Foodborne Transmission of Clostridium difficile

L. Hannah Gould and Brandi Limbago, of the Centers for Disease Control and Prevention (CDC), acknowledge that Clostridium difficile infection is increasingly recognized as a cause of diarrhea in outpatients and individuals lacking contact with a healthcare facility, and now, it may be transmitted via food in the community setting. Their research has been published in the September 2010 issue of Clinical Infectious Diseases. 

They note, "Food has been hypothesized as a possible source of C. difficile in community settings, but evidence to confirm or refute this hypothesis is incomplete. C. difficile is recognized as both a gut colonizer and cause of diarrhea in food animals. Recent studies have isolated C. difficile from retail foods intended for human consumption in the United States, Canada, and Europe and from meat products intended for consumption by pets. These findings support concerns about foodborne acquisition of this pathogen through consumption or handling of contaminated products; however, no published studies have documented consumption of any food product as a risk factor for CDI. An improved understanding of the relationship between animal and human strains of C. difficile will help to evaluate the potential for foodborne transmission and the role of animal-human contacts in C. difficile epidemiology."

In their study, Gould and Limbago summarize the available data on C. difficile in animals and food and discuss data gaps that must be addressed to clarify whether foodborne transmission of this pathogen might occur, and if so, whether this route might be important in the epidemiology of CDI.

The researchers point out, "Data on the presence and prevalence of C. difficile in food products are newly available, and there are limited epidemiologic data to connect C. difficile found in the food supply to human illness. However, the epidemiology of C. difficile infection is changing, including an increase in both incidence and severity of disease, emergence of a new epidemic strain (ribotype 027/NAP1), and an apparent increase in infections among persons in community settings. Increasing rates of CDI in the community have raised questions about origins of new human strains, sources of human C. difficile acquisition, and risk factors for the development of infection. In addition to causing human disease, CDI is recognized as a cause of epidemic disease in piglets, and C. difficile is also commonly found in other food animals, including cattle and chickens. Some of the C. difficile strains most commonly identified in food animals appear to be emerging as causes of disease in humans, especially among humans with communiassociated CDI. Although a link between C. difficile carriage in animals and disease in humans has not been adequately defined, some investigators have suggested that food animals may play an important role in the expansion of pathogenic C. difficile clones and in transmission to humans through food."

Gould and Limbago continue, "If transmission indeed occurs from animals to humans, it will be essential to characterize the dynamics of this transmission, including whether transmission occurs though direct animal-to-human contact or though indirect means, such as consumption of contaminated foods. Increasingly, foods such as produce have been recognized as vehicles for pathogen transmission in outbreaks. In many of these outbreaks, a contaminated environment (eg, soil or irrigation water) appears to be responsible for delivery of bacteria to the food plants. In some instances, pathogens are internalized by the plant during growth, limiting the efficacy of control measures based on sanitation or washing. C. difficile has also been isolated from produce and can be recovered from a wide variety of environmental sources, including soil, sea water, and fresh water. Thus, it is possible that humans and animals are frequently exposed to C. difficile spores from multiple sources. Whether, when, and how frequently this exposure leads to disease is a critical question for improved control of CDI."

Gould and Limbago emphasize that a number of questions must be answered to determine whether foodborne transmission of C. difficile occurs and to determine the possible impact of low-level spore contamination on the safety of the food supply: "For example, the infectious dose of C. difficile for humans is unknown; if the infectious dose were known, it could be compared with the microbial burden that is typically present on contaminated foods at the point of consumption. Infectious dose is likely to vary depending on host factors, including age, underlying medical conditions, and exposure to antibiotics and acidreducing medications, and these factors are likely to be very different between hospitalized and community populations. C. difficile is not considered to be part of the normal human intestinal flora, but limited studies have demonstrated presence of toxigenic C. difficile in 3 percent to 5 percent of asymptomatic persons in the community. It is unknown whether this finding represents subclinical infection, colonization, or transient pass-through of ingested spores. It is also unknown whether or how often C. difficile is transmitted from animals to humans, or vice versa, or whether presence of common strains in animals and humans reflects exposure to a common environmental reservoir. Surveillance for human and animal infections is needed and should include subtyping studies designed to distinguish between common sources of animal and human infection or animaltohuman transmission. Detailed strain typing and epidemiologic investigations designed to evaluate the role of foodborne transmission during C. difficile outbreaks might help to determine whether C. difficile strains found in humans are linked to the food supply. Additionally, studies are needed to characterize food and environmental exposures in persons with community-associated CDI who do not have any health are exposures and to clarify whether implicated risk factors also impact transmission in healthcare settings."

The researchers add that additional studies are needed to develop consensus best-practice methods to test meats and other foods for C. difficile, as well as to understand surface decontamination on C. difficile spores in and on meat and other food products and, if foodborne transmission proves to be a mechanism, to evaluate other possible approaches to limit transmission by this route. They note, "It is reasonable to assume that the general public is and has been often exposed to low numbers of potentially infectious C. difficile spores. There is currently limited epidemiologic evidence to support or refute the hypothesis that C. difficile is transmitted by the foodborne route; the presence of C. difficile on retail foods suggests but does not prove that some proportion of infections is acquired this way. The food supply may thus serve as a source of new strains causing human infections; alternatively, food could be another constant and normally innocuous exposure. It is very clear that more research is needed to better understand the dynamics of and risk factors for development of CDI among persons in the community, including the relevance and possible importance of foodborne transmission."

Reference: Gould LH and Limbago B. Clostridium difficile in Food and Domestic Animals: A New Foodborne Pathogen? Clinical Infectious Diseases 2010;51:577582.