More than 55 million people around the world are living with dementia, and almost 10 million people receive new diagnoses each year, according to the World Health Organization. Known risk factors for developing dementia (and Alzheimer’s disease, the most common form of dementia) include advanced age and genetic features, such as carrying the APOE gene that instructs the body to make a protein associated with the formation of harmful amyloid plaque in the brain.

Recently, Swedish researchers published a study in the Journal of Alzheimer’s Disease confirming previous work that revealed another possible risk factor: the herpes simplex virus—HSV-1, the type of HSV that commonly causes sores on the mouth, nose, and face.

More than 3.5 billion people worldwide are infected with HSV-1. For decades, scientists have suspected a possible link between HSV-1 and a higher risk of dementia, but the idea has remained controversial. Scientists have previously noted interactions between HSV-1 and the APOE gene associated with amyloid plaque.

Studies have suggested that once HSV-1 enters the body, it contributes to dementia risk by causing a genetic mutation. Medical researchers have also found that drugs used to treat HSV-1 can improve cognition in Alzheimer’s patients.

The Swedish researchers followed 1,002 men and women over 15 years who had not had dementia. All of the study participants were 70 years old when the study began. By the end of the 15-year study period, 107 participants had developed all-cause dementia or Alzheimer’s Disease.

Following the participants, until the day they died or the 15-year follow-up was completed (whichever came first), the researchers found that those who had been infected with HSV-1 at some point in their lives were twice as likely to develop dementia as those who had never been infected. While this repeats and helps confirm the results of previous research, it’s also the first study linking HSV-1 to dementia that looked at people all the same age, eliminating one confounding factor (a wide range of ages) that could negatively affect the validity of the results.

Some studies in the past have failed to find a significant link between herpes and dementia. Other researchers have looked at those studies to see why the results differ from others and have found possible faults.

While this new study helps move related research forward, officially confirming the connection between HSV-1 and dementia will still take time. What scientists have yet to discover is the exact role of the herpes virus in developing dementia and how the virus gets into the brain to begin with.

References

Erika Vestin, Gustaf Boström, Jan Olsson, Fredrik Elgh, Lars Lind, Lena Kilander, Hugo Lövheim, Bodil Weidung. Herpes Simplex Viral Infection Doubles the Risk of Dementia in a Contemporary Cohort of Older Adults: A Prospective Study. Journal of Alzheimer's Disease, 2024; 97 (4): 1841 DOI: 10.3233/JAD-230718