The world is experiencing an increase in illness and deaths associated with one of the most common nosocomial pathogens, Clostridium difficile. However, the sources of infection for people at risk, especially in the community which is also increasingly affected, are often unknown. In 2006 a study reported new hypervirulent and multidrug resistant epidemic C. difficile strains of relevance in human disease were present in livestock and retail meats highlighting the potential for foodborne transmission. The main goal of this PhD dissertation was to investigate and identify factors associated with C. difficile shedding in food animals, food contamination at harvest and its relation with wildlife and their environment with particular attention to inter/intra-species transmission and dissemination potential. Knowing how epidemic strains of bacteria disseminate among food animals and reach the food supply will help to identify strategies to mitigate the risk of food contamination and therefore transmissibility to humans.
By using observational, experimental and applied microbiological studies this dissertation documented: 1) the low prevalence of epidemic C. difficile in mature livestock at harvest, 2) the potential emergence of multidrug resistant strains that are unlikely to originate from food animal production systems, 3) fecal supershedding status relevant for infectious disease dynamics and environmental contamination, and 4) transient intestinal colonization and unaffected shedding dynamics despite enhanced use of antimicrobials in naturally infected cattle. Geographical clustering of captive white-tailed deer harboring emerging C. difficile PCR ribotype 078 was also documented. Nontoxigenic C. difficile strains, which can prevent the virulent effect of toxigenic strains, were unexpectedly prevalent in food animals at the time of harvest. Thermal studies with recovered spores conclusively documented the actual food safety risk highlighted in former publications, mechanistically showed that temperature affects cell division but not spore germination, and that hypervirulent strains are significantly favored with sublethal heating during cooking. Here it was also determined that wild birds are not clinically affected by human epidemic C. difficile strains and that dissemination at a regional scale is possible in anthropogenic and food animal ecosystems.
The identification of which food animal species and to what extent wild birds are active disseminators of emerging C. difficile strains is relevant to advance our ecological understanding of human diseases and to further develop strategies to reduce the risk of food contamination. These studies are expected to help increase global public and environmental health initiatives.