Some common germs might plant the seed for dementia later in life. Research out today has found a link between several infectious diseases, particularly herpes simplex-1 (HSV-1), and a greater risk of Alzheimer’s disease.
Scientists at Gilead Sciences conducted the study, published Tuesday in BMJ Open. They analyzed the medical records of Americans with health insurance, finding that people with Alzheimer’s and related conditions were also more likely to be diagnosed with HSV-1. The findings are the latest to suggest that the prevention and treatment of HSV-1 and similar viruses can be an effective measure against dementia.
Alzheimer’s is the most common form of dementia, affecting roughly 7 million Americans today. Most cases are caused by a complex mix of environmental and/or genetic factors. In recent years, many studies have suggested that certain infections—HSV-1 included—can be part of that equation. HSV-1 is the primary cause of oral herpes, commonly known as cold sores, though most infected people will experience few or no symptoms; it can also cause genital herpes (primarily caused by HSV-2).
Not every study has supported this viral link, however, so the authors decided to take a look for themselves, with the help of real-world data. According to lead researcher Luke Liu, this is the first study to use a large database of insurance claims in the U.S. to examine the connection between Alzheimer’s and HSV-1.
Liu and his team compared the health outcomes of nearly 400,000 Americans diagnosed with Alzheimer’s to people without Alzheimer’s who were matched in age, sex, and other factors. Only a small percentage of people with Alzheimer’s had a past HSV-1 diagnosis (a bit under 0.5%), but they still had it more often than controls (about 80% more likely).
The researchers then looked at a broader group of people with other forms of dementia related to Alzheimer’s, finding a similar pattern. They additionally found a link between Alzheimer’s and two other herpesviruses, herpes simplex-2 and varicella zoster virus, the cause of chickenpox and shingles. Interestingly, they also found that people who took antiviral treatment for their HSV-1 were less likely to develop Alzheimer’s than people with HSV-1 who didn’t receive treatment (antivirals can suppress and shorten active outbreaks, though not cure the infection).
Observational studies like this one can’t prove a cause-and-effect link between HSV-1 and Alzheimer’s, but the case for such a connection only appears to be getting stronger.
“Exposure to herpesviruses is likely to be associated with Alzheimer’s disease and other dementia, and antiherpetic medications can be protective against Alzheimer’s and related dementia,” Liu, a virologist at Gilead, told Gizmodo in an email.
There remain important questions about this association, including the actual mechanisms involved. Some research has suggested our brain’s immune system produces amyloid beta and tau to defend against herpesvirus infection, for instance (the misfolded forms of these proteins are the key drivers of Alzheimer’s). But there might be more ways that HSV-1 and similar viruses can set off or accelerate the condition.
Other factors almost certainly play a role, especially since many people contract HSV-1 in their lifetime but never develop Alzheimer’s. Roughly two-thirds of the world’s population under age 50 carries HSV-1, according to the World Health Organization. The researchers note that the HSV-1/Alzheimer’s link is even stronger in older people, and other research suggests that people carrying the ApoE-ε4 allele—a genetic variant that already increases Alzheimer’s risk—also seem more susceptible to HSV-1 infection. So these viruses are likely one of the many dominos that can fall and eventually lead to Alzheimer’s.
Still, the overall evidence does indicate that people can meaningfully lower their dementia risk by treating or preventing certain infections. Several studies have found a link between shingles vaccination and reduced Alzheimer’s, for instance. Clinical trials testing whether antiviral treatments for herpes can lower dementia risk could be warranted, though Liu notes that kind of research will take considerable resources to get off the ground. Research is also ongoing to develop vaccines for HSV-1 and HSV-2—research that might be more worthwhile than currently assumed.
“These findings place an even greater emphasis on viewing the prevention of herpesviruses as a public health priority,” the researchers wrote.
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