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New research has suggested an unexpected link between a common form of herpes and Alzheimer’s disease, indicating anti-viral treatments might offer a potential preventative measure.
Previous studies have shown that herpes simplex virus type 1 (HSV-1) can remain dormant within human cells, only to reactivate later in life, potentially triggering dementia-like symptoms.
Experts have observed that HSV-1 induces changes in the brain that mirror those seen in Alzheimer's patients, including the formation of amyloid plaque-like structures and inflammation.
A large-scale US study involving researchers from pharmaceutical company Gilead Sciences and the University of Washington in Seattle has now further explored this connection.
Analysing data from 344,628 individuals with Alzheimer's and an equal number without the disease, all over the age of 50, the researchers investigated the relationship between HSV-1 and Alzheimer's diagnoses between 2006 and 2021.
The study revealed that individuals with a history of HSV-1 infection had an 80 per cent increased risk of developing Alzheimer's, even after accounting for other contributing factors.
Of the participants with Alzheimer's, 0.44 per cent had a prior HSV-1 diagnosis, compared to 0.24 per cent of those without Alzheimer's. The study group comprised approximately 65 per cent women, with an average age of 73.
Crucially, the research also found that individuals with HSV-1 who used antiviral medications to treat the virus were 17 per cent less likely to develop Alzheimer's compared to those who did not receive antiviral treatment.
Among the 2,330 individuals with a history of HSV-1 infection, 40 per cent used antivirals following their diagnosis. These findings suggest that managing HSV-1 with antiviral medications could potentially mitigate the risk of developing Alzheimer's disease.
The authors, writing in the journal BMJ Open, concluded: “Findings from this large…study implicate HSV-1 in the development of Alzheimer’s disease and highlight anti-herpetic therapies as potentially protective for Alzheimer’s and related dementia.”
In the UK, the drug aciclovir is one of those available for treating cold sores, chickenpox, shingles and other herpes virus infections.
The researchers also looked at the potential role of other herpes viruses, including HSV-2, varicella zoster virus (which causes chickenpox), and cytomegalovirus.
Both HSV-2 and varicella zoster virus infections were also associated with a heightened risk of Alzheimer’s disease.
Exactly how HSV-1 and other viruses might heighten the risk of dementia is not clear, point out the researchers.
“However, studies have shown that inflammatory alterations in the brain caused by HSV infection are pivotal in (Alzheimer’s disease) development,” they added.
HSV-1 DNA is also found in the plaques characteristic of Alzheimer’s disease, and people carrying the most common genetic risk factor for the disease are more susceptible to HSV infections, they said.
Professor Tara Spires-Jones, from the University of Edinburgh, said: “This is a well-conducted study adding to strong data in the field linking HSV-1 and other viral infections to increased risk of developing Alzheimer’s disease, but it is important to note that HSV-1 infection, which is extremely common in the population, is by no means a guarantee that someone will develop Alzheimer’s.
“Why viral infections may increase risk of dementia is not fully understood, but the most likely explanation is that infections increase inflammation in the body and contribute to age-related brain inflammation.
“More research is needed to understand the best way to protect our brains from Alzheimer’s disease as we age, including a better understanding of links between viral infection and Alzheimer’s risk.”
Dr David Vickers, from the University of Calgary in Canada, said the “research exaggerates the role of HSV-1, failing to appreciate its absence in 99.56 per cent of Alzheimer’s disease cases”.
He added: “The observed 17 per cent hazard reduction with anti-herpetic drugs translates to a mere nine-month delay in Alzheimer’s disease onset.”
Dr Richard Oakley, director of research and innovation at the Alzheimer’s Society, said: “Results from this observational study suggested that people with recorded cold sore infections were more likely to develop Alzheimer’s disease, and interestingly those prescribed antiviral drugs had a slightly lower risk.
“But this doesn’t prove that cold sores cause Alzheimer’s disease, or that anti-virals prevent it.
“The data came from insurance records, often based on self-reported symptoms which may miss or misclassify infections, and didn’t track how often people had cold sores or how consistently they took medication.
“Much more research is needed to explore exactly how viruses might be involved and before we can draw firm conclusions.”
English (US) ·