A type of bacteria found in severe gum disease may trigger many cases of rheumatoid arthritis, researchers reported Wednesday.
The bacteria is found in about a third of people, and the researchers found evidence that 47 percent of patients with rheumatoid arthritis had evidence of an infection.
It's called Aggregatibacter actinomycetemcomitans, or "A a" for short.
The bacteria damages immune system cells called neutrophils in a way that seems to enrage the immune system, which then attacks them. It's the same process that appears to trigger the inflammation seen in rheumatoid arthritis, Maximilian Konig of Johns Hopkins University and colleagues found.
"We identified the bacteria in close to 50 percent of the patients," said Dr. Felipe Andrade, a rheumatologist at Johns Hopkins who oversaw the research.
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And while there's no immediate takeaway for patients who have rheumatoid arthritis, Andrade says there's no harm in flossing a little more carefully.
"Prevention is the best way to avoid any disease," Andrade told NBC News. "So yes, I think if we start with improving the oral hygiene in people that may have some predisposition for the disease it is possible that we may prevent the development of the disease."
Researchers have long believed that some sort of infection may cause rheumatoid arthritis, a so-called auto-immune disease caused when the body mistakenly attacks health tissue.
In the case of rheumatoid arthritis, it attacks tissues in the joints, and sometimes around the heart and lungs.
There's no cure, and treatments usually suppress the immune system, making patients vulnerable to infectious diseases and cancer.
The disease is rare — about 1.5 million Americans are believed to have rheumatoid arthritis, the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), part of the National Institutes of health, says.
It took a while for the team to find the culprit bacteria, and they still cannot say how, precisely, it might trigger rheumatoid arthritis.
First off, they found a distinct pattern of damage called hypercitrullination in the gums of patients with periodontal disease — the same pattern of damage seen in many rheumatoid arthritis patients.
"That was the first clue that something common may be happening in the patients," Andrade said.
They found the Aggregatibacter actinomycetemcomitans bacteria in patients with periodontal disease, and found these bacteria can cause the citrullination damage by secreting a toxin that punches holes in the neutrophil immune cells.
They had to develop their own test for infection with the bacteria. "We found that in patients with rheumatoid arthritis, almost half of the patients have evidence of infection by the bacteria," Andrade said. That compared to just 11 percent of people without rheumatoid arthritis.
The next question is how a mouth infection could cause inflammation in the joints. The bacteria might travel to the joints, or perhaps the damaged immune cells do, or maybe something else is going on, Andrade said.
Another question is whether antibiotics might help. But by the time patients get rheumatoid arthritis, which takes years and even decades to develop, it may be too late. And doctors know that using antibiotics wholesale can do more harm than good, by killing off beneficial bacteria and by helping breed drug-resistant superbugs.
And it might not just be the "A a" bacteria. Other bacteria also secrete toxins to defend against immune cells, and they may cause a similar pattern of damage. But a scan of the patients showed the bacteria normally found in the mouth did not appear to be linked with rheumatoid arthritis or the citrullination damage.