Research published this week provides the clearest indication yet that infection by the intestinal pathogen Clostridium difficile is not something that happens only in hospitals and other healthcare environments as a result of poor hygiene. The study, conducted by medical researchers in Oxfordshire in the UK, sequenced the genomes of more than 1200 isolates from infected patients over a three year period to try and establish where they where coming from. Remarkably, it seems that only 18% were acquired from another patient in a hospital setting. There was a lot more genetic diversity than might have been expected, suggesting a variety of sources of infection in the community.
One obvious source is animals, which would make C. difficile infection a zoonosis like Salmonella and Campylobacter and opens up the possibility of foodborne transmission. This suggestion has been discussed before. For example, in 2007 researchers at the University of Guelph in Canada showed that C. difficile strains known to infect humans could be found in a significant number of calves from dairy farms. They also looked at samples of ground beef on retail sale and found pathogenic strains there too. Other studies have since confirmed these results, notably in France where 5% of ground beef samples tested positive for C. difficile. Research in the Netherlands has also demonstrated that infection can pass from pigs to humans.
For most of us, C. difficile infection is nothing to worry about and won’t cause any symptoms. But for the elderly it can be a serious problem, occasionally causing a form of colitis with a reported mortality rate of up to 30%. People whose intestinal microflora has been suppressed by antibiotic treatment are especially vulnerable. It is not difficult to spot the similarities with Listeria monocytogenes, another pathogen that was not initially considered to be a food safety problem. Unlike Listeria, C. difficile also produces heat-resistant spores, making it potentially difficult to control in foods. The scientific evidence for C. difficile as a foodborne pathogen is accumulating to a point where a lot more research is called for. We need more data on the incidence of infection in food animals and on contamination rates in a range of foods. It might be necessary to look at potential controls too. As populations in developed countries grow older, protection against health threats of this kind can only become more important.