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.
Here are a few tips to keep in mind once you receive a result from your laboratory:
Advice to owners
Advise owners of pets with MRSA of the potential for zoonotic transmission (Figures 4A & 4B). Although the risk of clinical disease is probably low for immunocompetent people, reports of community-acquired MRSA in immunocompetent people are on the rise.2 Additionally, the concern for transmission is greater if a pet is exposed to immunosuppressed people or when household members are in contact with higher-risk people (i.e. healthcare workers).3 Involvement of physicians in these cases is prudent because veterinarians should not make specific recommendations for preventing or diagnosing disease in people.
Also advise owners of pets with MRSP and MRSA infections that, for most pets, the prognosis for cure is good with appropriate therapy and monitoring and, in cases of recurrent skin or ear infections, if the underlying cause is identified and addressed (Figure 5).4,5
Systemic antibiotics are not recommended to eliminate colonization. Topical antimicrobial shampoos and conditioners (i.e. those containing chlorhexidine) may be helpful but have not been specifically studied in animals. In one study in people, chlorhexidine bathing and intranasal mupirocin application in a 16-bed intensive care unit caused a 48% decrease in MRSA colonization and infection.6 However, topical use of mupirocin in the nasal cavity is unlikely to be successful as monotherapy in MRSA-colonized pets—as studies have shown identical MRSA carriage from the nares, mouth, anus, groin, and head.7
Mucosal carriage of S. pseudintermedius was shown to be significantly decreased in healthy dogs when fusidic acid was applied to the eyes, nostrils, anus, and vulva.8 However, in the absence of any controlled studies on spontaneous decolonization and long-term efficacy of antibacterial therapy for decolonization, it remains controversial whether decolonization of animals is necessary or warranted. If this measure is under consideration, it should involve collaboration between and advice from veterinary and medical infection-control experts and would need to include all in-contact people, in-contact animals, and their environments.1
Routine infections. Localized methicillin-resistant infections may be treated with topical medications such as chlorhexidine sprays or flushes, fusidic acid, or mupirocin applied twice daily until resolution. In cases of generalized pyoderma caused by methicillin-resistant staphylococci, always use aggressive topical antimicrobial therapies; in some cases, frequent (every one to two days) antibacterial (i.e. chlorhexidine) shampoos or conditioners and twice-daily antibacterial sprays (chlorhexidine, hypochlorous acid [Vetericyn—Innovacyn Inc.]) can resolve infection. A recent study comparing in vitro efficacy of antimicrobial shampoos found chlorhexidine to be superior in killing bacteria compared with benzoyl peroxide, ethyl lactate, and chloroxylenol.9
Refractory or severe infections. In refractory or severe pyoderma cases, systemic antibiotic therapy is used in combination with topical therapies (Figures 6A & 6B). Because of the variability of methicillin-resistant isolates, antibiotic choice should always be based on in vitro antibiotic susceptibility testing.10 Never treat methicillin-resistant infections with beta-lactam antibiotics (penicillins and cephalosporins), even if in vitro testing implies susceptibility. This is because methicillin resistance means resistance to all beta-lactams, but laboratory reporting errors can occur and erroneously imply sensitivity where it does not exist.11 For superficial pyoderma (whether methicillin susceptible or methicillin resistant), antibiotics are required for at least three weeks (one week beyond complete healing); for deep pyoderma, antibiotics may be needed for four to eight weeks or longer (or two to three weeks beyond complete healing) (Figures 7A & 7B).
Rechecks every two to four weeks are important to evaluate response to treatment, make treatment modifications if needed, and evaluate when antimicrobial therapy can be stopped. Depending on the bacterial strain, antibiotics that may be effective in methicillin-resistant infections include
Use of fluoroquinolones for the treatment of methicillin-resistant pyoderma is not recommended except as a last resort since, with the exception of moxifloxacin, they have the potential to select for high-level methicillin-resistant mutants (which are not only resistant to fluoroquinolones but also to other antibiotics).12,13 If fluoroquinolones are used, then veterinary-labeled products (which have near complete bioavailability)14 are recommended to be used at the high end of the label dose range.15
Use of ciprofloxacin may lead to treatment failure because of inconsistent absorption of this drug in dogs (in one study the oral absorption of ciprofloxacin in dogs varied from 28% to 98%),16 thus it is not recommended. Additionally, the use of vancomycin (administered intravenously only since oral vancomycin is not systemically absorbed) or linezolid for treating methicillin-resistant infections in animals is controversial and not recommended, as these drugs are often the last resort in human medicine.1
Finally, drugs for treating MRSA in human medicine include the streptogramins (quinupristin and dalfopristin), daptomycin, tigecycline, and ceftaroline fosamil; all are administered intravenously, and their use in animals has not been reported.17
Pets treated for a methicillin-resistant infection in a hospital environment must be treated as infectious and isolated from the general hospital population. Do not allow outpatients with suspected or known methicillin-resistant infections to contact other patients in the waiting room, but immediately usher them into an examination room. Use gloves and gowns or dedicated laboratory coats when handling the animals or any in-contact items such as bowls or bandages. Pens and stethoscopes must also be dedicated to the patient. Use disposable thermometer covers, or discard digital thermometers after the animal is discharged.
