Last year, a review paper in the Journal of the American Geriatrics Society, concluded that poor oral health is associated with brain degeneration over time. Like herpes, Porphyromonas gingivalis bacteria have also been identified in amyloid plaques inside the brains of Alzheimer’s patients.

Tomas Welsh, research and medical director at the Rice Institute in Bath, says that some bacteria can secrete enzymes that may be capable of helping them burrow into the brain.

“These bugs can work their way into the bloodstream, get to the brain where they start an inflammatory response, and the brain starts to develop some amyloid and tau tangles as part of that,” he says.

There are even suggestions that some gut infections such as C diff, a germ that causes diarrhoea and colitis, may be linked to Alzheimer’s. Yvonne Nolan, a professor in anatomy and neuroscience at University College Cork, is particularly interested in how shifts in the microbiome mediated by these infections could contribute to cognitive decline.

“Changes in the gut microbiome have been repeatedly shown to influence and regulate cognitive and memory behaviours,” she says. “We found that transferring gut microbiota from Alzheimer’s patients in a process called faecal microbiota transplantation to young rats caused the rats to perform poorly in certain memory tests.”

What can we do to reduce our risk of these infections and the issues they cause?

While there is no vaccine for HSV-1 or HSV-2, researchers believe that the emerging evidence linking pathogens with cognitive decline points to the importance of other midlife vaccinations, for example the shingles vaccine.

Research has indicated that the varicella zoster virus which causes shingles may be capable of reactivating oral herpes from a dormant state, and one study found that people who had received the jab were less at risk of Alzheimer’s.

According to Welsh, the evidence linking gum disease and brain health points to the importance of regular dental appointments and good oral hygiene to prevent the build-up of plaques which act as a home for the bacteria.

“It’s been established for many years now that poor dental health and more gum disease, means that your risk of Alzheimer’s progressing is much higher,” he says.

Nolan also points to simple lifestyle measures such as exercise and consuming a high fibre diet to ensure your gut is nourished with the right fuel to fight off potentially harmful pathogens.

“Given that it is becoming increasingly recognised that Alzheimer’s is substantially influenced by lifestyle and environmental factors, changing the composition of gut microbiota through diet or other lifestyle factors like exercise, or by administering substances like prebiotics, may help,” she says.

Could this lead to new treatments?

Perhaps most excitingly, these findings could point to an entirely different way of treating the disease. In a paper published in the Journal of the Alzheimer’s Association earlier this year, Schultek highlighted case reports of various dementias including Alzheimer’s which appeared to be caused by certain microbes.

“They were quite varied – bacteria, viruses, fungi, parasites and infections with more than one germ found in spinal fluid,” she says. “Some of these people responded to tailored antimicrobial treatment, restoring cognitive function.”

Dr Devanand is now looking to study this more rigorously through an ongoing clinical trial which looks to see whether a herpes antiviral drug called valaciclovir can slow down Alzheimer’s progression in patients in the early stages of the disease. If it demonstrates any benefit, this could even lead to herpes antivirals being offered to people in midlife or initiate new efforts to develop effective herpes vaccines.

Other researchers are looking at new ways of targeting harmful gum bacteria, and Welsh points to San Francisco-based biotech Quince Therapeutics which has developed a drug capable of blocking the action of some of the enzymes secreted by these bacteria. But while the therapy worked successfully in mice that had been engineered to show characteristics of Alzheimer’s, it failed when trialled in humans.

“I believe they’re developing some new molecules to try to pursue this hypothesis further,” says Welsh. “So I think this is still an exciting and cutting edge area of research. Our centre is keen to be involved in further trials and we’ve been having some conversations with the Bristol dental school which is interested in this area. It would be fantastic if we can find another solution to this disease.”