Canine pyoderma: the problem of meticillin resistance
This paper gives an overview of our current knowledge of meticillin-resistant Staphylococcus pseudintermedius (MRSP) as a causative pathogen of canine pyoderma and considers diagnosis of the disease, treatment options, prevention and zoonotic aspects.
Canine bacterial pyoderma is caused mainly by Staphylococcus pseudintermedius.
Meticillin-resistant S. pseudintermedius (MRSP) has a worldwide distribution. The bacteria is resistant to beta-lactam antibiotics and is also frequently resistant to other drugs commonly used to treat canine pyoderma.
Bacterial culture and antibiotic sensitivity are strongly recommended if MRSP is suspected.
Veterinary practices need to implement strict hygiene protocols to prevent dissemination of this pathogen.
Before the emergence of meticillin resistance, Staphylococcus pseudintermedius was susceptible to most of the antibiotic drugs available for animals. More recently, the bacterium has acquired genetic material and developed meticillin resistance; indeed, a multi-drug resistance pattern has emerged, limiting treatment options and highlighting the need for responsible antibiotic use. This paper gives an overview of our current knowledge of meticillin-resistant Staphylococcus pseudintermedius (MRSP) as a causative pathogen of canine pyoderma and considers diagnosis of the disease, treatment options, prevention and zoonotic aspects.
S. pseudintermedius – a pathogen?
Staphylococci bacteria are normal commensals of the skin and mucosae of healthy dogs, but they are also opportunistic pathogens. The most frequent clinical presentation of canine staphylococcal infections is pyoderma, followed by otitis externa. Staphylococcus pseudintermedius (formerly misidentified as S. intermedius) is the most common pathogen, and since 2007 has been classified within the S. intermedius group along with S. delphini and S. intermedius 1. Other coagulase-positive staphylococci that are considered pathogenic include S. aureus, S. hyicus and S. schleiferi subspecies coagulans. Coagulase-negative species, namely S. schleiferi subspecies schleiferi, have also been recognized as a cause of pyoderma 2.
What do we mean by meticillin resistance?
Meticillin (formerly known as methicillin) was introduced in 1959 and is a semi-synthetic penicillinase-resistant penicillin. The antibiotic was developed to overcome the resistance mediated by the enzyme beta-lactamase, which destroys the β-lactam ring of the penicillins. Meticillin resistance was first documented in S. aureus in 1961 3. Meticillin-resistant S. aureus (MRSA) evolved to produce a defective penicillin-binding protein mediated by the acquisition of the mecA gene; this is part of a larger mobile genetic element known as the “staphylococcal chromosomal cassette”, which can integrate into the staphylococcal chromosome. Nowadays, meticillin is no longer used clinically, and oxacillin has become the substitute for in vitro MRSA testing (Figure 1); resistance to oxacillin represents virtually total non-susceptibility to all β-lactams, including drugs that are commonly used to treat canine pyoderma 4 such as:
- Cephalosporins (e.g., cefalexin, cefpodoxime proxetil, cefovecin).
- Potentiated amoxicillins (e.g., amoxicillin-clavulanate).
- Penicillins (e.g., ampicillin, amoxicillin).
Cefoxitin can be used in human medicine to screen for MRSA, but it is not appropriate to determine non-susceptibility to β-lactams in S. pseudintermedius 5. Meticillin-resistant S. pseudintermedius (MRSP) was first reported in 1999 in North America and is now recognized as having a worldwide distribution 6,7,8. Referral practices, which commonly receive cases of pyoderma that are chronic or recurrent (and therefore have had previous antibiotic therapies) frequently report high levels of MRSP 6. Due to the zoonotic impact of MRSA, less attention has been paid to MRSP and meticillin-resistant S. schleiferi.
Is MRSP a challenge?
Historically, pyoderma has been treated empirically with beta-lactams, macrolides or potentiated sulfonamide antibiotics. The problem with MRSP is not only β-lactam resistance but also resistance to other antibiotics such as clindamycin, erythromycin, fluoroquinolones, gentamicin and tetracycline 9. The multi-resistant phenotype is associated with genetic changes due to transposable mobile elements which encode for antibiotic resistance 10. Two clonal MRSP lineages developed simultaneously in Europe and US with different resistant patterns; the North America clone is still susceptible to chloramphenicol, rifampicin and amikacin, whilst the European clone reveals susceptibility to fusidic acid and doxycycline/minocycline 9.
S. pseudintermedius with resistance to three or more classes of antibiotic is classified as a multidrug resistant staphylococcus, and it is therefore not advisable to empirically switch from one antibiotic class to another if treatment fails with the first-line antimicrobial. These cases should be cultured and sensitivity tested before a second antibiotic is prescribed 11. Differentiation between susceptible and resistant S. pseudintermedius strains based on the clinical picture is not possible, as MRSP is not more virulent compared to meticillin susceptible S. pseudintermedius (MSSP) 6.
