Possible link found between Alzheimer's disease and oral infections

Physicians and researchers all over the world are keen to discover a root cause for Alzheimer’s disease.

Nearly 44 million people worldwide have been diagnosed with Alzheimer’s and that number is expected to grow exponentially as the global population ages. More than 747,000 Canadians are currently living with Alzheimer’s disease, about the same number who will die of Alzheimer’s each year in the United States.

The growing elderly population has also seen some substantial changes to dental health, too. Severe systemic fungal infections in the mouth have increased dramatically for seniors over the last three decades.

Now a link may have been found between oral infections and Alzheimer’s disease.

University of Oslo professor Ingar Olsen has spent his career studying how oral micro-organisms invade local tissue. Their invasion wrecks havoc systemically as they spread.

Working with another senior researcher, Sim K. Singhrao at the University of Lancashire’s Oral & Dental Sciences Research, the team published a paper called "Can oral infection be a risk factor for Alzheimer’s disease" in the international peer reviewed Journal of Oral Microbiology.

Their conclusion: there is a possible link between late-onset Alzheimer’s disease and oral infection.

Doctors and researchers have known for years that one of the characteristic features of Alzheimer’s was inflammation of the brain. More recently, scientists such as Singhrao and Olsen have been looking for potential root causes for this characteristic and are now zeroing in on what’s known as “peripheral infections” such as those that originate in the mouth.

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The mouth is one of the most bacteria-ridden places in our bodies, for obvious reasons. Lots of foreign stuff goes in, mixing with other foreign stuff.

The oral cavity harbours a complex microflora in which bacteria predominate, according to Olsen. Only half of them have so far been cultivated.

“We have found that many of the uncultivable organisms play important roles in oral diseases. It is also important that we know which organisms in the mouth are compatible with health.”

It’s been known that bacteria in the mouth, particularly in people with gum disease, can find its way into the host bloodstream. Dr. Olsen said if these bacteria can pass the blood brain barrier – any number of them could be implicated in the Alzheimer’s enigma.

Alzheimer’s disease, despite being one of the top ten causes of death, is the only one that is not preventable and there is no cure for the disease, nor way to slow it down.

Olsen focused his research—after poring over hundreds of other reviews—on some of the lesser-known bacteria species that comprise the oral microbiome.

“So much of the research to date has been focused on a couple of groups of bacteria – namely spirochetes and Porphyromonas gingivalis,” said Olsen, “when there are well over 900 different bacteria in the oral cavity.”

Porphyromonas gingivalis, for example, has recently been linked to heart disease.

With the other bacteria still so mysterious, Olsen said he was convinced there was more to the story.

Half of the world’s human population has a typically harmless fungus in their mouth called Candida. But if Candida enters the bloodstream, it can lead to infection. Another virus, herpes simplex, is found in 70 per cent of the population over the age of 50. It persists latently in the peripheral nervous system and is periodically reactivated in the brain.

“Even oral Candida and herpes virus could possibly cause the inflammation in the brain that we see in Alzheimer’s patients,” said Olsen.

In the paper, Olsen and Singhrao wrote: “With a growing population of elderly, severe systemic fungal infections have increased dramatically in this age group during the last 30 years. Oral yeasts can be found in periodontal pockets, in root canals, on the mucosae and underneath dentures.”

The most likely pathway for dissemination of oral micro-organisms to the brain is through the blood stream.

Brushing, flossing, chewing and use of toothpicks in a patient with periodontitis will release a bacteremia which can occur several times during the day and has been estimated to last for up to 3 hours for oral bacteria .

The bacteremia is usually contained by immune cells of the body. However, in people with reduced immune defense, in particular seniors—the most likely candidates for Alzheimer’s--bacteria may localize to crevices of the oral cavity and vascular channels.

Increased risk of dementia in the elderly following multiple infectious episodes has been reported.

Dr. Olsen, a dentist, said researchers working in this area are keen to find a diagnostic biomarker for Alzheimer’s disease. Currently there is no biomarker and Olsen said the answers are still being sought and where future research may lead.