Can common infections trigger Alzheimer’s disease?

Much is still unknown when it comes to Alzheimer’s disease. One thing we do know is that it is the leading cause of dementia in developing countries.

Baycrest’s Kunin-Lunenfeld Applied Research Unit (KLARU) led a major review of published studies in order to determine whether certain types of infections or viruses may trigger Alzheimer’s disease later in life. The findings were recently published by Drs. Kie Honjo, Robert van Reekum (former KLARU researcher) and Paul Verhoeff.

“The cause of Alzheimer’s is probably multi-faceted.We are looking at several factors, which when combined, may put someone at higher risk of developing Alzheimer’s disease. What we have found is that certain viruses exist in the brains of people with Alzheimer’s. Research teams are therefore looking a little closer at this,” said senior author Dr. Verhoeff, clinician-scientist at KLARU and staff psychiatrist at Baycrest’s Brain Health Centre.

One infection that scientists are looking at is herpes simplex virus type 1 (HSV-1) – which is the virus behind cold sores. This infection is usually latent but certain triggers may cause it to reactivate sporadically. In the last two decades many scientists have found the HSV-1 genome in the brains of patients with Alzheimer’s as well as in normal older adults.

“What is interesting about this finding is that the virus only affected a very small number of young peoples’ brains, indicating that perhaps the virus can enter the brain when an individual becomes older,” explains Dr. Verhoeff. “Perhaps this is because of a decline in the immune system.”

“Keep in mind that HSV-1 may be a risk factor for developing Alzheimer’s but HSV-1 alone does not cause Alzheimer’s disease directly,” emphasizes Dr. Verhoeff. “Although it is not an independent risk factor, it might be a part of what causes Alzheimer’s by inducing inflammation in those areas of the brain that are vulnerable to Alzheimer’s, when it combines with other risk factors.”

Another infection that scientists are looking at is Chlamydia pneumoniae (C. pneumoniae) which causes acute respiratory infection and is associated with coronary artery disease and an increased risk of ischemic stroke. One study showed the presence of these bacteria in 89% of brains of people with Alzheimer’s disease, while 95% of non-Alzheimer’s patients were negative of the bacteria.

Although various infections can affect the human brain, we know that aging increases the risk of infection as well as the incidence of Alzheimer’s disease. Alzheimer’s disease may arise in older adults in part because of increased rates of infection or perhaps older adult brains react pathologically to infections.

“More research is clearly required,” says Dr. Verhoeff. “However the evidence provides hints about how some infections may precipitate or accelerate Alzheimer’s.

“The main findings indicate that no specific infectious agents cause Alzheimer’s directly, although a combination of infection with genetic background or some other risk factors may contribute to it. Therefore the timely treatment of infections may become an important part of Alzheimer’s prevention and therapy.”

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