Superficial pyoderma

What it is

Superficial pyoderma is a bacterial infection limited to the epidermis and the superficial part of the hair follicle. Its most common form is superficial bacterial folliculitis. The usual agent is Staphylococcus pseudintermedius, a normal inhabitant of canine skin and mucosae. Infection develops when the skin's defences fail, so in almost every case there is an underlying cause: atopic dermatitis, food allergy, flea allergy, ectoparasites, endocrine disease (hypothyroidism, hyperadrenocorticism) or seborrhoeic disorders. Methicillin-resistant strains (MRSP) are an increasing problem. When the follicle ruptures and infection reaches the dermis it becomes a deep pyoderma (furunculosis), a different and more serious condition.

Who it affects

Dogs of any age and breed with a predisposing skin disease. Impetigo, a non-follicular form with subcorneal pustules on the hairless abdomen, is typical of puppies before puberty.

Anatomy involved

The infection affects the skin (cutis): the stratum corneum and the infundibulum of the hair follicles. Lesions appear mostly on the ventral abdomen, inner thighs, axillae and trunk. The dog's relatively thin stratum corneum and the shape of its hair follicles have been proposed as reasons why pyoderma is more frequent in the dog than in other species.

Clinical signs

Follicular papules and pustules, epidermal collarettes (rings of scale left after a pustule ruptures), crusts, patchy alopecia with a "moth-eaten" coat in short-haired breeds, and post-inflammatory hyperpigmentation. Pruritus is variable and is often due to the underlying disease.

Diagnosis

• Recognition of typical lesions.
• Cytology of a pustule or under a crust: neutrophils with phagocytosed cocci confirm the infection.
• Bacterial culture and susceptibility testing when rod-shaped bacteria are seen, when the dog has received antibiotics recently, when the response is poor or when resistant strains are suspected.
• Investigation of the underlying cause, which determines whether the infection will come back.

Treatment

Topical antimicrobial therapy (shampoos, sprays or wipes with chlorhexidine and similar antiseptics) is recommended as the sole treatment whenever practical, because it reduces the use of systemic antibiotics. Widespread or unresponsive cases need systemic antibiotics chosen according to current guidelines and, ideally, culture results, given until the lesions have resolved. Treating and controlling the primary disease is essential; otherwise recurrence is the rule.