Management of pyoderma requires recognition of the type and depth of infection and, in many instances, identification of the pathogenic organism in order to make appropriate treatment recommendations. Treatment usually involves the use of systemic antibiotics, often accompanied by topical therapy. Treatment choices vary depending on whether the pyoderma is a first-time infection or whether it is recurrent in nature. Treatment choices also depend on whether the infection is focal or generalized, surface, superficial or deep.
General principles of antibiotic usage apply in all cases of bacterial skin infection. Antibiotic dosage should be based on the body weight of the animal and the full dosage administered. Length of antibiotic administration should be 7 days past clinical remission in uncomplicated infections and 10-14 days past clinical remission in complicated infections such as recurrent or deep infections and those associated with immunosuppression. This usually results in 3 to 5 weeks of antibiotics for superficial infections and 6 or more weeks for deep infections. Concurrent steroid use is contraindicated when treating skin infections. Corticosteroids will mask the clinical response, making the lesions look resolved when they are not. This may result in discontinuation of the antibiotic too soon, encouraging the development of recurrent infections. It will also interfere with diagnostic procedures. Last, many cases of pyoderma have an underlying cause such as allergy or endocrinopathy. Remember to look for an underlying cause for all cases of recurrent pyoderma.
Factors to consider when choosing an antibiotic
Choice of antibiotics will depend on many factors. These include the type of infection, depth of infection, the cost of the antibiotic, route and frequency of administration. Potential side effects of the drug need to be taken into account. In addition, the impact that the antibiotic may have on the normal flora may also need to be considered.
Empirical antibiotic selection
Antibiotics may be chosen without prior culture and susceptibility in a number of situations. Specifically, antibiotics may be empirically chosen in uncomplicated superficial skin infections, in recurrent infections when the previous antibiotic choice successfully cleared the infection, and in deep infections pending culture and susceptibility results.
Because Staphylococcus pseudintermedius (formerly known as S. intermedius) is the most frequent organism isolated in canine skin infections, antibiotics chosen should have a known spectrum of activity against Staphylococcus spp. Antibiotics that should be avoided for empirical treatment of pyoderma include penicillin, ampicillin, amoxicillin, and tetracycline.
Antibiotic selection based on culture and susceptibility testing
Cultures should be obtained from all cases of deep pyoderma, skin infections that fail to respond to empirical treatment, infections in dogs on immunosuppressive medications, infections in dogs with prior exposure to many classes of antibiotics (eg. recurrent pyodermas), and lesions in which intracellular rod bacteria are identified cytologically.
First-line antibiotics are those antibiotics that may be chosen empirically or based on culture and susceptibility that target Staphylococcus spp. primarily with minimal impact on other bacteria. Antibiotics included here are erythromycin, lincomycin, and clindamycin. These antibiotics have a narrow spectrum of action and are considered bacteriostatic for most bacteria. They are generally effective when chosen empirically to treat first-time skin infections; however, repeat exposure to these antibiotics results in the development of resistance. In addition, cross-resistance occurs among these three antibiotics. Therefore, use of these antibiotics to treat recurrent infections should be based on culture and susceptibility results.
Potentiated sulfonamide antibiotics (trimethoprim-sulfonamide; ormetoprim-sulfamethoxine) may also be considered as first-line antibiotic choices but their use should be avoided when long-term administration is required. Potentiated sulfonamides are bactericidal and demonstrate good efficacy against Gram-positive bacteria. Bacterial resistance to this class of antibiotics is quite variable, ranging from 0 to 33%; therefore, use of this antibiotic based on culture and susceptibility results may be more appropriate. Side effects in dogs associated with potentiated sulfonamides include hypothyroidism, keratoconjunctivitis sicca, neutropenia, hepatopathy and polyarthritis.
High levels of resistance to tetracyclines exist in S. pseudintermedius; therefore, this class of antibiotics cannot be regarded as a good empirical choice. Yet, doxycycline has been used effectively to treat bacterial skin infections caused by susceptible strains. Therefore, because of its narrow spectrum of action, doxycycline would be a good first-line antibiotic choice based on culture and susceptibility results.
