Common Childhood Virus Linked to Alzheimer's Development in Old Age

Yahoo

10-06-2025

A study published 30 years ago is striking up new conversation about the potential connection between the common cold sore and Alzheimer's disease. Published in The Lancet in 1997, researchers compared DNA extracted from the brains of elderly people with and without Alzheimer's, and in some of them, found herpes simplex virus 1 (HSV-1)—an infection often contracted in childhood—dormant in the nerves. Ultimately, researchers predicted that those with HSV-1 and a specific gene, called the ApoE-e4 gene, had a higher risk of developing Alzheimer's.
Meet the Experts: David Hunter, M.D., an assistant professor and neurologist with UTHealth Houston, Linda Yancey, M.D., an infectious disease doctor at Memorial Hermann Health System; and Mark Santos, Ph.D., an assistant professor of microbiology and immunology at Touro University Nevada.
Study co-author Ruth Itzhaki, Ph.D., summarized her team's findings in a recent article for The Conversation and noted that, over the years, additional work has surfaced to support hers.
But what does it all mean, exactly? If a cold sore pops up, should you be concerned about dementia? We reached out to experts who explained everything.
'HSV-1 is a common virus to which almost everyone is exposed early in life,' says David Hunter, M.D., an assistant professor and neurologist with UTHealth Houston. 'It is best known for causing cold sores and is closely related to chicken pox and mononucleosis.'
What these viruses have in common is that they are neurotropic, meaning they linger in the nervous system long after the initial illness, explains Linda Yancey, M.D., an infectious disease doctor at Memorial Hermann Health System, and can be reactivated by stress or lowered immunity. That's how, for instance, chicken pox can manifest later in life as shingles, adds Dr. Hunter.
The virus's ability to infiltrate the nerves is believed to get it past the blood-brain barrier, a membrane that shields the organ from infection via the bloodstream. Thanks to that entry point, Dr. Hunter says, HSV-1 is the most common cause of viral brain inflammation in the United States.
'There is no vaccine for HSV and almost everyone will get it by adulthood,' Dr. Hunter adds. Cold sores are a mild case of it, and symptoms can be treated with antiviral drugs.
Dormant HSV-1 may be reactivated by illness and reach the brain, explains Mark Santos, Ph.D., an assistant professor of microbiology and immunology at Touro University Nevada. This can trigger brain inflammation over time, known as herpes encephalitis,and in people with genetic risk factors, like the ApoE-4 gene, contribute to Alzheimer's development, he adds. 'We don't entirely know why ApoE-4 is a risk factor for Alzheimer's,' adds Dr. Hunter. 'The link to HSV could be that ApoE-4 impacts the blood-brain barrier and makes it easier for HSV to enter.' And the inflammation caused by that entry may heighten Alzheimer's risk. 'It is now universally accepted that inflammation in the brain is a key step in the development of Alzheimer's,' Dr. Hunter reiterates.
In the three decades since the original study, doctors and researchers have learned much more about Alzheimer's disease and its connection to viral inflammation. For instance, several studies have found that being immunized for any disease (like COVID-19 or the flu) reduces Alzheimer's risk, says Dr. Hunter. 'Several other studies have shown that reducing inflammation in the brain helps prevent Alzheimer's,' he adds.
Santos concurs. 'Since 1996, additional studies have found HSV-1 DNA in the brains of individuals with Alzheimer's. Lab studies have also shown that HSV-1 infection can trigger the buildup of amyloid-beta and tau—proteins strongly associated with Alzheimer's pathology,' he explains.
However, there are a few disclaimers to note. Firstly, the posed risk is associated with people who contract HSV-1 in early life, giving it time to progressively cause inflammation. The risk may be lower in adults, says Santos. 'That said, if someone has other risk factors (like a weakened immune system or APOE-e4), adult-onset HSV-1 could still have long-term consequences,' he adds.
Still, neurotropic viruses are extremely common, Dr. Yancey reiterates. 'This makes it difficult to tease out whether they are causing neurologic problems or are simply common in the general population, including people with neurologic problems,' she adds. In other words, because most people end up with HSV-1 doesn't necessarily mean that most people will get Alzheimer's.
'Most cold sores aren't a cause for concern,' Santos adds. 'They're common and usually well-controlled by the immune system. But for people with frequent outbreaks, weakened immunity, or APOE-e4, it may be worth discussing with a healthcare provider. The concern is not a single cold sore, but long-term viral activity in the brain.'
More research, including long-term studies, is needed to make any solid associations, Santos and Dr. Yancey conclude. 'The current data is based on population-level surveys. There is no way to predict on an individual patient's level whether this has a meaningful impact,' says Dr. Yancey.
Future research will, hopefully, at least help experts understand how antiviral treatment affects Alzheimer's risk, as well as identify better ways to detect when HSV-1 is active in the brain. 'The hope is that understanding how infections contribute to Alzheimer's will open up new ways to prevent or treat it—especially before symptoms start,' says Santos.
Being vaccinated, exercising, engaging in mental stimulation, and maintaining a heart-healthy diet are all known ways to lower dementia risk, our experts say. Avoiding smoking or nicotine use of any kind can also 'drastically' reduce the risk, says Dr. Yancey. All of these habits and choices work together to support both immune and brain health.
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