Common viral infection can cause Alzheimer's with nearly half of cases blamed on it, new report suggests

The virus in question, cytomegalovirus (HCMV), is one of nine herpes viruses, though it is not sexually transmitted.

While HCMV typically remains dormant in the gut after exposure during early life, researchers believe it can sometimes become active and make its way to the brain via the vagus nerve.

Once in the brain, the virus appears to trigger immune responses that may contribute to the formation of characteristic Alzheimer's plaques and tangles.

Post-mortem examinations revealed higher levels of immune cells linked to inflammation in the brains of Alzheimer's patients compared to those without the disease.

The virus was detected in patients' intestines, whilst their spinal fluid contained HCMV antibodies.

Researchers found HCMV present within the vagus nerve, suggesting this serves as the pathway for the virus to reach the brain.

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About 80 per cent of people show evidence of HCMV antibodies by age 80, though the virus typically remains inactive in healthy individuals.

Scientists are now developing a blood test to identify individuals with this type of chronic intestinal infection, hoping to prevent Alzheimer's through antiviral treatments.

Dr Ben Readhead, from Arizona State University, said: "We think we found a biologically unique subtype of Alzheimer's that may affect 25 to 45 per cent of people with this disease."

He added that this subtype includes the characteristic amyloid plaques and tau tangles, along with a distinct profile of viruses, antibodies and immune cells in the brain.

Dr Sheona Scales, research director at Alzheimer's Research UK, said: "This study adds to our growing understanding of the important role our immune system plays in the development of Alzheimer's."

She noted that while it's too early to draw definitive conclusions, the research "opens new possibilities for future treatments and ways to diagnose the disease."

Britain currently has an estimated 944,000 people living with dementia, with Alzheimer's being the most common form.

This number is projected to exceed one million by 2030, with one in three people born in the UK today expected to develop dementia in their lifetime.

Prof Will McEwan from the UK Dementia Research Institute said: "There has been a long-standing suspicion that viral infections may increase the risk of Alzheimer's disease... The finding of infection in the brain is novel and highly deserving of follow-up research."

Dr Richard Oakley from Alzheimer's Society cautioned that while the research found a link between immune cells and HCMV, "it only established a connection however, and did not directly show that the virus leads to Alzheimer's disease in people".