Archie, a 4-year-old Labrador retriever, is presented for evaluation of a wound infection.
History. Five days previously, Archie sustained a shallow laceration over the ventrolateral aspect of the abdomen while playing unobserved outside. His owner has been cleaning the wound with antibacterial soap and applying a topical antimicrobial (polymyxin B). Archie is the only pet in the household and his vaccinations and parasite prevention are up to date.
Physical Examination. Archie is bright, alert, responsive, and afebrile. A 2-cm wound is present over the abdomen, and a small amount of purulent discharge is present. Warmth and erythema extend approximately 5 to 10 cm from the site of the wound, and palpation of the area elicits pain. The wound is cleaned and a swab of the discharge is taken for culture. Amoxicillin/clavulanic acid is prescribed, and the owner is advised to clean the wound daily with dilute chlorhexidine solution.
Laboratory Results. Two days later, the laboratory results shown in the Table are received.
Organism: Staphylococcus aureus
Antimicrobial - Sensitivity
Penicillin - Resistant
Cephalothin - Resistant
Cefoxitin - Resistant
Oxacillin - Resistant
Cefotaxime - Resistant
Tetracycline - Resistant
Enrofloxacin - Sensitive
Gentamicin - Resistant
Chloramphenicol - Resistant
Trimethoprim–sulfamethoxazole - Sensitive
Amoxicillin/clavulanic acid - Resistant
Erythromycin - Resistant
Clindamycin - Sensitive
What is the best antimicrobial choice in this case?
On the basis of oxacillin resistance, methicillin-resistant Staphylococcus aureus (MRSA) has been isolated from this dog. Methicillin susceptibility was not tested because methicillin is poorly stable in vitro; therefore, oxacillin and/or cefoxitin are used. A 2-week course of trimethoprim–sulfamethoxazole (30 mg/kg PO Q 12 H) is prescribed, and a follow-up appointment is scheduled to determine whether treatment will need to be continued. Household infection-control practices are discussed to reduce the risk of zoonotic transmission.
Antibiotic Selection. MRSA is an emerging problem in veterinary medicine and selecting an appropriate antimicrobial can sometimes be complicated.
The use of fluoroquinolones for treatment is considered suboptimal in humans due to the potential for resistance developing during treatment and the poor correlation between in vitro susceptibility and clinical response.1 The same principle most likely applies to animals; therefore, other options are probably better in this case given the apparently invasive nature of this infection.
Clindamycin susceptibility is reported, but this isolate could be resistant. A phenomenon of inducible clindamycin resistance can be present in S aureus2 whereby isolates appear susceptible with routine testing but are truly resistant. In a recent study, over 50% of erythromycin-resistant MRSA from dogs and cats was inducibly resistant to clindamycin.3
A D-test (Figure 1) is used to identify resistance to clindamycin with exposure to erythromycin. With a positive test, as shown in the figure, there is a blunting of the zone of inhibition around the clindamycin disk adjacent to the erythromycin disk, creating a “D” shape. However, this test is not readily available in most veterinary laboratories. Because this isolate is erythromycin-resistant, it should be considered to be clindamycin-resistant until proven otherwise.
Imipenem susceptibility data are not available, but the isolate would be resistant because MRSA is resistant to all beta-lactams (eg, penicillins, cephalosporins, carbapenems).
Another option, vancomycin, is commonly used in human medicine; however, principles of prudent antimicrobial use4 dictate that drugs important for serious infections in humans should be used sparingly in veterinary medicine. There is some controversy as to whether vancomycin should be used in animals at all, but most experts would agree that it should be reserved for cases where there are no other reasonable options.5 In addition, vancomycin must be administered intravenously, which is not desirable in a case such as this.
Trimethoprim–sulfamethoxazole (TMS) is left as the best option. Although TMS is an “old” drug, it can be highly effective and is sometimes used to treat MRSA infections in people. It is typically used sparingly in dogs because of concerns about adverse effects, including keratoconjunctivitis sicca (“dry eye”), arthropathy, hepatic necrosis, and bone marrow suppression.
However, TMS is cost-effective and can be administered orally, and Labrador retrievers are not a breed at increased risk for keratoconjunctivits sicca. In Archie’s case, invasive infection may be present (and there are few other options), so TMS is a reasonable choice.
Follow-Up. A Schirmer’s tear test should be performed before initiating treatment, periodically thereafter (ie, every 10–14 days) if treatment is prolonged, and for a few weeks after cessation of therapy. The client should also be advised to watch for mucoid ocular discharge and to return for evaluation of the wound in a week or sooner if any signs of deterioration are noted.
Outcome. Archie recovered uneventfully. After 2 weeks of appropriate treatment, the infection was eliminated and the wound healed normally.