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Suspicion lingers C. diff transmitted by meat
No clusters, but could sporadic transmission occur?
Investigators are trying to verify or refute the lingering suspicion that an undefined level of Clostridium difficile is being transmitted to humans through meat in the food supply. The link has not been sufficiently established to unequivocally say C. diff is a foodborne pathogen, but a leading researcher says nearly 20% of cases may not be accounted for by traditional risk factors.
Concern is being fueled by cases of community-acquired C. diff and the fact that researchers have found that some retail meat samples tested positive for C. diff strains. Research and analysis are continuing, but the most disconcerting finding is that C. diff strains detected include the highly toxigenic strain that has caused hospital outbreaks and infections in the community. A troubling footnote is that cooking may not kill the resilient spore-forming pathogen.
"We know it is in retail meats," says L. Clifford McDonald, MD, acting chief of the prevention and response branch at the CDC division of healthcare quality promotion. "But we do believe from the epidemiology that we can sort out that about 80% of all C. diff in humans appears to be health care-associated."
Most of the cases are probably acquired the old-fashioned way, through nosocomial transmission after admission to a health care facility. "Some of the other [cases] that seem to be transmitted in the community setting may actually have a doctor's office involved [or other ambulatory settings]," he says. "What could the percent be that is transmitted directly through the meats? We don't know. We think it's less than 20% and it may be as low as 5% if there is any [transmission] at all. No one has proven it."
Foodborne pathogens such as Salmonella usually aren't carried by humans so they are easier to detect in clusters and outbreaks. Not so with C. diff, which can remain dormant until triggered by a course of antibiotics.
"It won't [show] clusters very well because you could probably eat C. diff in most instances and not get disease until you take antibiotics or until you are sick from something else," McDonald says. "So that makes it hard to detect a cluster. It's probably going to be sporadic disease and that is going to make it hard to prove that the food supply is playing a role. It doesn't mean that it can't be [ascertained], but it is going to take some more artful things."
The CDC first reported the phenomenon of community-associated C. diff in 2005, citing reports in four states of infections in patients previously thought to be at low risk for C. diff. Considered in the context of high-morbidity, hospital-associated outbreaks in North America, Great Britain, and the Netherlands, the cases of severe C. diff appear to reflect a "changing epidemiology," the CDC reported.1 More evidence is being accumulated in the form of continuing cases in otherwise healthy people in the community, many of whom have no history of recent hospitalization or prolonged antibiotic use that typically precedes C. diff infection.
The CDC recently conducted a study at 10 of its FoodNet surveillance sites, looking for community-acquired C. diff-associated disease (CA-CDAD) and performing stool cultures for C. diff over a three-month period. Presumed CA-CDAD was defined in an ambulatory patient with a C. difficile toxin-positive stool who, based upon available medical record review, had no overnight stay in a health care facility in the preceding three months. A total of 175 presumed CA-CDAD cases were identified. C. diff was isolated from the stool specimens of 92 (57%) of 162 patient specimens cultured. The isolates from presumed CA-CDAD cases were genetically diverse, indicating that CA-CDAD is not caused by a single strain, the CDC found. Again, the epidemic strain (ribotype 027) that has caused severe hospital outbreaks was among the C. diff detected in the community cases.
"These [community strains] look more diverse, which suggests that wherever they are coming from there is a more diverse pool out there," McDonald says. "Whether they are from the food supply or just person-to-person in the community, they are more diverse. There is some of the epidemic strain [in the community] — the one that is causing all the problems in the hospitals. There are also strains that have been a problem in food-producing animals, too."
Strains of C. diff seen in pigs also were identified in humans, though it's unclear if the animals were the original source or people have also carried such strains. "We don't know if it was there before in the community in humans [because] nobody was looking 10 years ago," he says. "These are not the 'purest' community cases. Going forward, we would like to get cases that have been fully studied from an epidemiology standpoint to make sure that they weren't recently in a health care facility or a doctor's office. Then we can start asking about food consumption."