Methicillin-resistant Staphylococcus pseudintermedius as causative agent of dairy cow mastitis

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) strains have emerged as a significant health problem in companion animals, but the microorganism has never been reported as agent of bovine mastitis. The aim of this study was to report the occurrence of intramammary infections with MRSP in a dairy herd. Cyto-bacteriological analysis of quarter milk samples of all 250 lactating cows were performed and coagulase-positive staphylococci were detected in 1.2% of the animals, with somatic cell counts greater than 1x106 cells/mL. The isolates were further tested by PCR for specie identification and sequenced. The strain carried mecA gene and was therefore classified as MRSP and further characterized by MLST, spa typing and SCCmec typing. The pathogen resulted in a relevant inflammatory response of the infected quarter, but was not diffusive. The genomic profile displayed was ST71, t02, II-III, identical to the most common MRSP genotype detected in dogs in Europe and North America, raising concern about the potential spread of MRSP also among farm animals and humans. Finally, the phenotypical variability of S. pseudintermedius and the potential for mis-identification of this pathogen as S. aureus by routine identification systems, would advise the use of molecular methods to confirm S. aureus isolates from bovine mastitis.