Infection with the herpes simplex virus may boost the likelihood of developing Alzheimer's, researchers suggest.
The new paper, published in the Frontiers in Ageing Neuroscience journal, also suggests that antiviral drugs may reduce the risk of senile dementia — which is mostly caused by Alzheimer's disease — among people who have severe cases of herpes.
Herpes simplex virus 1 (HSV1) is the type of herpes that results in cold sores.
HSV1 is a common virus, and the majority of people will have contracted it by the time they reach old age.
However, the virus remains permanently in the body and cannot be decisively removed either by the body's natural defense mechanisms or by drugs.
The virus is inactive most of the time, but when a person has HSV1, they may find that flare-ups occur when they are stressed or sick, resulting in characteristic blisters.
Medical News Today have reported on several studies this year alone that have provided evidence of a connection between Alzheimer's and herpes.
In June, we looked at a study in which postmortem tests on brain tissue support a mechanistic link between Alzheimer's and the herpes viruses HHV-6A and HHV-7.
And in July, we brought you news on a study that found the use of antiherpetic medication may dramatically reduce dementia risk.
'Strong evidence' links herpes with dementia
Study author Professor Ruth Itzhaki, from the University of Manchester in the United Kingdom, found in previous studies that cold sores caused by HSV1 are more prevalent among people that carry a gene variant called APOE-e4, which may increase a person's risk of developing Alzheimer's.
"HSV1 could account for 50 percent or more of Alzheimer's disease cases," she states.
"Our theory is that in APOE-e4 carriers, reactivation is more frequent or more harmful in HSV1-infected brain cells, which as a result accumulate damage that culminates in development of Alzheimer's."
Prof. Ruth Itzhaki
For this review, she looked at three recent studies on the relationship between Alzheimer's and herpes or chickenpox that analyzed population data from Taiwan, a country which enrolls almost all citizens in the National Health Insurance Research Database.
Because this database is so comprehensive in collecting health data from the Taiwanese population, it has become an important resource for researchers who are interested in examining the relationships between microbial infections and diseases.
Itzhaki says that these studies provide "striking" evidence that people who have herpes viruses are at a much higher risk of developing senile dementia.
Also, the results of these studies suggest that the use of antiviral medication may result in "a dramatic decrease" in dementia risk.
'Findings justify use' of herpes drugs
However, researchers need to conduct further studies to confirm that HSV1 causes Alzheimer's. So far, these studies can only show that there is a relationship between the two conditions.
Despite this, Itzhaki believes that the data so far supports the case of using antiviral drugs to deter Alzheimer's.
"Considering that over 150 publications strongly support an HSV1 role in Alzheimer's," she says, "these Taiwan findings greatly justify usage of antiherpes antivirals — which are safe and well-tolerated — to treat Alzheimer's disease."
"They also incentivize the development of an HSV1 vaccine, which would likely be the most effective treatment," the researcher adds.
Itzhaki says she would like to now study dementia rates among people who have mild HSV1 or mild genital herpes because the findings from the studies based on the Taiwanese data only examine the link between dementia and severe HSV1 and chickenpox infections.