Many people use first-generation cephalosporins (cephalexin, cefadroxil) as first-line antibiotics. These antibiotics are bactericidal and have a broad spectrum of action but primarily target gram positive bacteria. Resistance to this class of antibiotics is only now being identified. Ideally, use of these antibiotics should be reserved for those cases in which culture and susceptibility indicate they are the antibiotic of choice. For cases in which there have been multiple antibiotic exposures (e.g. recurrent infections) and success of other antibiotics is questioned, then first-generation cephalosporins can be used empirically.
Two third-generation cephalosporins, cefovecin and cefpodoxime proxetil, have recently been registered in the USA and Europe. Cefovecin is a long-acting injectable antibiotic which lasts 14 days and cefpodoxime proxetil is an oral antibiotic that can be administered once daily. While both drugs offer good activity against staphylococci, their activity against S. pseudintermedius is not superior to first-generation cephalosporins. In addition, they are active against a wide range of Gram-negative bacteria. Therefore, their use has the potential for selection of both methicllin resistance in staphylococci and multi-resistant E. coli. In spite of their convenient dosing, these drugs should only be used as first-line antibiotics if compliance is anticipated to be a problem.
Amoxicillin with clavulanate is also a broad-spectrum bactericidal antibiotic that primarily targets Gram-positive bacteria. It, too, should be reserved for those cases of pyoderma in which culture and susceptibility indicate it is the preferred antibiotic. In addition, it can be used empirically similar to first-generation cephalosporins.
Second-line antibiotics should be based solely on culture and susceptibility results. Antibiotics included here include chloramphenicol, rifampin, and amikacin. Chloramphenicol is a bacteriostatic, narrow spectrum antibiotic. While it meets the criteria to be a first-line antibiotic, it is reserved for treatment of methicillin resistant S. pseudintermedius which are sometimes only susceptible to this antibiotic. Chloramphenicol is administered three times daily. In addition to this inconvenient frequency of administration, it often causes gastrointestinal disturbances, is associated with drug interactions, and may cause bone marrow suppression.
Rifampin is a bactericidal antibiotic with excellent tissue penetration. It has a broad spectrum of activity against many Gram-negative and most Gram-positive microorganisms and is the most active antibiotic known against staphylococci. Resistance to rifampin readily develops with monotherapy; therefore, it is best to use in combination with another antibiotic such as clindamycin or cephalexin. Rifampin is potentially hepatotoxic and this side effect appears to occur more commonly in dogs than in people. Mild increases in alkaline phosphatase activity occur frequently and appear to be benign; however, treatment should be discontinued if there are concurrent increases in other hepatic enzyme activities. Other rare signs associated with rifampin administration in dogs include thrombocytopenia, hemolytic anemia, anorexia, vomiting, diarrhea, and death.
Amikacin is an aminoglycoside that is not typically considered for treating dogs with skin infections. It is an injectable antibiotic and is nephrotoxic. Therefore, use of this antibiotic is only based on culture and susceptibility results when no other antibiotic would be effective.
Third-line antibiotics should be chosen last because of the pressure they place on bacteria in terms of selecting for antimicrobial resistance. These include the third-generation cephalosporins (see above) and fluoroquinolones. While staphylococci often are susceptible to fluoroquinolones, resistance develops rapidly. In addition, fluoroquinolone use appears to select for methicillin resistance. Use of this class of antibiotics should be preserved for cases of deep infections associated with Gram-negative organisms.
Topical therapy may be used as both an adjunct to systemic therapy or, in some cases, as the sole therapy for cutaneous skin infections. Topical therapy includes whirlpool baths, especially for dogs with deep pyodermas, and antibacterial shampoos. Chlorhexidine, benzoyl peroxide, and ethyl lactate containing shampoos all have demonstrated beneficial responses in dogs with infections. Bathing should be done two to three times per week with a 10 minute contact time. In a study using topical therapy alone to treat superficial pyodermas, 50% of the cases were treated effectively when bathed three times per week. In cases of recurrent or resistant infections, a topical chlorhexidine spray is very beneficial when used once to twice daily.
Focal lesions can be treated with chlorhexidine spray, mupirocin ointment, benzoyl peroxide gel, or fusidic acid (not available in the USA).
