27 Staphylococcal pyoderma
Pyoderma is one of the most common causes of skin disease in dogs and yet one of the most underdiagnosed, perhaps in part because the clinical signs are so variable between individuals, and because pyoderma can mimic almost any dermatosis. It is defined as a pyogenic infection of the skin but to some extent the definition is a misrepresentation, because in many cases there are no pustules and/or pus. The clinical signs include papules, pustules, follicular papules, crusting, comedones, epidermal collarettes, scaling, follicular casting, alopecia, erythema, excoriations, ulceration, hyperpigmentation and lichenification. The lesions seen depend on the depth of the infection and also on the duration of disease.
The majority of cases of canine pyoderma involve infection with resident bacteria, in particular, Staphylococcus intermedius, and arise because of an underlying systemic or cutaneous disease that renders the cutaneous microenvironment more favourable for bacterial proliferation and infection. Ectoparasitic disease, hypersensitivity disorders, endocrinopathies and primary scaling disorders are amongst the recognized underlying diseases. Some cases of canine pyoderma, however, are idiopathic. Pyoderma is classified as either surface, superficial or deep depending on the depth of infection. Surface infection is limited to the surface of the stratum corneum (Fig. 27.1), and includes intertrigo (skin fold dermatitis), pyotraumatic dermatitis (acute moist dermatitis) and some mucocutaneous pyodermas. Superficial infection involves the epidermis and/or the follicular infundibulum (Fig. 27.2), and includes impetigo, superficial folliculitis, superficial spreading pyoderma and some mucocutaneous pyoderma. Deep pyoderma involves all the portions of the hair follicle and the dermis (Fig. 27.3). This group includes deep folliculitis, furunculosis and cellulitis, German shepherd pyoderma and pyotraumatic folliculitis and furunculosis. In addition, localized deep infections are classified as: pedal pyogranulomas, or as nasal, muzzle, chin, callus and pedal folliculitis, or furunculosis. This case describes a case of superficial folliculitis, which is a common presentation in short-coated dogs and results in a characteristic moth-eaten appearance.
The level of pruritus in dogs with pyoderma varies from none to severe but dogs with an underlying allergic skin disease are usually pruritic. However, a ventral pyoderma in a young dog without a previous history of pruritus may be the first harbinger of the onset of atopic dermatitis. A history of a generalized scaling skin disease and possibly alopecia prior to the onset of more typical pyoderma lesions might suggest a primary seborrhoea or a genodermatosis such as colour dilution alopecia. The owner should be questioned regarding symptoms suggestive of systemic involvement. For example, lethargy and weight gain might suggest hypothyroidism, and polyuria, polydipsia and polyphagia should alert the clinician to the possibility of hyperadrenocorticism or diabetes mellitus. Owners may report that the dog’s skin is greasy and malodorous, especially when deep pyoderma is present.
Pyoderma commonly involves the ventral abdomen and groin, but a wide variety of cutaneous signs may be evident and it is important to thoroughly examine the entire integument. Similarly, a full physical examination is required in order to carefully evaluate the patient for evidence of underlying systemic disease. Deep pyoderma may result in localized lymphadenopathy of the draining lymph node – for example, prescapular or popliteal lymph node enlargement is usually evident when the feet are affected.
Although the clinical signs and previous response to antibiotic treatment supported a pyoderma, there were other possible differential diagnoses. The presence of multifocal, patchy alopecia is suggestive of a folliculitis which is most commonly the result of staphylococcal pyoderma, demodicosis or dermatophytosis. The age of the dog and the lack of pruritus were important factors to consider.
If this was a pyoderma, possible underlying causes included an adverse food reaction, particularly given the history of predisposition to gastrointestinal disease, and despite the lack of pruritus, atopic dermatitis. In addition, underlying immunosuppressive disorders should also be considered.