In people, the most critical step for reducing MRSA transmission is hand hygiene18 ; frequent hand washing was shown to reduce MRSA colonization in equine veterinarians,19 and the same likely applies to small-animal veterinarians. Perform hand hygiene, whether done by hand washing or alcohol-based hand sanitizers, before patient contact, before aseptic procedures, after contamination of the hands, after removing gloves, and after patient contact.20 MRSA can survive up to months on inanimate surfaces, depending on environmental conditions,21 and long-term survival of MRSP is also likely, as MRSP was isolated over a six-month period from household environmental sites and, in some households, beyond resolution of MRSP infection in the pet.22
After discharge, just as with any patient, disinfect all examination room tables, floors, door and sink handles, light switches, scale surfaces, cage items, and medical equipment used on animals with methicillin-resistant infections. Use appropriately diluted disinfectants allowed to contact surfaces for the time listed on the product label (typically five to 10 minutes or longer), after removal of any organic debris that could potentially inactivate disinfectants. Staphylococci, including methicillin-resistant staphylococci, are susceptible to most commonly used disinfectants.20
The importance of environmental cleaning was highlighted by the finding that MRSA carriage in a kennel of rescue dogs resolved spontaneously with regular kennel cleaning alone.23 The development of a good general infection control program in the hospital (rather than focusing solely on methicillin-resistant staphylococci) is probably the most important factor for reducing methicillin-resistant staphylococcal transmission20 (see "Avoid the spread: Methicillin-resistant staphylococcal infection control" below in this article for helpful guidelines).
Methicillin-resistant infections in pets are an increasing problem in veterinary medicine and are driven by antibiotic pressure. In patients with recurrent skin and ear infections, evaluation and treatment of the underlying disease is essential to reduce the need for antibiotic treatment of secondary infections. Increased use of topical antiseptics, basing treatment decisions on bacterial culture and antimicrobial susceptibility testing as much as possible, and clearly communicating with clients about the need for full treatment compliance may help reduce selection pressure.20 Additionally, veterinarians must ensure, via frequent hand sanitation and effective hospital infection control programs, that the risk of interpatient spread of infection or colonization is minimized. Also see Checklist: Environmental control of infectious disease  to download a list of measures you can take to precent the spread of infectious disease in your practice.
Avoid the spread: Methicillin-resistant staphylococcal infection control
Useful recommendations for methicillin-resistant infection management can be found in the following online resources:
Kimberly S. Coyner, DVM, DACVD
Dermatology Clinic for Animals of Las Vegas
5231 W. Charleston Blvd.
Las Vegas, NV 89146
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3. Manian F. Asymptomatic nasal carriage of mupirocin-resistant, methicillin-resistant Staphylococcus aureus (MRSA) in a pet dog associated with MRSA infection in household contact. Clin Infect Dis 2003;36(2):e26-28.
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10. Bemis DA, Jones RD, Frank LA, et al. Evaluation of susceptibility test breakpoints used to predict mecA-mediated resistance in Staphylococcus pseudintermedius isolated from dogs. J Vet Diagn Invest 2009;21(1):53-58.
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12. Dalhoff A, Schubert S. Dichotomous selection of high-level oxacillin resistance in Staphylococcus aureus by fluoroquinolones. Int J Antimicrob Agents 2010;36(3):216-221.
13. Venezia RA, Domaracki BE, Evans AM, et al. Selection of high-level oxacillin resistance in heteroresistant Staphylococcus aureus by fluoroquinolones exposure. J Antimicrob Chemother 2001;48(3):375-381.
14. Papich MG, Riviere JE. Fluoroquinolone antimicrobial drugs. In: Riviere JE, Papich MG, eds. Veterinary pharmacology and therapeutics, 9th ed. Ames, Iowa: Wiley-Blackwell Publishing, 2009.
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18. Pittet D, Hugonnet S, Harbarth S, et al. Effectiveness of a hospital-wide programme to improve compliance with hand hygiene. Lancet 2000;356(9238):1307-1312.
19. Anderson ME, Lefebvre SL, Weese JS. Evaluation of prevalence and risk factors for methicillin-resistant Staphylococcus aureus colonization in veterinary personnel attending an international equine veterinary conference. Vet Microbiol 2008;129(3-4):410-417.
20. Weese JS. Staphylococcal control in the veterinary hospital. Vet Dermatol 2012;23(4):292-298.
21. Kramer A, Schwebke L, Kampf G. How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infect Dis 2006;6:130.
22. Laarhoven LM, de Heus P, van Luijn J, et al. Longitudinal study on methicillin-resistant Staphylococcus pseudintermedius in households. PLoS One 2011;6(11):e277788.
23. Loeffler A, Pfeiffer DU, Lindsay JA, et al. Lack of transmission of methicillin-resistant Staphylococcus aureus (MRSA) between apparently healthy dogs in a rescue kennel. Vet Microbiol 2010;141(1-2):178-181.