This is the first study investigating Barn Swallow as a possible source of CD for farm animals and humans. In the past, there has been increasing concern about animals as potential CD reservoirs and sources of human exposure [12, 24]. Of particular note is the finding of ribotype 078. This strain is common in food animals [4, 25, 26] and has been reported as an emerging and increasing cause of community-associated Clostridium difficile infection (CDI) in humans [25, 27, 28]. Two of the three other toxigenic ribotypes identified in this study (002, 014) can be found both in humans and animals [26, 29]. Three new CD ribotypes were also identified (SB3, SB159, SB166), one possessing A + B + CDT- (SB166).
It was interesting that CD was only found in juvenile birds. The sample population in this study was predominantly juvenile birds, which is in concordance with the expected ratio of juvenile birds on migration (>80%) [16, 30]. In most studied animal species, CD tends to predominate in younger animals [31, 32]. There is no evidence that CD causes disease in Barn Swallows.
Currently, the epidemiology of community-associated CDI is poorly understood. Food animals and food have been suggested as sources of human exposure [9, 33]; however, other potential forms of exposure in the community must be considered. While the high prevalence of CD in some farm animal groups clearly indicates that they could be reservoirs of CD, the potential for other animals to act as a source of CD from farms to the broader human or animal population is intriguing. The biology of Barn Swallows potentially makes them a very efficient vector for CD dissemination. They are the most common and widespread migratory bird in Europe and cohabit with farm animals and humans [17, 18, 20].
Passerine birds were previously not determined to be the source of CD; however, only passerines unlikely to come in close contact with humans have been investigated . In contrary, House Sparrows, a non-migrating passerine, had previously been associated with CD on pig farms in the Netherlands . European Barn Swallows, the species studied here, preferably nest within the cattle barns during warm months of the year, and spend the winter in Sub-Saharan Africa. Some can migrate to Arabia and to the Indian sub-continent [17, 18]. In our study, a total of 4% of all captured and sampled Barn Swallows were positive for CD, which reflects the prevalence of adult cattle in some reports. Given the presence of CD in the bovine population, with reported prevalence of 2.4-6.3% in adult cattle and 7.6-51% in calves [3, 5, 34] and the nesting locations of Barn Swallows, it is certainly plausible that these birds could acquire CD on farms. Interestingly, to date, including this study, CD was isolated only from wild birds that are associated with intensively farmed habitats [16, 19]. Therefore, it is more likely that Barn Swallows have a more indicative than perpetuating role in CD epidemiology.