How is pyoderma diagnosed?
It is possible to diagnose a pyoderma based on previous history and clinical signs. Minimal diagnostic tests include cytology, bacterial culture and antibiotic sensitivity testing. The differential diagnosis includes demodicosis and dermatophytosis and, more rarely, sterile pustular diseases. Other diagnostic procedures, such as skin scrapings, dermatophyte culture and histopathology, should be applied on a case-by-case basis.
S. pseudintermedius colonizes the skin and mucosa (nose, mouth and anal mucosa) of healthy dogs and around 80% of the infections originate from the patient carriage sites 12. Canine skin infections caused by S. pseudintermedius include superficial and deep pyoderma. The most common form of canine superficial pyoderma is bacterial folliculitis; typical lesions include small pustules and erythematous papules that are associated with hair follicles (Figure 2). Epidermal collarettes and target lesions are also frequently observed, whilst crusts, alopecia, erythema and hyperpigmentation may also be seen. In short-coated breeds the clinical presentation may be characterized by multifocal circular areas of alopecia (giving a “moth-eaten” appearance). Signs of deep pyoderma include hemorrhagic bulla, draining sinuses, ulcers, edema and severe inflammation (Figure 3). A hemorrhagic and/or purulent discharge may be observed with associated pain. It is crucial to distinguish between bacterial folliculitis and deep pyoderma; the latter is more penetrating, with hair follicle rupture and involvement of the dermis and subcutis, and therefore requires a longer treatment duration 13.
Cytology is a reliable, fast, and minimally invasive in-house test to confirm the presence of a bacterial infection. The presence of neutrophils with intracytoplasmic phagocytosed cocci confirms a pyoderma (Figure 4). Where there is deep pyoderma, the inflammatory pattern is characterized by the presence of degenerate neutrophils, macrophages and sometimes eosinophils. Rods can also appear in rare cases. Lack of micro-organisms in skin cytology does not rule out an infection, and whilst cytology is the first diagnostic test to be performed, it cannot replace bacterial culture or histopathology 14. Culture and sensitivity can be performed for any case, but is strongly recommended in the following situations:
- If clinical signs and cytological findings are not consistent with each other, e.g., if no microorganisms are seen with cytology but the clinical signs are still suggestive of pyoderma.
- If rod-shaped bacteria are seen on cytology, as antibiotic susceptibility for bacilli is difficult to predict.
- For any case of deep pyoderma, as it requires a longer treatment duration.
- Any life-threatening infection.
- If MRSP infection is suspected.
When should MRSP infection be suspected?
MRSP should be suspected if one or more risk factors (Table 1) are recognized 4,11,14,15,16. Clinicians should be aware that MSSP infections, confirmed by culture, can turn into MRSP infections during antibiotic treatment. This can either be due to transmission of genetic factors, or because although multiple clones of both MSSP and MRSP were present in the patient, only MSSP was cultured at the first attempt 17.
Table 1. Risk factors associated with MRSP.
How should material for bacterial culture be collected?
Several types of lesions can be cultured but contamination of the samples must be avoided. The skin should firstly be wiped with alcohol and left to air dry. Intact pustules, papules and furuncles are suitable lesions and can be carefully opened with a sterile needle before collecting the content with a sterile swab (Figure 5). If no intact lesions are present, it is still possible to collect material by swabbing an epidermal collarette or from under a recently formed crust. Recently, three sampling techniques (dry cotton swab, saline-moistened cotton swab and surface skin scraping) were reported to deliver similar results when used for bacterial culture 18. In the case of draining tracts, the lesion should be squeezed gently to collect fresh material. For nodular lesions, obtain material by puncturing the nodule with a needle and aspirating the content with a syringe. Skin biopsy is helpful to collect material from deeper tissues and can be done using a biopsy punch or a scalpel blade for a full-thickness wedge biopsy; this allows examination of subcutaneous or deeper tissues. The material is sent to a microbiological laboratory in a sterile container using appropriate transport media.
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.
|Primary choice for empirical therapy based on suspected sensitivity or if susceptibility proven by culture and sensitivity
|5.5-10 mg/kg PO q12H
|15-25 mg/kg PO q12H
|12.5-25 mg/kg PO q12H
|15-30 mg/kg PO q12H
|15-30 mg/kg PO q12H
|First or second tier
|Third generation cephalosporins
|8 mg/kg SC every 2 weeks
|5-10 mg/kg PO q24H
|Reserve to use after proven susceptibility and if first tier drugs are not an option
|5 mg/kg PO q12H or 10 mg/kg PO q24H
|10 mg/kg PO q12H
|5-20 mg/kg PO q24H
|2.75-5.5 mg/kg PO q24H
|3 mg/kg PO q24H
|Use after proven susceptibility; should be used with caution due to potential severe side effects
|40-50 mg/kg PO q8H
|15-30 mg/kg IV/IM/SC q24H
|5-10 mg/kg PO q12H
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.
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