A collage image of viruses under the microscope and the hands of an older adult holding a caneShare on Pinterest
A new study links multiple viruses to a heightened risk of neurodegenerative conditions, including Alzheimer’s and Parkinson’s disese. Design by MNT; Photography by Aleksandar Zdravkovic/EyeEm/Getty Images & Smith Collection/Gado/Getty Images
  • There are currently 219 viral species known to infect humans, all of which cause inflammation in the body.
  • Researchers from the National Institutes of Health (NIH) have discovered a correlation between viral illness and an increased chance of developing neurodegenerative diseases like Alzheimer’s and Parkinson’s disease.
  • Although the study found potential links between viral exposures and neurodegenerative disease risk, more research is still required to confirm causality.

For as long as history has been recorded, there have been viruses — microscopic infectious particles that are parasites of cells and therefore can only multiply when inside living cells, such as in human and other animal cells.

There are currently 219 species of viruses known to be able to infect humans. Viruses cause different diseases associated with various symptoms. However, one thing all viruses have in common is that they cause an inflammatory response as the body works to defend itself from invasion.

Previous studies show that viral inflammation can negatively affect areas of a person’s health, such as cardiovascular health and lung health.

Now, researchers from the National Institutes of Health (NIH) have found a correlation between having a viral illness earlier in life and an increased risk of developing neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease.

The study appears in the journal Neuron.

A neurodegenerative disease affects the body’s central nervous system, including the brain. Such conditions can impact certain body functions such as movement, balance, speaking, thinking, and memory.

Types of neurodegenerative diseases include:

There is currently no cure for neurodegenerative diseases. Doctors advise people to modify certain risk factors for these conditions to help prevent or slow their progression. And many have medications available to help treat and manage disease symptoms.

For this study, researchers first searched about 300,000 medical records in the Finnish biobank FinnGen, looking for people who had one of six neurodegenerative diseases — Alzheimer’s disease, ALS, generalized dementia, multiple sclerosis, Parkinson’s disease, or vascular dementia.

Scientists analyzed the narrowed-down records to see if any of these people had also been checked for a viral infection at a hospital.

The research team identified 45 significant associations between a neurodegenerative disease diagnosis and a previous viral infection through this first research stage.

The team then narrowed those associations down to 22 after performing a second medical records search of the 500,000 medical records in the UK Biobank.

Of the six neurodegenerative diseases selected, researchers reported generalized dementia had the most viral exposure associations. They found links between dementia and more than six different viral diseases — viral encephalitis, viral warts, all influenza, influenza and pneumonia, viral pneumonia, and other viral diseases.

Scientists reported that people who had viral encephalitis were at least 20 times more likely to receive an Alzheimer’s disease diagnosis than those who had not had the virus. And researchers found severe cases of influenza were associated with the widest range of neurodegenerative disease risks.

“Over the years, several lab-based experiments have suggested that viruses may be a risk factor for neurodegenerative disorders,” Dr. Mike Nalls, leader of the NIH Center for Alzheimer’s and Related Dementias (CARD) Advanced Analytics Expert Group and senior author of this study told Medical News Today.

“What struck us is that we were able [to] obtain similar results by data mining medical records,” he said.

While this study examines the potential links between viral exposures and neurodegenerative disease risk, researchers point out a causal link cannot yet be confirmed.

“At this point, we are not addressing the mechanistic link, we are simply showing an association — a lot more work needs to be done,” Dr. Nalls cautioned. “The results of this study provide researchers with several new critical pieces of the neurodegenerative disorder puzzle.”

“In the future, we plan to use the latest data science tools to not only find more pieces but also help researchers understand how those pieces, including genes and other risk factors, fit together,” he added.

Dr. Melita Petrossian, a neurologist and director of the Pacific Movement Disorders Center at Pacific Neuroscience Institute in Santa Monica, CA, who was not involved in this study, commented on the findings for MNT:

“The way I think of these kinds of viral exposures is they are one part of the puzzle and are indicative that whatever the driving forces are of these neurodegenerative diseases, including Alzheimer’s and Parkinson’s, there isn’t any one thing that causes somebody to get those conditions.”

“What it points out is that there are factors — in this case we’re talking specifically about environmental factors, but we know from other studies that there are genetic factors as well — that play a role in the development of these neurodegenerative diseases and that these processes take decades before the disease manifests,” she added.

Currently, vaccines are available for various viral diseases. How might these findings change how doctors advise patients to receive certain vaccinations?

For example, a previous study linked the shingles vaccine to a reduced risk of dementia. And other research associates the influenza vaccine with a lowered risk for Alzheimer’s.

“The obvious thought would be that if you can reduce the viral load, reduce the inflammatory process of the virus, that you could reduce the systemic inflammatory processes that eventually over the years can be associated with neurodegeneration,” Dr. Petrossian detailed.

“It’s something that should ideally be in the minds of people who are deciding on vaccines about having yet another tool in their pocket for reducing the risk of neurodegenerative diseases,” she said.

She added that sometimes it is difficult for patients to understand that vaccines provide not only short-term effects but potentially long-term ones.

“It’s something that isn’t on the mind of people as they’re considering their vaccine decisions, but ideally would be really great to see people having more of a motivation for getting vaccinated to protect themselves against potentially the risk of neurodegenerative disease, but also to protect the community,” noted Dr. Petrossian.

MNT also spoke with Dr. Clifford Segil, a neurologist at Providence Saint John’s Health Center in Santa Monica, CA, who was also not involved in the study, and had a different take on the findings.

“I’m going to use their exact words — no causal link has been found from this study, there are significant associations, [and] medical decision-making shouldn’t be based on significance associations,” he cautioned.

Dr. Segil said certain viruses — such as the herpes virus and West Nile virus — can cause confusion, which may lead to some memory loss down the line.

“When those viruses do cause encephalitis, people are confused,” he explained. “And later on in life, people that have had encephalitis may have more memory loss. It’s a stretch to say that a patient that had viral encephalitis has Alzheimer’s dementia. If they have confusion later in life, the confusion is from having viral encephalitis, it’s not from having Alzheimer’s dementia.”

For the next steps in this research, Dr. Segil said he would like to see a correlation with neuroimaging. “I’d like to see the results for neuroimaging to correlate that, if any of these patients had abnormalities from any of these viruses,” he added.