Carriage frequency and genetic diversity of methicillin-resistant and methicillin-susceptible Staphylococcus aureus in Danish horses

The current knowledge of Staphylococcus aureus in horses is largely biased in favor of methicillin-resistant S. aureus (MRSA) and very little is known about the prevalence and genetic diversity of methicillin-susceptible S. aureus (MSSA) in this animal species. Even though pigs are the main reservoir of the MRSA clonal complex (CC) 398, this livestock-associated lineage has been reported in various animal species, including horses. The objective was to improve the current understanding of the frequency and genetic diversity of S. aureus in horses, including MRSA and MSSA. Nasal swabs were collected from 401 horses originating from 74 farms and analyzed by standard methods for isolation and typing of S. aureus and MRSA, and MRSA CC398 isolates were compared by whole-genome phylogenetic analysis to 16 porcine isolates representing the three major MRSA CC398 lineages in Denmark and to all 11 human MRSA CC398 t011 isolates carrying the immune evasion cluster (IEC) collected in Denmark between 2004-2015. S. aureus and MRSA were isolated from 54 (13.5%) and 17 horses (4.2%) originating from 30 (40.5%) and seven (9.5%) farms, respectively. MSSA isolates differed genetically from MRSA isolates, with CC1 t127 being the most prevalent spa type in the horse study population as well as in human cases of S. aureus bacteremia in Denmark. MRSA isolates comprised (i) 10 CC398 with spa type t011 and carrying IEC (3 farms), (ii) four IEC-negative CC398 t034 (2 farms), and (iii) three IEC-negative, mecC-positive CC130 t528 (2 farms). The IEC-positive t011 isolates were closely related to the isolates from three equine veterinarians and belonged to a recently identified horse-adapted clone in Europe. In contrast, the IEC-negative t034 isolates belonged to pig-adapted clones. The occurrence in horses of multiple S. aureus genotypes implicated in human infections, including both horse-adapted and pig-adapted MRSA CC398, another livestock-associated MRSA such as CC130, and a common cause of human MSSA bacteremia such as CC1, suggests that the risk of MRSA and MSSA infection should be assessed in people exposed to horses.