Clinical appearance of recurrent bacterial pyoderma can vary by patient

Although recognizing a bacterial pyoderma in a canine patient is relatively easy, eradicating the pyoderma can be difficult.

"Motheaten" appearance of a bacterial pyoderma in a Dachshund.

In some patients, underlying diseases such as allergy, hypothyroidism and Cushing's disease are present which perpetuate the ongoing infection. Yet, in other cases, resolution of the pyoderma is as simple as using the appropriate antibiotic for long enough periods of time.

"Pyo," the prefix for pus and "derma" meaning skin is a word used to clinically describe a bacterial infection of the skin. In the canine, Staph intermedius, is the species of bacteria most commonly causing bacterial infections. It exists as part of the normal skin flora yet becomes a problem when there are changes in the patient's environment, ectoparasite infections, or underlying diseases that may encourage the overgrowth of the organism either by attracting the bacteria to the skin (atopy) or immunosuppression (hypothyroidism, Cushing's disease, diabetes mellitus).

Diagnosis

Most of us have seen our share of bacterial pyoderma in our canine patients and certainly the most common method of diagnosis is by clinical appearance and response to therapy.

Clinical appearance of the lesions can vary with the patient, some having intact pustules, others with large superficial circular lesions surrounded by crusting (epidermal collarettes), or a motheaten appearance.

Many of our clients are convinced their pet has "ringworm" because of the circular nature of the lesion, yet the majority of these lesions are, in fact, bacterial in origin. Dermatophytosis or "ringworm" can certainly be ruled out by performing a fungal culture. If an intact pustule is present, cytology should be performed to confirm the diagnosis of bacterial pyoderma. A small amount of pus is placed on the microscope slide, the slide is heat fixed then stained with Dif-Quik and observed under an oil immersion lens. Neutrophils with either intracellular or extracellular cocci will be present and in severe infections, a few acantholytic cells may be seen. Culture and sensitivity is not usually necessary unless the patient is not responding to appropriate antibiotics or if demodicosis is present.

Pustules on the abdomen of a Dachshund with bacterial pyoderma.

A skin biopsy should be considered particularly in older patients (especially Golden Retrievers) if you are treating what you feel is a bacterial infection appropriately, yet not making any progress. Epitheliotropic lymphoma can appear clinically similar to a pyoderma, yet the patient will not respond to antibiotics and continue to get worse. Pemphigus foliaceus can also present with pustules and one of the difficulties for the veterinarian and pathologist is differentiating between a bacterial pyoderma and pemphigus. Usually in pemphigus, a fever is present and on cytology or skin biopsy, numerous acantholytic cells are present.

Antibiotic therapy

The best case scenario for emergence of a bacterial pyoderma following antibiotic therapy is that the patient was not treated long enough with appropriate antibiotics or that steroids were used in conjunction with antibiotics because the patient was pruritic. (Pruritus varies among patients with bacterial pyoderma.) Antibiotics that have no role against Staph intermedius include: ampicillin, amoxicillin, and tetracycline. Antibiotics that are effective against this organism include the sulfonamides, cephalosporins, macrolides, amoxicillin with clavulanic acid, and the quinolones. The choice of which to use depends on if this is a first-time pyoderma, the patient's gastrointestinal tolerance, breed, and cost to the owner. Whichever appropriate antibiotic is selected, it should be used until total clearing of the lesions plus an additional week. Seldom is seven, 10 or 14 days of antibiotic adequate in eradicating the infection. An antibacterial shampoo should also be used at least once weekly in conjunction with the antibiotic. Resist the temptation to use steroids, however small the dose, in the treatment of bacterial pyodermas. Even though steroids tend to make everything "look" better, they will ultimately make the patient worse as even small doses in some patients can cause immunosuppression. If you are uncertain as to whether the bacterial infection accompanies atopy, treat the pyoderma first, then if pruritus continues to be present, perform a work-up for atopy.

Underlying problems

Underlying allergy such as flea allergy, atopy, or food allergy can be the reason for recurrent pyoderma in some patients. With the new adulticide flea preparations, flea allergy is more easily treatable than in the past. Usually bacterial pyodermas associated with flea allergy dermatitis have lesions confined to the groin, rear legs, or dorsal lumbar area. Recurrent pyoderma that occurs seasonally initially but progresses to nonseasonal in a young patient of an atopic breed may signal underlying atopy. Atopic breeds include retrievers, terriers, herding breeds, Dalmatians, German Shepherds, Setters, and Spaniels. Additional clues to the presence of atopy usually include the symptoms of face rubbing and foot chewing. Food allergy can occur at any age and symptoms of bacterial pyoderma will most likely be nonseasonal since the patient is eating the offending food daily all year round.

Patients with endocrinopathies such as hypothyroidism and Cushing's disease may present with recurrent bacterial pyoderma.

In hypothyroidism, alterations of the skin's normal lipid structure may compromise the patient's skin immunity whereas in Cushing's disease, the excessive cortisol compromises skin immunity, resulting in a bacterial infection. In some patients with Cushing's disease, large pus-filled circular outwardly expanding Staph lesions are suggestive of this disease. The appearance of these lesions is the same no matter if iatrogenic or naturally occurring Cushing's disease is the cause. Again, remember some patients are exquisitely sensitive to steroids and even small doses will cause or perpetuate the infection!

Large coalescing epidermal collarettes in a cockermix with Cushing's disease (also note diffuse hyperpigmentation and abnormal coat).

Bacterial hypersensitivity (BH) is uncommon, but should be considered in patients with recurrent pyoderma when the above differentials of allergy and endocrinopathies have been ruled out. Skin biopsies can also be supportive or suggestive of BH. Choices for treatment include injectable immunostimulants or pulse dosing with bactericidal antibiotics.

Recurrent pyoderma can be frustrating for both the veterinarian and the owner, but digging a bit deeper looking for underlying causes as those discussed above may be the clue to resolving the infection. Most importantly, avoid the use of steroids when treating a pyoderma, no matter how small the dose! The temptation to use steroids in a bacterial pyoderma is often elected due to the patient's pruritus. However using the appropriate antibiotic for a long enough period of time (varies for each patient) and antibacterial bathing will get you off on the "right foot" and not get you into the "steroid trap" which is often difficult to escape.