In a study of more than 3,000 people, researchers investigated whether exposure to four common pathogens (disease-causing virus or bacteria) increased the risk of cardiovascular disease. The pathogens are CMV, Chlamydia pneumoniae (respiratory infections), Helicobacter pylori (ulcers), and hepatitis A (liver disease).
The researchers found that CMV exposure carried a higher risk for cardiovascular events or death, while the other pathogens, by themselves, were not related to increased risk. However, they also found that a person's "total pathogen score" predicted a small increase in risk for cardiovascular events. Thus, people exposed to all four pathogens had a higher risk compared with those exposed to just one of the four, says lead author Marek Smieja, M.D., Ph.D., assistant professor in the department of pathology and molecular medicine, McMaster University, Hamilton, Ontario, Canada.
Researchers have grappled with whether exposure to common infections contributes to heart disease. While there have been a large number of studies on such topics, most have been small and many lacked long-term follow up, Smieja says.
Smieja and colleagues studied blood that had been banked from 3,168 Canadians who participated in the Heart Outcomes Prevention Evaluation (HOPE) study. Patients in the study had cardiovascular disease or were at high risk for cardiovascular disease, stroke or diabetes. The researchers measured antibodies to the four pathogens. They measured levels of two different antibodies for Chlamydia pneumoniae (CP) - the CP IgG antibody, which is believed to stay in the body for several years after infection, and CP IgA, a much shorter-lived antibody. CP IgG was detected in 82.9 percent of participants and IgA was found in 63 percent. Researchers found that 61.7 percent of the participants had antibodies for Helicobacter pylori, 76 percent for hepatitis A and 70.4 percent had them for CMV.
The risk associated with blood levels of the various pathogens and patient outcomes of heart attack, stroke or cardiovascular death was assessed during 4.5 years of follow-up.
Heart attack, stroke or cardiovascular death occurred in 494 patients. The researchers found no connection between CP and cardiovascular disease.
"We're pretty comfortable saying that chlamydia is not an independent risk factor for heart disease," Smieja says. Helicobacter pylori and hepatitis A also showed no relationship to increased risk for cardiovascular events. The only pathogen linked to cardiovascular disease risk was CMV, which increased the risk of heart attack, stroke or cardiovascular death by about 24 percent.
In addition, the researchers found that people who had antibodies to all four pathogens were 41 percent more likely to suffer heart attack, stroke or die from cardiovascular disease than those who had antibodies for zero or one pathogen. Smieja concludes that the association between these infections and cardiovascular disease is too modest to recommend any routine testing for them. Future studies, he says, should look at additional infections, such as herpes 1 and 2, to collectively determine if they impact risk.
In an accompanying editorial, Joseph B. Muhlestein, M.D., associate professor of medicine at the University of Utah and director of cardiology research at LDS Hospital, Salt Lake City, Utah, says that while this study didn't show the same extent of burden from these pathogens that previous studies had, it does confirm that infections, especially cytomegalovirus, are associated with worsening cardiovascular events in people at high risk of cardiovascular disease. "Therefore, I would take it as confirmation that we need to continue to look at and study the relationship between a variety of infections and atherosclerosis," he says. "In addition, this study appears to have demonstrated a greater importance, perhaps, with viral infections rather than bacterial infections. This may send us in a little different direction - looking more carefully at viruses than we had before."
Co-authors are Judy Gnarpe, Ph.D.; Eva Lonn, M.D.; Hakan Gnarpe, M.D., Ph.D.; Gunnar Olsson, M.D., Ph.D.; Qilong Yi, Ph.D.; Vladimir Dzavik, M.D.; Matthew McQueen, M.D., Ph.D.; and Salim Yusef, M.B.B.S., D.Phil.
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