What tests should a microbiological laboratory perform?
The laboratory will identify the micro-organism and perform appropriate antibiotic testing. It is advised that S. aureus is discriminated from other coagulase-positive staphylococci for two main reasons; S. aureus bacteria have zoonotic implications, and antibiotic sensitivity breakpoints differ between S. aureus and S. pseudintermedius. Recently published guidelines 11 recommend initial antibiotic testing should include erythromycin, clindamycin, amoxicillin-clavulanate, tetracycline (for testing susceptibility to doxycycline), trimethoprim-sulfamethoxazole, gentamicin, cephalothin (or cefazolin, as a first-generation cephalosporin), cefpodoxime proxetil (as a third-generation cephalosporin) and enrofloxacin. Oxacillin is included to detect meticillin resistance in S. pseudintermedius. Inclusion of other fluoroquinolones (difloxacin, marbofloxacin and orbifloxacin) may be considered if enrofloxacin is not the fluoroquinolone of choice. The results should be compared with breakpoints as defined by the Clinical and Laboratory Standards Institute* (CLSI). Antibiotics with “intermediate susceptibility” should be reported as resistant, as they are unlikely to achieve therapeutic concentrations in the affected sites 11. Finally, the D-zone test for inducible clindamycin resistance is performed if in vitro results reveal resistance to erythromycin and susceptibility to clindamycin, as 2% of clindamycin inducible resistance was reported in MRSP 9. If a meticillin-resistant staphylococci is identified, additional testing susceptibility for amikacin, chloramphenicol, minocycline and rifampicin can be performed by the laboratory 11.
*The CLSI standards include information from the Subcommittee on Veterinary Antimicrobial Susceptibility Testing and the European Committee on Antimicrobial Susceptibility Testing.
How is S. pseudintermedius pyoderma treated?
Systemic therapy is frequently employed for the treatment of canine superficial and deep pyoderma. Before starting antibiotic therapy it is important to determine if the pyoderma is deep, severe or/and generalized enough to require systemic antibiotics 13. The treatment of MRSP and MSSP follows the same basic principles, with recognition of the pathogen and susceptibility pattern 19. Patient factors, such as the underlying cause, immunosuppression and concurrent disease, all need to be addressed. Owner compliance and drug availability, cost, and side effects should also be taken in consideration. Some drugs may be unlicensed for animal use in certain countries, and if off-label use is proposed the clinician should first discuss implications with the owner.
A recent systematic review identified good evidence for high efficacy of subcutaneously injected cefovecin in superficial pyoderma and for oral amoxicillin-clavulanate in deep pyoderma 20. A fair level of evidence was identified for moderate to high efficacy of oral amoxicillin-clavulanate, clindamycin, cefadroxil, trimethoprim-sulphamethoxazole and sulfadimethoxine-ormetoprim in superficial pyoderma and oral pradofloxacin, oral cefadroxil and subcutaneously injected cefovecin in deep pyoderma 20. A recent publication provides clinical guidelines for the diagnosis and treatment of canine superficial bacterial folliculitis 11.
How is first-occurrence superficial pyoderma/folliculitis treated?
A first occurrence of superficial pyoderma/folliculitis can be treated empirically or after bacterial culture and sensitivity. The recommended antibiotics for empirical use are amoxicillin-clavulanate, cefadroxil/cefalexin, clindamycin, lincomycin, trimethoprim- or ormetoprim-sulfonamides, and these options are licensed for veterinary use in most countries 11. If compliance is poor, cefovecin and cefpodoxime proxetil can also be considered for first occurrence pyoderma. It is important to keep in mind that these latter antibiotics have a broader spectrum of activity, including some gram-negative bacteria, and should only be used when appropriate and after culture and sensitivity tests 13.
How should MRSP be treated?
Systemic antibiotic options for MRSP or multi-drug resistant staphylococci are more limited. It is recommended that suitable drugs are selected after culture and susceptibility and when there are no alternatives. When choosing a treatment plan, it is important to consider that there is a risk that further resistance of the infective strain may develop 4. Another consideration is that MRSP can be treated only with diligent topical therapy. The drugs available for MRSP are tetracyclines (e.g., doxycycline and minocycline), fluoroquinolones (e.g., enrofloxacin, marbofloxacin, orbifloxacin, pradofloxacin and ciprofloxacin), chloramphenicol, rifampicin and aminoglycosides (e.g., gentamicin and amikacin). The use of drugs such as linezolid, teicoplanin, or vancomycin is strongly discouraged, regardless of the susceptibility, as these drugs are reserved for the treatment of serious MRSA infections in humans 11.
Some of the drugs used for MRSP have potentially serious side effects. Chloramphenicol is a bacteriostatic antibiotic which must be handled with gloves due to possible irreversible aplastic anemia in humans. Side effects in the dog include vomiting, hepatic toxicity and (reversible) bone marrow suppression. More recently, hind limb weakness has been also reported 21. Aminoglycosides can cause nephrotoxicity and ototoxicity and are best avoided in animals with renal insufficiency. Monitoring of renal function for prevention of aminoglycoside-induced acute kidney injury is advised**. Rifampicin can cause hepatoxicity and requires hepatic function monitoring before starting therapy and then at weekly intervals during treatment; other side effects include anemia, thrombocytopenia, anorexia, vomiting, diarrhea and orange discoloration of body fluids. It has been reported for S. aureus that rifampicin resistance can be prevented by association with certain antibiotics like clindamycin and cefalexin. It is unknown if this also occurs with MRSP, since development of resistance has been reported even with association with another antibiotic 22.
