This study showed that C. difficile can be isolated from a variety of gastrointestinal compartments of healthy horses and that several distinct ribotypes may be isolated from the same horse. Explanations for the different ribotypes in the same horse include simultaneous transient passage of multiple strains through the gastrointestinal system or colonization of different compartments by diverse strains. Regardless, these data indicate that the typical practice of selecting one colony for typing, an almost universal procedure in studies that have reported C. difficile types in horses [23, 24], might underestimate the true ribotype diversity and distribution. Human studies have reported co-existence of different ribotypes in faecal specimens from healthy humans and those affected with CDI [25, 26]; however, the presence of multiple strains is rarely reported. In a study of healthy humans, Kato and co-workers (2001) reported that only 1/94 (1.1%) faecal samples positive for C. difficile contained more than one ribotype per sample. Similar data have not been available for horses, but this study suggests that methods able to detect different ribotypes in a single sample might be required to get a true understanding of the ribotype distribution in equine samples. Regardless, the potential presence of multiple C. difficile strains in the same animal must be considered when designing and interpreting studies.
Clostridium difficile was found in various locations throughout the intestinal tract, which is not surprising given its association with both small and large intestinal disease. As with any study of the intestinal microbiota, it is impossible to definitively state that C. difficile was truly colonizing these locations as opposed to passing through transiently following ingestion. This study cannot differentiate between these two, but demonstrates that this organism is adapted to survive and perhaps proliferate in various intestinal locations. Further investigations into the dynamics of C. difficile and ways to differentiate transient passage from colonization are required.
Inconsistent detection of C. difficile in different intestinal compartments of the same horse could be due to non-homogenous distribution of C. difficile in truly colonized animals, intestinal site predilections for C. difficile (or specific strains), or presence of C. difficile below detection thresholds at some sites.
Overall the prevalence of C. difficile in the gastrointestinal tract of horses found in this study (53%) was higher compared to other studies (typically 0-10%), including those performed in the same geographical region [5, 7–9]; however, prevalence data should be interpreted with caution, as the study was not designed to determine population prevalence. Also, the presence of multiple animals per farm may have introduced a clustering effect. Six horses in this study originated from one farm, and C. difficile was isolated from 3/6 (50%) of them, potentially slightly over estimating overall prevalence data. Given the small sample size and potential for clustering in one farm, this study should not be taken as a population prevalence study, but rather as a comparison among intestinal compartments and assessment of ribotype distribution. The detection of C. difficile in feces of healthy horses shows that horses are exposed to this pathogen frequently, however only a small subset of horses eventually develop disease. Better understanding of the epidemiology of this pathogen is required to better prevent and treat disease.
Assessment of faecal samples is the standard practice for diagnosis of disease caused by enteropathogens, because of the ease of access and the impractical nature of sampling other intestinal compartments in most cases. Therefore, understanding how faecal samples reflect proximal compartments is critical for interpretation of the data. Agreement between rectal samples and proximal compartment was good (κ = 0.61), although these data do indicate that rectal samples might not absolutely reflect the status of proximal compartments, both in the presence or absence of C. difficile and the ribotypes that are recovered. These findings should be considered when interpreting results from other studies, as prevalence of C. difficile in the gastrointestinal tract of horses might be higher than faecal prevalence would suggest.
The presence of a variety of ribotypes in this study is not surprising. Previous studies have reported a wide diversity of ribotypes in horses [24, 27]. The most common ribotype in this study was ribotype 078, commonly found in pigs and cattle , and increasingly implicated in community-associated CDI in humans [29, 30]. While ribotype 078 has been found in horses [9, 23, 28], the prevalence here is higher compared to previous studies of horses in this region, such as a 2006–2007 surveillance study where ribotype 078 only accounted for 3/52 (5.8%) of isolates from healthy horses . This could indicate that this ribotype is emerging in this region. The second most common strain in this study was ribotype 001. This ribotype has been previously reported in horses [9, 27] and is also a predominant human clone, being the most common ribotype in a study of people in hospitals in the region . Overall, nine of the 14 (64%) isolates found in this study were types that have been implicated in CDI in humans in the region . Data such as these raise concern that C. difficile may be a zoonotic pathogen ; however, proof of interspecies transmission is currently lacking.
The low prevalence of C. perfringens in intestinal content samples obtained in this study is consistent with previous studies. While C. perfringens is commonly found in horses with enteric disease, foals and pregnant mares, the low prevalence reported here is consistent with other studies of adult horses [13, 16]. Methodological issues should always be considered with studies reporting low prevalence. Standard and validated methods are not available for C. perfringens isolation from equine faeces; however, the method used here had a detection threshold of 9 cfu/ml of faeces (data not presented) and identical protocols were used in a study that identified C. perfringens shedding in 88% of healthy dogs . Thus, it is unlikely that poor test sensitivity accounted for the low prevalence reported here, and these results suggest that C. perfringens is uncommonly shed by healthy horses, or shed intermittently or at very low levels. Whether the higher isolation rates from horses with gastrointestinal tract disease indicates cause or simply opportunistic overgrowth of this organism is still unclear [13, 15].
A limitation of this study was its small sample size and therefore care has to be taken when interpreting these results; however, this is the first study determining the prevalence and ribotype distribution of C. difficile in different compartments of healthy horses and despite the small sample size it gives new insight into the epidemiology of this pathogen.
Information on antimicrobial use in the weeks prior to euthanasia was not available; however, none of the horses had any clinical condition reported that would likely have been treated with parenteral antimicrobials. It is certainly plausible that prior antimicrobial use would have affected results of this study; however, any effect would probably have been on overall prevalence, not an alteration of distribution of C. difficile in different intestinal locations. If there were any impact on C. perfringens, it would have been expected to increase the prevalence, something that was not observed here given the very low prevalence.