Common viruses may trigger Alzheimer’s disease

Researchers have shown that a virus which commonly causes chickenpox and shingles may activate another common virus, setting in motion the early stages of Alzheimer’s disease.

The researchers at Tufts University and the University of Oxford found varicella zoster virus (VZV) may activate herpes simplex (HSV), starting the process of cognitive decline.

Normally one of the main variants of the HSV virus lies dormant within the neurons of the brain, but when it is activated, it leads to a loss of neuronal function – a signature feature found in patients with Alzheimer’s.

‘Our results suggest one pathway to Alzheimer’s disease, caused by a VZV infection which creates inflammatory triggers that awaken HSV in the brain,’ said Dana Cairns, GBS12, a research associate in the Biomedical Engineering Department. ‘While we demonstrated a link between VZV and HSV-1 activation, it’s possible that other inflammatory events in the brain could also awaken HSV-1 and lead to Alzheimer’s disease.’

‘We have been working off a lot of established evidence that HSV has been linked to increased risk of Alzheimer’s disease in patients,’ said David Kaplan, Stern Family Professor of Engineering and chair of the Department of Biomedical Engineering at Tufts’ School of Engineering.

‘We know there is a correlation between HSV-1 and Alzheimer’s disease, and some suggested involvement of VZV, but what we didn’t know is the sequence of events that the viruses create to set the disease in motion,’ he said. ‘We think we now have evidence of those events.’

According to the World Health Organization, an estimated 3.7 billion people under the age of 50 have been infected with HSV-1—the virus that causes oral herpes. In most cases it is asymptomatic, lying dormant within nerve cells.

When activated, it can cause inflammation in nerves and skin, causing painful open sores and blisters. Most carriers—and that’s one in two Americans according to the CDC—will have between very mild to no symptoms before the virus becomes dormant.

Varicella zoster virus is also extremely common, with about 95 percent of people having been infected before the age of 20. Many of those cases are expressed as chicken pox. VZV, which is a form of herpes virus, can also remain in the body, finding its way to nerve cells before then becoming dormant.

Later in life, VZV can be reactivated to cause shingles, a disease characterized by blisters and nodules in the skin that form in a band-like pattern and can be very painful, lasting for weeks or even months. One in three people will eventually develop a case of shingles in their lifetime.

The link between HSV-1 and Alzheimer’s disease only occurs when HSV-1 has been reactivated to cause sores, blisters, and other painful inflammatory conditions.