** According to the International Renal Interest Society (IRIS) guidelines (www.iris-kidney.com).
The recommended drugs and dosages for treating superficial folliculitis are shown in Table 2. Deep pyoderma with extensive scarring and necrosis may limit drug penetration in the tissues, therefore antibiotics that penetrate sites of inflammation such as clindamycin, cefovecin and fluoroquinolones can be used in these cases 13. In general, for uncomplicated superficial pyoderma, therapy is given for 3-4 weeks plus one week after clinical resolution. In recurrent cases, deep pyoderma or concomitant immunosuppression, treatment should be given for 6-8 weeks plus 10-14 days after clinical resolution. Failure to diagnose and control the underlying cause can also prevent complete resolution of the infection and predispose to future infections. Longer treatment regimes might be necessary for MRSP in many cases 23. Re-checks are usually rescheduled every 2-4 weeks until clinical remission is achieved.
Table 2. Recommended antibiotics and dosages for superficial bacterial folliculitis in the dog 11.
Topical therapy – does it help?
Topical treatment for pyoderma hastens recovery and/or reduces the need for systemic therapy. Topical agents may be the only treatment required in some cases, or can be adjunctive to systemic antibiotics. Topical products may be divided into antimicrobial products and topical antibiotics; both may be used for generalized or localized lesions.
Topical antibacterials include chlorhexidine, benzoyl peroxide, ethyl lactate and sodium hypochlorite based products. 2-4% chlorhexidine concentration has been reported to be effective as a sole therapy, and chlorhexidine shampoo revealed more efficacy when compared to benzoyl peroxide shampoo 24. These products can be used in the form of shampoos, conditioners, sprays, wipes or diluted in the bath water. No biocide resistance has been reported for chlorhexidine in MRSP 25. For localized lesions, other topical anti-bacterial alternatives include honey-based ointments, which have an antibacterial effect against MSSP and MRSP 26. Nisin is an antimicrobial peptide, available as wipes to treat localized pyoderma and bacterial surface colonization 27.
When necessary, topical antibiotics can be used for focal lesions. They include fusidic acid, silver sulfadiazine, gentamicin, fluoroquinolones and mupirocin, and may be useful even when resistance is reported by the laboratory. Fusidic acid is a concentration-dependent antibiotic and high concentrations can be achieved locally, and may be an effective option for MRSP even when in vitro testing reveals non-susceptibility. Mupirocin is used for topical nasal infection and decolonization of MRSA in humans, but some countries restrict its use in animals.
What are the zoonotic implications of MRSP?
With the emergence of MRSP there has been a renewed interest in zoonotic implications of S. pseudintermedius. It has been shown that nasal colonization can occur in humans, and owners with dogs affected by deep pyoderma can carry the same genetic MRSP strain that occurs in their pets, which supports inter-species transmission 28. Veterinarians in contact with infected animals also seem to have a higher risk of being MRSP nasal culture positive when they share the environment 29. Humans are not natural hosts for S. pseudintermedius, which explains the lower impact of MRSP compared to MRSA, but it is unknown if S. pseudintermedius strains containing mobile genetic elements could represent a reservoir for the spread of resistant genes to the human commensal skin flora 4.
How can MRSP dissemination in the practice be prevented?
Guidelines are available on how to maintain high standards of clinical practice and hygiene in order to reduce the risks of MRSA and MRSP and manage infected patients 30. Prevention of MRSP is based on responsible antibiotic use, strict hand hygiene and environment disinfecting measures. All surfaces and equipment must be effectively cleaned and disinfected between patients; if surfaces are soiled, detergent and water must be used first as soiling can compromise the efficacy of disinfectants. All surfaces should be easily cleanable (e.g., by using washable computer keyboards) and team involvement is crucial, with cleaning and disinfection procedures displayed at appropriate places and recording of the protocol tasks. One MRSP hospital outbreak has been reported with colonized and infected canine and feline patients 31. The report suggested that rigorous control measures are needed to control an outbreak, and recommends the implementation of a search-and-isolate policy and standard precautions including hand disinfection, barrier nursing, environment and clothing hygiene to prevent MRSP transmission between patients.
Small animal clinicians often encounter dogs with bacterial pyoderma, and most first occurrence cases can be treated empirically. However, an MRSP infection should be suspected if there is a poor response to previous antibiotherapy or other risk factors are present, and culture and antibiotic sensitivity should be performed as MRSP offers limited systemic antibiotic options. Topical treatment is advised as a sole or adjunctive therapy to systemic antibiosis to hasten recovery. MRSP has zoonotic implications and practices should implement protocols to avoid dissemination of this pathogen.