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NEW YORK (Reuters Health) - Despite earlier evidence tying an outbreak of E. coli infections in Canada to later heart disease, an expanded follow up study finds no link between the two.
"Although we definitely want to avoid anyone getting infected in the first place, this new information is reassuring for those who develop an infection from E. coli O157:H7," Dr. Amit Garg, one of the authors of the study, said in a press release issued by the Canadian Medical Association Journal (CMAJ), which published the study.
This strain of E. coli bacteria polluted the drinking water supply of Walkerton, Ontario in May of 2000, sickening more than 2,300 people and resulting in seven deaths.
Food-borne E. coli infections - which affect about 265,000 people each year in the U.S., according to the Centers for Disease Control and Prevention - can damage the kidneys and lead to high blood pressure. That has raised concerns that they might also contribute to heart disease and stroke.
"There's anecdotal evidence that certain infections immediately predate heart attack or stroke," said Dr. Deepak Bhatt, the chief of Cardiology at VA Boston Healthcare System, who was not involved in the study.
"It's not been clear whether it's the infection or inflammation (from the infection) or coincidence," Bhatt, also a professor at Harvard Medical School, told Reuters Health.
To see whether an E. coli outbreak could increase the risk of heart disease and stroke, Garg, a professor at Western University, Lawson Health Research Institute in London, Ontario, and his colleagues collected data on affected individuals from the 2000 event at a health clinic where they had annual visits.
Initially, the group seemed to have a higher risk for heart disease and stroke compared to people who had not suffered an E. coli infection. The researchers point out, however, that nearly half of the participants dropped out of the study, making those findings difficult to interpret.
In the current study, the group included 153 people who experienced severe illness during the outbreak, 414 people with mild illness, 331 people from Walkerton who did not get sick and more than 11,000 people who lived in neighboring towns that were spared from the E. coli outbreak.
In the decade following the outbreak, people who became severely sick were no more likely to later suffer a heart attack or stroke than people who lived outside of Walkerton.
In contrast, people who suffered a mild illness were actually 36 percent less likely to die from heart disease or stroke than residents of the surrounding communities.
Among people with a mild reaction to the infection, about 6 percent died during the study period, compared to about 10 percent of people who lived outside of the outbreak.
The reason is not totally clear. The authors write in their study that perhaps people in the mild-illness group didn't get that sick from the infection - and also had a lower risk of cardiovascular death - because they were healthier than average.
(Garg would not agree to an interview with Reuters Health unless he was able to review major portions of this article in advance, a practice that violates Reuters' policy to protect journalistic independence.)
The results from the study don't necessarily mean infections don't increase the risk of cardiovascular disease, said Dr. Liam Smeeth, a professor at the London School of Hygiene and Tropical Medicine, who was not part of the study.
Smeeth told Reuters Health that research has shown that any impact on the coronary arteries from infection and subsequent inflammation is short-lived, and perhaps the numbers in the Walkerton study were not big enough, or the increased risk not large enough, to be detected.
"It's not crystal clear because it was a relatively small study," he said.
Bhatt agreed that the findings don't prove or disprove the idea that infections could be involved in heart disease, and it's also possible that the type of infection might matter.
He said that it's important to rule out the types that don't contribute.
"I think the study's important because it makes it very, very much less likely that gastrointestinal infections in some way are linked to atherosclerosis, and I think that finding is useful because probably investigators in the future shouldn't focus on this area as far as causes of atherosclerosis and heart attack and stroke," he said.
SOURCE: bit.ly/Te450j CMAJ, online November 19, 2012.