Y-024. Phylogenetic Evidence for Possible Reverse Zoonosis and Species-Specific Strains of Methicillin-Resistant Staphylococcus aureus in Domestic Animals
Background: Methicillin-resistant Staphylococcus aureus (MRSA) infection is becoming an increasingly important community-acquired (CA) public health problem. The original source of CA-MRSA strains is often unknown, with many such strains apparently unrelated to known hospital-associated isolates. We have observed an increasing incidence of primary MRSA infections in domestic animals over the years 2000-2007. We hypothesized that local human and animal clinical isolates were epidemiologically linked, either through animal-to-human (zoonosis) or human-to-animal (reverse zoonosis) transmission. Methods: We performed mec, REP-PCR and spa sequence typing on 41 veterinary (22 equine, 16 canine, 2 feline, 1 cetacean), 39 local hospital, and 7 human reference MRSA isolates. REP-PCR band data was extracted from image files and used to infer phylogenies by the parsimony method. mec and spa sequences were obtained from PCR products using standard primers; repeat types were identified using custom software. Results: REP-PCR, mec and spa type data strongly supported independently evolving clusters of MRSA isolates. Human isolates fell into 5 independent clusters. Each cluster contained closely related animal isolates. About half of the canine and equine isolates clustered independently of human isolates. Canine and equine isolates tended to group with other canine and equine isolates, respectively. Overall, isolates did not cluster with respect to year of isolation. Conclusions: High genetic relatedness between human and animal strains supports a hypothesis either of zoonotic or reverse zoonotic transmission. Strong clustering of animal isolates with isolates from like animals, independent of time, may indicate host species-specific adaptation in these strains.