A new study found a link between a common type of herpes virus and Alzheimer's disease

New Study Reveals Chronic Gut Infection Could Be Linked to Alzheimer's Disease Experts explain the finding—but there's still more to learn

A new study found a link between a common type of herpes virus and Alzheimer's disease

• A new study found a link between human cytomegalovirus (HCMV), a common type of herpes virus, and Alzheimer’s disease.
• The findings suggest that HCMV can infect the gut and travel to the brain, researchers said.
• While infections may play a role in the development of Alzheimer’s, experts say there are more questions to explore about the complex processes.

A new study has provided further evidence of the “gut-brain connection”—chronic gut infections could be linked to the development of Alzheimer’s disease.

The research, published in Alzheimer’s and Dementia on Dec. 19, found that a common virus can take hold in some people’s gut and later travel to the brain. Further investigation showed a link between a high concentration of the virus in brain cells and certain tau and amyloid proteins associated with Alzheimer’s disease.

The virus in question is called human cytomegalovirus (HCMV). About one-third of Americans have antibodies to HCMV by age 5, and about half of Americans have been infected by age 40. The virus is more common in developing countries, where it infects about 90% of the population.

HCMV is a type of herpes virus, and it lies dormant in the body, even after a person recovers from their initial infection. In healthy people, these infections usually cause mild symptoms such as fever and sore throat or no symptoms at all.

“When you are infected, you are infected for life. Your immune system can control the virus but never clear it from your system,” Kevin Zwezdaryk, PhD, assistant professor of microbiology and immunology at the Tulane University School of Medicine, told Health.

Because the body can’t completely get rid of HCMV, it’s possible for the virus to be reactivated, reinfecting the gut and potentially moving to the brain, Zwezdaryk explained.

This new study adds to a growing body of research suggesting that herpes viruses, including HCMV, could be a risk factor for the development of Alzheimer’s disease in certain people, possibly decades before their symptoms manifest.

“We don’t have direct evidence of that yet, but we do know that Alzheimer’s disease has a multi-decade preclinical course,” study author Benjamin Readhead, PhD, associate research professor at the Arizona State University’s ASU-Banner Neurodegenerative Disease Research Center, told Health. “That means by the time someone comes to the doctor complaining of memory problems, there is likely a 20- or 30-year-long process [at play].”

How a Virus in the Gut Can Affect the Brain

In this new study, researchers looked at brain tissue samples from 101 people who had died, 66 of whom had been diagnosed with Alzheimer’s disease.

Readhead and his colleagues examined the brain tissue for a certain type of protein on the surface of microglia cells—these are cells in the brain that regulate development, help maintain neuronal networks, and repair injuries to tissue.

The protein, called CD83(+), is an indicator that HCMV was present in the outer layer of a person’s brain, as well as the vagus nerve.

The vagus nerve plays a key role in the parasympathetic nervous system, helping manage mood, immune response, and digestion. It also connects the gut and the brain, and researchers believe it could serve as a sort of highway that allows viruses such as HCMV to travel to the brain.

After looking at the tissue samples, the researchers noticed that, in Alzheimer’s patients, the presence of the CD83(+) protein was linked to HCMV in both the gut and the brain.

“It was very tightly correlated—if we saw CD83(+) in the brain, we almost invariably saw HCMV in the gut,” said Readhead.

Further investigation showed microglia cells only produced CD83(+) when they were exposed to HCMV, not other viruses.

The researchers also studied brain organoids—tissues grown in a lab that resemble those in a human brain—and infected them with HCMV. The results of their experiment showed the virus could accelerate the cells’ production of two amyloid and tau proteins associated with Alzheimer’s disease.

It’s important to note that the findings of this study only show a correlation—it’s not clear if HCMV is necessarily causing Alzheimer’s disease.

Also, it’s not clear which factors might trigger an active or chronic HCMV infection in a person’s gut, though the virus is more likely to reactivate in immunocompromised patients.

However, the research does suggest that this movement of the HCMV virus from the gut to the brain may be tied to the disease in some way.

“We are describing what is probably a pretty modest slice of Alzheimer’s disease, [and] we have not established that this infection and CD83(+) cause Alzheimer’s disease,” said Readhead. “Having said that, I think it’s pretty plausible that this process is somehow contributing to pathology.”

Many Questions Remain

HCMV isn’t the only pathogen that’s been linked to Alzheimer’s disease. In fact, there are a number of infections caused by viruses, bacteria, and fungi that have been linked to neurodegenerative issues—these include infections from the common yeast Candida albicans, as well as viral encephalitis and influenza with pneumonia.

But understanding how a single infection may be playing a role in a person’s brain health is an extremely difficult thing for researchers to study.

“It is hard to do in a human being because we don’t know when they [were] infected. We only find it in post-mortem analysis,” Brian Balin, PhD, director of the Center for Chronic Disorders of Aging at the Philadelphia College of Osteopathic Medicine, told Health.

Humans are also infected with many, many pathogens throughout their lifetimes, making the waters increasingly murky. It may be that co-infection with another virus could allow HCMV to better infect the gut or the brain in some way, Readhead said.

It’s also possible that when pathogens get into the brain—either through a weakened blood-brain barrier, or through the vagus nerve—they might cause issues that look like Alzheimer’s but are not, leading to a misdiagnosis, Balin said. These infections could also be opening up the door for other issues that are the real drivers of neurodegeneration, he added.

“It’s important for us to understand what [viruses in the brain] are really doing,” Balin said. “Is it just the signature of a virus there that really isn’t causing pathology? Or is it something that is causing pathology or causing inflammation that may allow other agents to infiltrate the brain?”

Julia is a news reporter and editor for Health, where she covers breaking and trending news on health and wellness topics. Her work has been featured in The Heights, an independent student newspaper at Boston College, and Minnesota Monthly.

This story originally appeared on: Health News - Author:Kaitlin Sullivan