In dogs, Staphylococcus pseudintermedius (formerly recognized as S. intermedius) is the most common resident organism of the skin and the most common cause of canine pyoderma. Occasionally, S. pseudintermedius and related species (S. intermedius, S. schleiferi) acquire a genetically coded, penicillin-binding protein that confers resistance to all beta-lactam antimicrobials and their
derivatives. Affected organisms are termed methicillin-resistant Staphylococcus. In older articles, these organisms were known as MRSI; however, the term MRSP is now considered more appropriate.
Healthy dogs and people can serve as carriers for MRSP. In one reported outbreak of surgical wound infections in pets, MRSP
carriage was detected in a surgeon, two surgical nurses, and staff members' dogs that were frequent visitors to the clinic.
Colonized personnel were likely the source of postoperative wound infections. Almost half of owners of dogs with MRSP deep
pyoderma are colonized with the same bacteria. Infections within humans, however, are rare.
Diagnosis of MRSP is based on susceptibility testing of deep tissue cultures obtained by aspirate or punch biopsy after aseptic
skin preparation. Definitive diagnosis of the specific species requires DNA sequencing. Evaluate animals with MRSP for underlying
diseases that could predispose them to infection or interfere with wound healing.
In general, animals with wound infections are treated similarly, regardless of the underlying cause. Because of the risk for
disease spread, however, handle patients with MRSP similar to animals with other contagious diseases: if possible, isolate
infected animals from other patients. Restrict the use of any supplies and tools (e.g., stethoscopes, thermometers, writing
utensils, leashes), used during patient handling to the affected individual during its stay, and disinfect them before use
on other patients. MRSP is sensitive to many common detergents and disinfectant cleaners as long as the manufacturer's suggested
contact time is used. Wear gowns and gloves when handling the patient and masks and protective eye wear when flushing wounds.
Use alcohol-based hand sanitizers before and after handling each patient; in fact, hand hygiene is the most important factor
in reducing spread of methicillin-resistant bacteria of any type.
In patients with focal MRSP wound infections, administer supportive care as needed, and clean and drain the wound. In some
patients, removal of implants (e.g., plates, screws, or sutures) may be required to clear the infection. Open wound management
is preferable in wounds that are effusive or contain necrotic tissue or debris. Debride, clean, and flush the wound, and cover
it with a topical dressing effective against MRSP. Options include antiseptics (chlorhexidine, povidone iodine, acetic acid),
antimicrobials (fusidic acid, mupirocin, tea-tree oil, silver), and hyperosmotic agents (honey, sugar, dextrans, hypertonic
saline). Tie-over bandages are helpful for securing topical dressings and primary bandage layers to the wound. Cover the tie-over
bandage with an iodine-impregnated adhesive drape to reduce wound and environmental contamination. Daily bandage changes are
usually necessary for the first seven to 14 days, and wounds may take two to four months to heal.
A tough infection: Bite wounds infected with methicillin-resistant Staphylococcus species. Despite systemic antibiotics and
local wound therapy, the wounds dehisced. The dog eventually healed four months later after open wound management and punch
grafting. (PHOTO: COURTESY OF KAREN M. TOBIAS)
Systemic antimicrobials should be based on results of culture and susceptibility testing; however, some strains of MRSP may
display in vitro susceptibility that does not correlate with in vivo response. Microbiology personnel can be contacted to
determine the most common sensitivity pattern of local strains while results are pending. In our hospital, MRSP is usually
susceptible to chloramphenicol (100 percent) or aminoglycosides (97 percent). Systemic antimicrobials should be administered
at least one week beyond remission of clinical disease (usually a minimum of 21 days). Repeated antimicrobial exposure at
subtherapeutic concentrations or inappropriately short duration may select for resistance. In some patients, topical wound
therapy is sufficient to clear the infection. Deep tissue cultures should be repeated during antibiotic therapy if healing
is not progressing as expected.
If an outbreak of MRSP wound infections is detected in your hospital, evaluate your hand hygiene and surgical preparation
practices. Updating these practices will provide the best method for preventing future infections.
Dr. Karen Tobias is an ACVS board-certified veterinary surgeon and professor of small-animal surgery at the University of Tennessee. She is
the author of Manual of Small Animal Soft Tissue Surgery (Wiley-Blackwell, 2009), and the owner of several incorrigible pets.