Scientists believe they have discovered how a common bacterial infection can trigger heart disease -- a controversial hypothesis that is generating enormous interest among heart researchers.
The key finding, reported in the journal Science, is that chlamydia bacteria, which cause lung and eye infections and a sexually transmitted disease, have a protein on the outer coat that mimics a protein found in the heart muscle of mammals.
Normally this molecular mimicry allows the germ to evade the chlamydia-infected individual's immune system. But sometimes the immune cells mount an attack against the chlamydia germs that gets misdirected against heart cells as if they were the enemy.
"What we now see is that we don't have to have bacterial damage to the heart" in order for bacteria to cause heart disease, said Dr. Josef M. Penninger of the University of Toronto, who led the research team.
Although the Canadian researchers' experiments involved mice, Penninger said there is good reason to believe the same mechanism may explain some human heart disease. The heart muscle protein that chlamydia mimics is identical in mice and humans.
Several previous studies have suggested a connection between chlamydia infections and human heart disease. However, these studies were epidemiological -- based on associations between heart disease and the prior incidence of infection -- and could not suggest a way to explain the link.
Moreover, chlamydia infections are very common; at least two out of three adults have antibody evidence of a past infection. Since heart disease is also common, it has been impossible to know from epidemiology alone if the two diseases were really linked.
Boston University researchers reported this month that British patients who had been treated with certain antibiotics were less likely to develop heart disease.
Two large human studies are under way to see whether antibiotic treatment will prevent heart attacks among individuals who have already survived one.
Still, even some of those who believe that infections may play a role in heart disease remain unconvinced.
"The epidemiologic studies have not clearly linked the two together," said Dr. Paul Ridker of Brigham and Women's Hospital in Boston, who studies the role of inflammation in heart disease.
A study of chlamydia's role in heart disease, conducted by Ridker and his colleagues and due for publication next month, failed to find a link between prior chlamydia infection and later heart problems. Neither was chlamydia infection associated with certain markers of inflammation that Ridker has found in heart patients.
Still, Ridker called the Toronto findings "very intriguing" and said, "Any open-minded scientist has to be excited by this type of data."
Pub Date: 2/26/99