Experts have long thought that medications that reduce chronic infection might reduce heart disease risks, too. But while this theory looked good in the laboratory, it has failed in real-life trials.
Heart patients infected with Chlamydia pneumoniae, a bacterium that causes respiratory illness, did not fare better when they were given regular doses of antibiotics than the patients who did not get the drugs, according to two recent studies.
Infection with C. pneumoniae usually causes transient symptoms similar to those of the common cold. But when the bacterium takes up permanent residence in the body, it can get into the arteries and create inflammation that can increase the risk of a plaque rupturing.
The First Study
Dr. Christopher P. Cannon, a cardiologist at Brigham and Women's Hospital conducted one trial that included nearly 4,200 people who had been hospitalized for a heart attack, stroke or other cardiovascular event. Half of them were given regular doses of the antibiotic gatifloxacin for two years, and the other half got a placebo. There was only a slightly lower incidence of deaths and severe cardiovascular events for patients who received the antibiotic-23.7% for the antibiotic group compared with 25.1% for the placebo group.
Maybe antibiotic treatment is just too little and too late for these people, Cannon says. People who are hospitalized for heart conditions usually have a variety of risk factors, such as high cholesterol levels or high blood pressure. Perhaps, "trying to treat one part of the problem doesn't have a big impact," he speculates.
Another explanation is that a chlamydia infection might help start the process of artery blockage, but then 20 or 30 or 40 years later, it is too late to treat the problem," Cannon adds.
The Second Study
The second study, led by researchers at the University of Washington, looked at 4,012 people who had stable coronary artery disease.
Using a different antibiotic—azithromycin—they found no difference in the occurrence of adverse events such as heart attacks between those who got the antibiotic and those who got a placebo.
In explaining the results of both studies, Dr. Jeffrey L. Anderson, a professor of medicine at the University of Utah, suggests several possibilities.
"One is that this whole idea of infection increasing the risk of heart disease and stroke is wrong," he says. "More likely, we've taken the wrong approach. Giving antibiotics to everyone isn't going to work.
"But maybe we just used the wrong antibiotics or the disease was so advanced that it was just too late" in the human trials, Anderson says. Or maybe chlamydia hides so well in arterial plaque that the antibiotic can't get to it, he adds.