July 09, 2018

Antibiotic Selection in Pyoderma

Growing antibiotic resistance has complicated the treatment of bacterial skin conditions. Here’s what to consider when making therapeutic decisions for your patients.
By JoAnna Pendergrass, DVM
(Left) Draining tracts on the foot of a dog with pyoderma. (Right) Epidermal collarette.

Bacterial pyoderma is one of the most commonly diagnosed dermatologic diseases in dogs. Although β-lactam antibiotics such as cephalexin were once effective in treating pyoderma, the growing emergence of resistant bacteria, particularly methicillin-resistant Staphylococcus, has made treatment of this condition much more complicated. At the 2018 Midwest Veterinary Conference in Columbus, Ohio, Paul B. Bloom, DVM, DACVD, DABVP (Canine & Feline), discussed the diagnosis and treatment of pyodermas, with a particular focus on using a bacterium’s minimum inhibitory concentration (MIC) to select the proper antibiotic.

The frequency of antibacterial resistance in both human and veterinary medicine has been increasing. This increased resistance heightens the risk of spreading antibacterial resistance genes between animals and people, said Dr. Bloom, an adjunct assistant professor in the Department of Small Animal Clinical Sciences at Michigan State University College of Veterinary Medicine and a practitioner at the Allergy, Skin and Ear Clinic for Pets in Livonia, Michigan. Previous studies, for example, have reported the transfer of drug-resistant Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA), and methicillin-resistant Staphylococcus pseudintermedius (MRSP) between pets and people.1-4

Characterizing Pyodermas

Pyodermas are typically caused by a staphylococcus. In recent years, methicillin-resistant Staphylococcus—MRSA, MRSP, MRSS (schleiferi), MRSI (intermedius)—has been identified increasingly in canine pyodermas.

Superficial bacterial folliculitis (SBF), a superficial pyoderma, is characterized by such clinical signs as variable pruritus, multifocal alopecia, follicular papules or pustules, and epidermal collarettes. In addition, serous serocelluar crusts can be found on the trunk and abdomen, and in axillary regions. Short-haired breeds with SBF may present with a moth-eaten appearance. Notably, cocker spaniels with SBF can present with vegetative plaques that can be mistaken for seborrheic plaques.

Deep pyodermas are characterized typically by nodules, hemorrhagic bullae, and serosanguineous to purulent draining tracts. Deep pyoderma lesions can be found on the nose, chin, elbows, hocks, and interdigital spaces.

Diagnosing Pyodermas

Diagnosing bacterial pyodermas requires a systematic approach, which includes obtaining a thorough history and performing a dermatologic examination.

Obtaining a detailed history involves asking numerous questions about a dog’s condition:
  • When did the signs first occur? Have they progressed rapidly or gradually?
  • Has the dog been treated previously for skin or ear disease? If yes, what was used and for how long? What was the dog’s response?
  • Does the dog live indoors, outdoors, or both?
  • Is the dog pruritic? If so, did the rash develop first, or did the pruritus and rash begin simultaneously?
  • Compared with the first appearance of signs, is today’s clinical presentation better, worse, or about the same?
  • Are there other pets living in the household? If so, do they have skin problems?
  • What is the dog’s diet?
  • What medication is the dog currently receiving?

Dermatologic Examination
The dermatologic examination serves to identify primary lesions (eg, papules, pustules, nodules) and secondary (eg, ulcerations, crusts) lesions, detect ectoparasites, observe evidence of self-trauma, and determine hair coat quality and skin appearance. The exam should include an impression smear of a lesion, in which infectious agents, neoplastic cells, inflammatory cells, and pemphigus foliaceus–associated acantholytic keratinocytes can be evaluated. Skin scrapings for ectoparasites and flea combing are essential parts of a dermatology minimum database.

The patient history and results of the dermatologic examination form the basis for the differential diagnosis. This list is important, given that canine pyodermas are never primary diseases and are always “due to” something, Dr. Bloom noted. Potential differentials include hypersensitivities (eg, atopy), endogenous or exogenous steroid exposure, ectoparasites, hypothyroidism, and follicular dysplasias.

It is especially important to rule out demodicosis in dogs with lesions that are consistent with SBF. In addition, the presence of pustules requires ruling out pemphigus foliaceous.
(Top) Mini-tip culturette versus standard culturette. (Bottom) Demodex mites.

Culture and Susceptibility Testing
Culture and susceptibility (C/S) testing is an important diagnostic component and is recommended for poorly responsive SBF. It is also warranted at the first visit for the following scenarios:
  • Deep bacterial pyoderma with predominantly rods on cytology
  • Pyoderma accompanied by systemic illness
  • Recent antibiotic therapy

There are several ways to obtain samples for C/S testing. For example, mini-tip culturettes, rather than large swabs, are ideal for sampling from draining tracts and pustules. A papule, Dr. Bloom mentioned, can be sampled like a pustule in which the papule or pustule is ruptured with a needle and then the swab is touched carefully to the surface of the lesion. For patients with epidermal collarettes, he recommended swabbing just underneath the collarette’s edge. Because lesions can have bacterial strain heterogeneity, which can lead to different antibiotic susceptibilities, samples should be taken from 3 or 4 locations but all can be sampled with the same swab.

Sampling can be done while a patient is on antibiotics; to date, there is no empiric evidence to support waiting until the end of antibiotic therapy to obtain a sample for C/S testing. “When a client is there and you can do something,” Dr. Bloom emphasized, “go ahead and do it [right] then,” because clients may be too busy or may not see the importance of returning to the veterinary office for a recheck of the pyoderma, particularly if the signs have improved.

That said, if a dog is not currently on antibiotics, Dr. Bloom advised waiting until after culture results come back before starting antibiotic therapy, unless the dog has a fever or a life-threatening illness. Starting antibiotic therapy before the results come back could result in altered susceptibility patterns, thus complicating results.

The old method of C/S testing was the disk diffusion method (Kirby-Bauer method), which determines susceptibility (susceptible, intermediate, resistant) based on the zone of inhibition in a solid medium. This method, however, does not provide an accurate measure of how susceptible a bacterial strain is to a particular antibiotic.

The tube dilution method, which is quantitative, is the preferred method of C/S testing. Each test tube contains the same concentration of bacteria (105/mL) and serially increasing antibiotic concentrations (which vary among antibiotics); each tube contains double the antibiotic concentration as the previous tube. This method tells how susceptible a bacterial strain is to an antibiotic by reporting the MIC: the concentration of the first tube in which there is no visible bacterial growth.

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