Can herpes cause dementia?

By Jules Murtha
Published October 5, 2021

Key Takeaways

While we know that a herpes infection lingers in the body for a lifetime, the secondary and tertiary effects of herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) have remained elusive. For decades, researchers have wondered whether there is a hidden link between common viral infections, such as HSV-1 and HSV-2, and neurodegenerative diseases like dementia. While some answers have emerged, the nature of the link leaves many unanswered questions.

Most people who contract the disease experience an initial infection that the immune system quickly suppresses, preventing future outbreaks. In a sick twist, researchers are finding that the body’s measures to control infection could also expose patients to a heightened risk of dementia over time. 

So, what is the link between herpes and dementia? And what do physicians need to know about immune responses and their potential effects on brain health?

The make-or-break protein

Herpesvirus can manifest in several ways. Most patients have an immune system that’s strong enough to keep the virus from damaging the central nervous system during the initial infection, preventing some, but not all, future outbreaks. This varies from person to person. When the mechanisms responsible for this process break down, trouble can occur, according to a new study published in September in Nature Communications.

For HSV-1, a protein called optineurin (OPTN) does the dirty work of stopping the spread, according to the authors, who believe this could be true of the seven other types of human herpesviruses. OPTN acts as the ultimate defender: To combat herpes, the protein presents small vesicles, known as autophagosomes, which engulf particles of the virus, preventing them from reaching other parts of the body. This process is called autophagy, in a nod to those heroic vesicles. 

But when OPTN is mutated or absent, HSV-1 has the opportunity to penetrate the central nervous system. That places infected individuals at risk of long-term degenerative disorders of the brain and eyes.

Researchers came to this finding in the above study of mice by comparing those with functioning OPTN genes to others without the protein. Both groups received HSV-1 ocularly, and the mice without OPTN experienced higher levels of neuronal death than those with the protein. Make no bones about it, the authors concluded: OPTN appears to play a crucial role in sidelining neurodegenerative damage.

When neurodegenerative damage does occur, however, patients experience a number of undesirable symptoms that worsen with time. Alzheimer disease, Parkinson disease, and amyotrophic lateral sclerosis (ALS) all share a devastating range of symptoms caused by progressive dysfunction of the nervous system. Patients with these diseases tend to experience decreased cognition and behavioral and motor skills, as well as neuroinflammation and brain atrophy, all of which signal developing dementia.

When the cure becomes the killer

The human immune system sometimes fights off microbial and viral infections in ways that can harm body systems. That’s where the link between herpes and Alzheimer comes in.

When a patient gets infected with HSV-1, their immune cells rush to the rescue, inducing inflammation. This reaction wouldn’t necessarily be of any concern—that is, if HSV-1 didn’t exist in neurons indefinitely. Because the virus makes a permanent home in its host, the immune system must periodically stamp out resurgences. Consistent inflammation can cause mild to severe encephalitis, among other issues, in patients.

In the brain, the hippocampus, temporal cortex, and frontal cortex receive most of this damage. Alzheimer also tends to strike these areas, according to a 2019 Nature study. Researchers have pondered the potential relationship between HSV and Alzheimer since 1982, when M.J. Ball, MD, noticed similarities in symptoms between patients who survived herpes simplex encephalitis and patients with Alzheimer disease.

When peripheral inflammatory reactions occur (as is the case in those with herpes), the body is simply responding to a call for help. The long-term effects of the immune system’s emergency response may pave the way for uglier neurological outcomes—many of which end in dementia.

Other viral infections and neurodegenerative diseases

The connection between common viral infections and the development of neurodegenerative diseases is slowly emerging. But the risk could extend beyond herpes and Alzheimer disease. 

Other viruses that appear to be linked to Alzheimer disease include human immunodeficiency virus Type 1 (HIV-1), periodontitis, and chlamydia. HIV-1 is also loosely connected to ALS and Parkinson disease, which is the second most prevalent neurodegenerative disease (after Alzheimer) and is characterized most notably by motor dysfunction. Additionally, bacteria such as H. pylori, usually located in the gut, are linked to both Alzheimer and Parkinson, researchers have found

But researchers caution that human neurodegenerative disease must be explored holistically with the understanding that environmental, genetic, and socioeconomic conditions influence neurological health. Because of the time it takes for such illnesses to rise—for many, decades—it’s impossible to draw a direct line from, say, HIV-1 to Parkinson. The apparent connection between these conditions may inspire further research.

COVID-19 could cause more complications

The virus behind the most recent global pandemic, SARS-CoV-2, and its accompanying disease (COVID-19) could also threaten the brain. Early in the pandemic, researchers found that 36% of COVID patients suffered from neurologic symptoms, though they didn’t pinpoint a cause. Some patients exhibited Parkinsonian behaviors, including involuntary movement, difficulty maintaining a solid posture or gait, and general motor dysregulation. 

Immunocompromised and aging patients could be at risk of developing neurodegenerative diseases as a result of COVID-19. Only time—and plenty more research—may reveal the true scope of the link.

Until then, physicians can encourage their patients with HSV to get seasonal vaccinations to avoid other viral infections that may catalyze neurodegeneration

Click here to learn more about dementia on MDLinx.

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