Evidence of Multiple Virulence Subtypes in Nosocomial and Community-Associated MRSA Genotypes in Companion Animals from the Upper Midwestern and Northeastern United States

Objective Not much is known about the zoonotic transmission of methicillin-resistant Staphylococcus aureus (MRSA) in companion animals in the United States. We report the rate of prevalence of S. aureus and MRSA recovered from clinical samples of animals requiring treatment at veterinary clinics throughout the upper midwestern and northeastern United States.

Design We compared phenotypes, genotypes, and virulence profiles of the MRSA isolates identified in cats, dogs, horses, pigs, etc., with typical human nosocomial and community-associated MRSA (CA-MRSA) genotypes to assess implied zoonotic transmission or zooanthroponosis. Five hundred thirty-three coagulase positive staphylococci (CPS) isolates recovered between 2006 and 2008 from a variety of animal-source samples were screened for S. aureus by S. aureus-specific 16S rDNA primers and for methicillin-resistance. All MRSA isolates were genotypes by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and spa typing. They were also screened for common staphylococcal enterotoxin toxin and adhesion genes by multiplex and singleplex PCR.

Results Among the 533 CPS isolates recovered, 66 (12.4%) were determined to be S. aureus and 24 (4.5%) were MRSA. The percent of animals that were positive for S. aureus were as follows: 6.6% (32 of 487) dogs, 39.6% (19 of 48) cats, 83.3% (10 of 12) horses, and 100% of pigs, rabbit, hamster and rat. Notably, 36.4% of all S. aureus identified were MRSA. Methicillin-resistant S. aureus was present in clinical samples from 12 of 487 dogs (2.5%), six of 48 cats (12.5%), five of 12 horses (42%), and one of two pigs. The 24 MRSA isolates resolved into four PFGE clones (USA100 (50%), USA300 (16.7%), USA500 (20.8%) and USA800 (12.5%) and six sequence types (ST5, ST8, ST105, ST830, and ST986) or two clonal complexes, CC5 and CC8. Five major virulence profiles (clusters A to E) were observed in these MRSA isolates. Genotypic and virulence profiles of cats and dogs were more similar to each other than to those of horses. A Panton-Valentine leukocidin positive isolate with ST8:USA300 was identified in a pig causing skin and soft infection.

Conclusion The presence of human MRSA clones in these animals suggests possible reverse zoonotic transmission. This study reports the first case of a USA300 genotype in a pig. Presence of multiple virulence profile within a MRSA genotype suggests the potential of emergence of new MRSA clones by gaining or losing additional virulence genes.