounter a couple of different factors before they develop Parkinson’s, Bega said. He puts it in terms of “hits” — a hit could be genetic, or it could be environmental, such as exposure to a virus or pesticide. These “hits” build up over time until there are enough factors to cause the neuron degeneration specific to Parkinson’s.

It’s possible that this is why Parkinson’s risk increases with age, Bega said. The longer you live, the more of these “hits” you take, as you encounter different Parkinson’s risk factors throughout your life.

“I always caution people who try to blame their Parkinson’s on one thing,” he said. “Rest assured, it’s never one thing that you could have done or should have done differently.”

The work to treat and hopefully one day prevent Parkinson’s is being done through finding these factors and eliminating them one by one. “The more targets that we have, the more likely we are to be able to achieve a treatment that actually can slow things down,” Bega said.