Can Herpes Cause Dementia?

Debating the Link Between Cold Sores and Alzheimer's

Girl with Cold Sore
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Herpes dementia is a controversial concept in which the herpes virus may directly or indirectly cause the profound and irreversible loss of cognitive function.

While contentious, there is some evidence that a prior history of herpes may increase the risk of Alzheimer's disease. The risk is primarily related to the herpes simplex virus 1 (HSV-1) and less so to herpes simplex virus 2 (HSV-2).

About Herpes and the Brain

HSV-1 is the virus primarily responsible for oral herpes (cold sores), while HSV-2 is the cause of most cases of genital herpes.

In rare instances, HSV-1 can affect more than just the skin and peripheral nerves and move to the central nervous system where it can trigger the inflammation of the brain, known as infectious encephalitis. Symptoms of this include:

  • Fever
  • Headaches
  • Weakness
  • Vomiting
  • Changes in behavior
  • Memory loss
  • Seizures
  • Partial paralysis or isolated areas of paralysis

Herpes encephalitis is a concern among newborns who had been infected with HSV while in the womb. Also at risk are individuals whose immune system has been severely compromised, such as those with advanced HIV infection.

In adults, HSV encephalitis can affect the frontal and temporal lobes of the brain and manifest with bizarre behavior and gustatory hallucinations (unreal taste sensations). If it affects the hippocampus and causes localized brain cell death (necrosis), it can, in rare cases, cause dementia.

Herpes and Alzheimer's Disease

One of the associations that have gotten a lot of attention lately is the hypothetical link between HSV-1 and Alzheimer's disease.

In this instance, HSV is not believed to be the direct cause of dementia but rather a culprit.

A recent study coordinated by the National Research Council in Italy has suggested that the virus may effectively "turn on" a genetic switch that triggers the eventual development of Alzheimer's.

Alzheimer's, a progressive disease characterized by advancing dementia, is caused by the build-up of lesions (plaques) on the brain which prevent nerve cells from communicating with each other, leading to memory loss and cognitive decline.

It has long been believed that either a viral or bacterial agent may instigate this process, with HSV-1 being one of the prime suspects.

In their research, the scientists stated the HSV-1, which can lie dormant on nerve cells for decades, may suddenly reactivate in that brain as a person get older (in the same way that it can reactivate on the lip and cause a cold sore). It is possible that such a reactivation may somehow trigger the production of beta-amyloid, a type of protein which causes nerve cells to stick together and form plaques.

The researchers also point to a specific gene mutation, known as APOE-4, which is not only linked to Alzheimer's but may potentiate the harmful effects of HSV.

If proven correct, scientists may be able to develop an antimicrobial agent able to block this process before it starts. While the jury is still out as to actual role HSV plays, further research may help clarify the interrelationship, if any, between infections and unexplained brain disorders like Alzheimer's and Parkinson's disease.

Sources:

Honjo, K.; van Reekum, R., and Verhoeff, N. "Alzheimer's disease and infection: do infectious agents contribute to "progression of Alzheimer's disease?" Alzheimers Dement. 2009; 5(4):348-60. DOI: 10.1016/j.jalz.2008.12.001.

Itzhaki, R. "Herpes simplex virus type 1 and Alzheimer's disease: possible mechanisms and signposts." FASEB J. 2017; 31(8):3216-26. DOI: 10.1096/fj.201700360.

Piacentini, R.; De Chiara, G.; Li Puma, D. et al. "HSV-1 and Alzheimer’s disease: more than a hypothesis." Front Pharmacol. 2014; 5:97. DOI: 10.3389/fphar.2014.00097.