Once thought to be an inevitable part of natural aging for many people, researchers are now discovering that in many cases, dementia is preventable.
Interventions known to reduce the risk of dementia include exercising regularly, eating a healthy diet, quitting smoking and drinking alcohol (or drinking some alcohol in moderation), maintaining healthy blood pressure, and treating other underlying conditions.
Now a new study has revealed another factor that may affect dementia risk: being infected with two forms of a very common virus at the same time.
Herpes can cause dementia
Researchers of the tufts university and the Oxford University published a preview this month in the Journal of Alzheimer’s Disease. Using a three-dimensional culture model of human brain tissue, they showed that herpes simplex virus (HSV) can, under certain conditions, cause Alzheimer’s disease, the most common form of dementia.
Specifically, they identified HSV-1, the type of herpes simplex that causes cold soresas responsible for initiating some cases of dementia.
However, the HSV-1 virus does not act alone. The researchers say that people infected with herpes simplex were more likely to develop Alzheimer’s disease only when they were also infected with varicella-zoster virus (VZV), a second herpes virus that commonly causes chickenpox and shingles. The researchers say that the presence of the second virus can “activate” herpes simplex and start the early stages of Alzheimer’s disease.
How does all this happen?
Although researchers have explored the link between HSV-1 and Alzheimer’s for decades, this new study describes the sequence of events that viruses create to initiate Alzheimer’s.
«Our results indicate a pathway to Alzheimer’s disease caused by VZV infection. The virus creates inflammatory triggers, which arouse HSV in the brain said Dr. Dana Cairns, PhD, a Tufts investigator and study co-author. «Although we have shown a link between VZV and HSV-1 activation, it is possible that other inflammatory events in the brain may also arouse HSV-1 and lead to Alzheimer’s disease“, he added.
The study authors add that HSV is usually in a dormant (inactive) state in the brain: but when activated it leads to accumulation of T-proteins and beta-amyloid and loss of neuronal function, traits characteristic features found in the brains of Alzheimer’s patients.
These viruses are extremely common
If it seems unlikely that one person would get both viruses, let’s take a look at infection rates. The World Health Organization (WHO) estimates that 3.7 billion people under the age of 50 (67% of people in this age group) are infected with the herpes simplex virus (HSV-1), but most of these cases are asymptomatic. Varicella-zoster virus (VZV) is also common.
The authors note that 95% of people are infected with VZV before the age of 20, leading to chickenpox or shingles. Like HSV-1, VZV can also remain inactive, in this case in nerve cells.
However, it is important to note that this combination of viruses only causes dementia if a specific inflammatory response is triggered in the body. «The link between HSV-1 and Alzheimer’s appears only when HSV-1 has reactivated and causes sores, blisters and other painful inflammatory conditions», the authors of the study point out.