Patients with staphylococcal skin infections are seen every day in small-animal general practice. However, recent developments in treatment options, along with concerns about emerging antibiotic resistance, are changing the way we diagnose and treat these conditions. These six key points highlight the current principles and practices:
1. Treating a staphylococcal infection is not enough. It is critical to search for the underlying cause of the infection and treat or prevent it.
Coagulase-positive staphylo-cocci are normal organisms on canine and feline skin. Infection occurs only in the presence of an underlying cause. It's important to explain to clients that the staphylococcal infection is not something that their pet contracted from the environment or another animal. Some physiologic or micro-environmental change had to occur in the skin to allow colonization and infection. Thus, particularly in the case of recurrent infections, it is critical to search for this underlying cause and treat or prevent it.
2. The choice of systemic antibiotic treatment is based on considerations of efficacy, safety, cost, and client compliance.
Other classes of antibiotics (e.g., macrolides, lincosamides, potentiated sulfa drugs) are generally associated with resistance rates of 10% to 30%, and resistance can develop rather rapidly with repeated use. Fluoroquinolone antibiotics, though effective against many staphylococcal infections, should be reserved for more unusual situations, such as Pseudomonas infections or organisms resistant to other drugs. Interestingly, chloramphenicol is making a comeback as an antibiotic useful for highly resistant strains of Staphylococcus. Because this antibiotic is, in my opinion, one of our last remaining hopes for some infections, it should never be used unless it is clear that no alternative exists, as documented by culture and susceptibility testing.
Based on all these factors, veterinary dermatologists generally consider cephalo-sporins the antibiotics of choice for staphylococcal skin infections (Table 1). These drugs combine high efficacy and safety with reasonable cost and relatively infrequent development of resistance.
3. Staphylococcal skin infections should be treated for at least one to two weeks past clinical resolution of the lesions.
Some practitioners advocate prolonged, continuous, or pulse treatment schedules for patients with recurrent infections, but their advisability has been questioned recently. Generally, visible lesions of staphylococcal infection disappear before the infection is fully cured. Superficial infections are usually treated for three to six weeks, depending on severity and patient response. Deeper infections (e.g., furuncles, deeper abscesses, draining tracts) usually require much longer treatment—six to 12 weeks is common. Because of these rather long treatment durations, clients may become weary of administering oral medications, particularly those that must be administered several times daily (usually in addition to multiple other recommended treatments such as shampoos and topicals).
Using once-daily products, such as cefpodoxime, may enhance compliance with the treatment regimen and unburden the pet owner. Recently available is a long-acting injectable cephalosporin, cefovecin. Each single subcutaneous injection of this aqueous product is equivalent to a two-week course of an orally administered cephalosporin in the treatment of superficial staphylococcal pyoderma.
4. Increasingly, staphylococcal skin infections may require a culture and sensitivity test prior to treatment.
The rule of thumb for when to culture includes resistance, response, and recurrence. A few years ago, culture and susceptibility tests were rarely necessary before treating staphylococcal skin infections—their susceptibility patterns were predictable, as nearly 100% of isolates were susceptible to a cephalosporin.
This situation has now changed. Emergence of resistant strains dictates that any animal with apparent resistance to treatment or inadequate response to treatment must be cultured. Likewise, a recurrent infection should always be cultured, especially if the animal has been treated frequently in the past with various antibiotics. The goal is not only to choose the correct antibiotic, but also to identify the presence of methicillin-resistant strains. If the laboratory reports a coagulase-positive Staphylococcus that is methicillin-resistant, order a staphylococcal speciation test.
This test identifies the particular strain of Staphylococcus present and if it is a human or animal strain. This information will allow you to take necessary isolation precautions and to advise the client about any zoonotic concern.