Can the Herpes Virus Cause Alzheimer’s Disease?

Researchers are trying to determine the exact link between herpes and dementia.

Source: Guilherme Gomes/Pixabay, used with permission

Most research on Alzheimer’s disease looks at the abnormal accumulation of B-amyloid and tau proteins that form amyloid plaque and neurofibrillary tangles in the brain. These protein clumps decrease neuroplasticity, or the brain’s ability to grow, adjust, and adapt to change. They can also cause the death of brain cells that leads to symptoms of neurodegenerative disease, such as memory loss and poor judgment.

Herpes May Play a Role in Alzheimer's

Researchers also know that other factors play a role in the development of the condition. Some have proposed that the progressive brain deterioration of Alzheimer’s disease is somehow associated with the inflammatory immune response in the body that results from chronic herpes simplex virus 1 (HSV-1). In other words, though the idea remains somewhat controversial, many scientists now believe that infection with HSV-1 increases the risk of developing Alzheimer’s disease and related dementia.

The exact cause and mechanism of that link, however, has yet to be determined. It is known that people with HSV-1 are more than twice as likely to develop dementia than those who don’t have HSV. It is also known that HSV-1 can live and lie dormant inside the neurons (brain cells) of people with and without Alzheimer’s disease. A recent collaborative review performed by scientists in the United States and China suggests that once HSV-1 infects the body, it contributes to the risk of developing Alzheimer’s by means of genetic mutation. At the same time, another study found that valacyclovir, a drug used to treat HSV-1, can improve cognition in Alzheimer’s patients.

The World Health Organization (WHO) estimates some 66 percent of the world’s population aged 49 and under are infected with HSV-1. That’s approximately 3.7 billion people. After initial infection, the virus lies dormant in the body throughout one’s life, surfacing at times in the form of oral lesions, causing itchy and sometimes painful blisters and sores on the mouth or other parts of the face. HSV-1 can also be transmitted as genital herpes via oral sex. Many people who are infected with HSV-1 remain asymptomatic throughout their lives, and according to the WHO, most people who harbor the virus don’t even know they’ve been infected.

The Link Is Confirmed but Not the Details

In the 1990s, scientists studied people who developed cold sores and began to speculate that HSV-1 in the brain, in combination with the inherited presence of the APOEe4 gene—the gene that instructs the body how to make the specific protein found in amyloid plaque—significantly increases the risk of developing Alzheimer’s disease. Since then, many studies have confirmed a link between HSV-1 and Alzheimer’s disease, but details as to the exact role of HSV-1 and how it gets into the brain to begin with have remained elusive. Not everyone with HSV-1 develops Alzheimer’s disease, and a question remains as to whether HSV-1 is a cause of or a reaction to the development of amyloid plaque and subsequent changes in the brain.