Clostridium difficile (CD) is present in almost all human environments and is a potential zoonotic pathogen that can be isolated from a variety of animal species [1–5]. It is the most commonly diagnosed cause of antimicrobial- and hospital-associated diarrhoea [6] and is an emerging cause of community-associated disease [7, 8]. The presence of CD in animals, including those that come in close contact with humans as pets or through the food chain [1, 9], and the significant overlap between the types of CD isolated from humans and animals [10–12], has lead to suggestion that CD might be a zoonotic and foodborne pathogen [9, 12].

Recognition of different potential sources of CD transmission remains a pressing clinical and investigative quest. Migratory birds travel between continents twice each year and are responsible for the transmission of several pathogens [13] and/or disease vectors [14]. The world’s largest bird migration system is the Palaearctic–African flyway, which involves an estimated 2 billion passerines and near-passerines migrating between the European continent and sub-Saharan Africa [15].

An epidemiological study on mostly migrating passerine birds in Europe suggested that they are unlikely to serve as a carrier or reservoir of CD [16]. However, the study [16] was conducted in the region where Barn Swallows (Hirundo rustica) were not congregating during their migration south, and included only garden birds unlikely to dwell in habitats intensively cultivated by humans. On the other hand, Barn Swallows, come in close contact with human habitats [17, 18], as do House Sparrows (Passer domesticus), which are a non-migrating passerine bird, that have been associated with CD when sampled on pig farms [19].

Barn Swallows are among most common and widespread migratory birds in the Palaearctic. In Europe they most commonly nest in cattle farms inside the barns or stables, which had a significant evolutionary effect on Barn Swallow biology with regards to their nesting habits. Any changes in their environment significantly affect their population [17, 18]. Its habit of nesting in buildings associated with human habitation has made this species one of the most familiar bird species in the world [20] as well as a potential source for international and local CD perpetuation. The aims of the present study were to determine the prevalence of CD in Barn Swallows, and to characterize CD isolates.

Sampling of Barn Swallows was conducted once in September 2011 in the central part of Slovenia (46° 0′ 2.12″ N, 14° 22′ 58.27″ E) in an area identified as a Barn Swallow congregation point during their autumn migration across Europe (average week temperature 21.8°C, average week humidity 69.5%). One hundred seventy-five (n = 175) Barn Swallows were captured with mist nets and placed individually in clean custom made hot pressed plastic containers with air holes. Bird faeces, if present, were collected from containers using sterile gloves (Ansell Ltd, UK) after the birds were removed for standard general inspection, ringing and release to continue with their migration. Faeces were transferred into 2 mL sterile tubes (Eppendorf Tubes, Germany) and stored at -20°C until analysed for the presence of CD spores.

The study was carried out with animal ethics approval by the Ministry of the Environment and Spatial Planning (document No.: 35601-10/2010-6).

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 [16]. In contrary, House Sparrows, a non-migrating passerine, had previously been associated with CD on pig farms in the Netherlands [19]. 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.

Results of this study showed that Barn Swallows could potentially be a source of CD in humans and animals. During their congregation at migration destinations inter-individual and interspecies horizontal transmission can occur [13]. However, based on this and previous studies on the prevalence of CD in wild passerine birds [16, 19], and studies associated with farm animals and humans [10, 27, 31, 32, 35], Barn Swallows may more realistically be an indicator and not the source of the contamination of CD in the environment. It seems that intensive farming and hospital environment are the sources of CD, and only humans and/or animals associated with such environment perpetuate CD.