Duration of therapy
Length of therapy depends of the depth of infection and is determined by clinical cure. For superficial infections the average duration of therapy is 3 to 4 weeks with treatment continued 1 week past clinical cure. For recurrent superficial infections the average duration of therapy is a bit longer and is continued for 10 to 14 days past clinical cure. Deep infections require 6 to 12 weeks of antibiotics with treatment continued 2 weeks past clinical cure. If lesions recur within 1 week after discontinuing therapy, it is likely that treatment was not long enough.
In order to determine if clinical cure is achieved, the dog should be re-examined before the antibiotic course is completed. Discontinuation of therapy too soon leads to the selection of resistant bacteria. A common reason why pyodermas fail to respond to treatment or recur is if the length of therapy is too short.
Reasons for treatment failure
Pyoderma may fail to respond to treatment if the bacteria are resistant or develop resistance to the chosen antibiotic. This may occur because an inappropriate antibiotic was chosen initially (e.g. amoxicillin) or resistant bacteria were selected during the antibiotic course. As stated before, treatment failure also occurs if the antibiotic course was not long enough.
Management of methicillin resistant staphylococcal infections
An infection caused by methicillin resistant staphylococci is managed similar to management of any pyoderma. An antibiotic is chosen based on culture results. Staphylococci exhibiting resistance to methicillin or oxacillin are also resistant to other penicillins, cephalosporins, and amoxicillin/clavulanate despite apparent in vitro susceptibility. Not all methicillin resistant staphylococci are multi-drug resistant. Typical antibiotic choices may include clindamycin, chloramphenicol, doxycycline, minocycline, amikacin, and rifampin. Unfortunately, antibiotic choices are becoming limited for some of the methicillin resistant Staphylococcus pseudintermedius (MRSP). We have managed some cases with aggressive topical therapy as the sole treatment. Use of the antibiotics reserved for MRSA in people, such as vancomycin and linezolid, is ethically questionable.
Staphylococcus pseudintermedius has been shown to colonize people temporarily, yet human infections with this organism are rarely reported. Since Staphylococcus pseudintermedius is not a common human pathogen, the primary recommendation to owners involves frequent handwashing after handling their pets. There is a potential increased risk to immunocompromised members of the household, but this is not known.
It is likely that other pets in the household will be colonized with the same resistant bacteria; therefore, pets with infections from households with known MRSP should always be cultured rather than empirically treated.
Reasons for recurrence
Common reasons for recurrent pyodermas include too short a duration of therapy, inappropriate dose of antibiotics, and concurrent glucocorticoids. If lesions recur within 1 week after discontinuing therapy, it is likely that treatment was not long enough. If the skin infection recurs after 3 to 4 weeks of discontinuing therapy, then there is likely an underlying condition predisposing the patient to infections that needs to be addressed. Common causes of recurrent infections include allergies and endocrinopathies. Occasionally, an underlying cause is not identified.
Managing idiopathic recurrent infections
Management of idiopathic recurrent pyoderma will depend on the frequency and severity of the infections. If the infections only occur a few times per year, then each episode should be treated with antibiotics as described above.
In some cases aggressive topical therapy can increase the interval between pyoderma episodes. Owners can bath their dogs with antibacterial shampoos containing chlorhexidine, ethyl lactate, or benzoyl peroxide three times per week at the first sign of the infection returning. In addition, chlorhexidine spray can be applied to the infected areas.
In cases where the infection is severe, the goal may be to prevent recurrence of the infection. This can be attempted using pulse antibiotic therapy. The antibiotic is administered until the infection is cleared and then is given either at weekly intervals or every other day to prevent the infection from returning. Because of the increasing incidence of MRSP, pulse therapy is unlikely to be effective because it will select for resistant staphylococci.
Immune modulation with bacterins has been used to manage some cases of idiopathic recurrent pyodermas. Bacterins contain various cellular products of staphylococci or other bacteria and are available commercially as Staphage Lysate (Delmont Laboratories) and ImmunoRegulin (Neogen Vet). In addition, some microbiology laboratories will make autogenous bacterins from the patient's staphylococci. Bacterins will not clear an existing infection and must be started concurrently with antibiotics. Once the infection is cleared, the antibiotics are discontinued while the bacterin is continued. If the infection recurs, then a complete course of antibiotics will again be needed.