Researchers from Tufts University and the University of Oxford have found that two common viruses —the varicella zoster and herpes simplex viruses— could trigger Alzheimer’s disease.

Varicella zoster (VZV) is an extremely common virus causes which causes chickenpox. Once cured of the first infection, the virus tends to linger in peripheral nerves where they remain dormant. When these dormant viruses are reactivated, they cause shingles.

HSV-1, the subtype of the herpes simplex virus, causes both oral and genital herpes. It is a very common infection, affecting nearly 4 million people worldwide under the age of 50 years. The American Sexual Health Organization estimates that around one in two adults has oral herpes in the United States. 

Cytokines are produced in response to VZV. Cytokines are part of a healthy immune system. These small proteins help control the growth and activity of your blood cells and immune cells. Cytokines tell your immune system to do its job. But when too many cytokines are released, it can cause your immune system to go into overdrive, resulting in cytokine storm.

In their findings, published in the Journal of Alzheimer’s Disease, researchers found that when VZV infect neurons, they trigger an inflammatory response due to this overproduction of cytokines. This inflammatory response in turn awakens the herpes simplex viruses which typically lie dormant and harmless in the brain. With both viruses now active, inflammation throughout the brain is aggravated, potentially leading to the formation of plaque and the slow deterioration of neurons—both hallmarks of Alzheimer’s.

The study’s leading author, Dana Cairns, along with her team of collaborators gathered data by using lab grown cultures of brain nerve, or neural, stem cells. They found that infecting neurons with varicella zoster alone was not enough to trigger Alzheimer-like properties. However, when the herpes simplex was already lying-in wait, varicella zoster initiated a series of events that resulted in plaques, tangled fibers and brain damage.

“It’s a one-two punch of two viruses that are very common and usually harmless, but the lab studies suggest that if a new exposure to VZV wakes up dormant HSV-1, they could cause trouble,” explains Cairns. One of her collaborators, Oxford’s Ruth Itzhaki, was one of the first scientists to suggest a link between herpes infections and Alzheimer’s.

The California Institute for Regenerative Medicine (CIRM) has already invested almost $35 million in 21 different Alzheimer’s projects. In addition, we are committed to investing at least $1.5 billion in treatments that target conditions affecting the brain and central nervous system (CNS), including Alzheimer’s.