Cold sore virus linked to Alzheimer’s disease in large study

Symptomatic infection with cold sore virus herpes simplex 1 may be implicated in the development of Alzheimer’s disease, researchers have reported.

A study published in BMJ Open also found that treatment with antiviral therapy appears to be associated with a lower risk of the condition.

The research done on 700,000 patients from US database of health insurance claims found the likelihood of an HSV-1 diagnosis was 80% higher among those with Alzheimer’s disease, after adjusting for potentially influential factors. 

In all just under 0.5% people with Alzheimer’s disease had been diagnosed with HSV-1 (0.44%) compared with 823 (just under 0.25%) of those in the comparison group.

The analysis also found that among the 2330 people with a history of HSV-1 infection, 931 (40%) used antiherpetic medication after their diagnosis and they were 17% less likely to develop Alzheimer’s disease than those who did not use these treatments.

The researchers also looked at other herpes viruses and found HSV-2 and varicella zoster virus infections were also associated with an increased risk of Alzheimer’s disease.

Last month researchers reported the ‘strongest evidence yet’ that shingles vaccination reduces the risk of dementia, using data from Wales.

It is not the first study to suggest a link between infections – including with HSV-1 – and Alzheimer’s disease but it is not clear how the viruses may heighten the risk.

But studies have shown that inflammatory alterations in the brain caused by HSV infection are pivotal in [Alzheimer’s disease] development, the team from the University of Washington and Gilead Science reported. 

HSV-1 DNA is also found in the plaques associated with Alzheimer’s disease, and people carrying the ApoE ε4 allele, the most common genetic risk factor for the disease, are more susceptible to HSV infections, they added.

‘While the molecular mechanisms remain to be fully elucidated, these results are indicative of a possible role for antiherpetic therapy in mitigating dementia risk,’ they concluded.

Dr Sheona Scales, director of research at Alzheimer’s Research UK, said there was an increasing amount of evidence that the body’s response to certain viruses could increase the risk of developing Alzheimer’s disease in later life. 

‘These recent findings from a large study using US health records propose that infection with HSV-1 – a common virus that causes cold sores – may be associated with an increased risk of Alzheimer’s disease.

‘The researchers also state that taking medicines to treat HSV-1 infections could reduce the risk, but this is still very early work and needs more investigation.’  

She pointed out that people infected with HSV-1 don’t have any symptoms so some infections might not have been recorded and although cases were matched with controls, diagnosing Alzheimer’s disease, especially in the early stages, remains a challenge.    

‘We know there are 14 established risk factors for dementia, and there’s not enough evidence to include infections in this list. 

‘This study doesn’t tell us if infections are causing the risk, it only shows an association. Further research is needed to understand what the underlying biology around this is.’