Chlamydia (C.) pecorum, an obligate intracellular bacterial species commonly found in ruminants, can also occur in pigs. However, its significance as a potential porcine pathogen, or commensal, is still unclear. In a previous study (Hoffmann et al. 2015), mixed infections of C. suis and C. pecorum were detected in 14 Swiss fattening pig farms. Using these samples, we aimed to investigate the infection dynamics of C. suis and C. pecorum mixed infections in these farms. In addition, we analyzed the genetic diversity of Swiss porcine C. pecorum strains in relation to globally circulating strains. In total, 1284 conjunctival and rectal swabs from 391 pigs, collected at the beginning and end of the fattening period, were tested during the course of this study. We determined the bacterial loads of C. suis and C. pecorum using species-specific real-time PCR (qPCR) and compared these results to already existing DNA-microarray and Chlamydiaceae qPCR data. Overall, C. suis and Chlamydiaceae copy numbers decreased in the course of the fattening period, whereas C. pecorum copy numbers increased. No association was found between clinical signs (conjunctivitis, lameness and diarrhea) and the bacterial loads. Preventive antibiotic treatment at the beginning of the fattening period significantly lowered the chlamydial load and outdoor access was associated with higher loads. Proximity to the nearest ruminants correlated with increased C. pecorum loads, indicating that C. pecorum could be transmitted from ruminants to pigs. Multi-locus sequence typing (MLST) and major outer membrane protein (ompA) genotyping revealed two novel sequence types (STs) (301, 302) and seven unique ompA genotypes (1-7) that appear to form a specific clade separate from other European C. pecorum strains.
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