In part 1 of this series ("The emergence and prevalence of MRSA, MRSP, and MRSS in pets and people" in the December 2012 issue), we took a look at how methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus pseudintermedius (MRSP), and methicillin-resistant Staphylococcus schleiferi (MRSS) strains have become an increasing problem and the risk factors for infecton with a resistant staphylococcal strain. Here is what to do if you think one of these stubborn strains has taken hold in one of your patients.
Although some of the photos in this article show dramatic lesions, it is important to remember that in most cases methicillin-resistant staphylococcal infections in animals present no differently than methicillin-susceptible infections (Figure 1). Clinical signs of superficial pyoderma include papules, pustules, crusts, scaling, erythema, and hair loss. Patients with deep pyoderma may have nodules or bullae, thick crusts, or ulcerated areas.
Figure 1. An atopic Lhasa apso with a spreading superficial pyoderma caused by MRSP.
A methicillin-resistant infection should be suspected (Figures 2 & 3) whenever there is poor response to empiric antibiotics, especially if a patient has a history of treatment with multiple prior antibiotics or a previous methicillin-resistant infection. Cytologic examination of lesions should be performed to document the presence of bacteria and to aid in interpretation of culture results. Prompt sample submission for bacterial culture and sensitivity testing should be performed rather than an empiric antibiotic change.1
Figure 2. Acral lick dermatitis in an atopic dog, omplicated by deep infection with MRSP.
Methicillin-resistant infections are diagnosed by bacterial culture and antimicrobial susceptibility testing of appropriate samples submitted to a veterinary reference laboratory. Depending on the clinical presentation of the infection, culture samples can be obtained by sampling otic exudate or an intact pustule with a culture swab, by rubbing a sterile saline-moistened culture swab under crusts or scaly rims of epidermal collarettes, or by obtaining a punch biopsy of a lesion for macerated tissue culture (especially recommended for deep pyoderma).
Figure 3. An atopic Labrador retriever with methicillin-resistant blepharitis.
Here are a few tips to keep in mind once you receive a result from your laboratory:
- If a methicillin-resistant infection is identified, and if the laboratory does not automatically test sensitivities to chloramphenicol, amikacin, and doxycyline, call the laboratory to make sure these antibiotics are added to the sensitivity panel.
- If a coagulase-positive Staphylococcus species is identified but not speciated, call the laboratory to make sure this essential test is done.
- If a coagulase-negative Staphylococcus species is cultured as the only organism from a suspected resistant infection but is not speciated or tested for antibiotic sensitivities, call the laboratory and request that the bacteria be fully speciated and that an antibiotic sensitivity panel be performed.