The role of allergy in the development of canine pyoderma

Pyoderma is commonly seen in canine veterinary practice and usually occurs as a complicating factor in other primary conditions. Allergic skin disease is often the underlying cause but the precise relationship between allergy and infection is unknown. Our studies have investigated the relationship between bacterial proliferation at the skin surface and hypersensitivity reactions with the skin. Hypersensitive dogs were shown to have significantly higher surface counts of staphylococci than normal controls and these bacteria were concentrated in the more superficial layers of the stratum corneum. Intradermal injection of staphlococcal antigens in normal dogs elicited epidermal damage similar to that seen in clinical disease. Preliminary autoradiographic studies using a model of canine skin hypersensitivity reactions showed that percutaneous absorption of radiolabelled staphylococcal antigens was increased by mast cell degranulation. These findings suggest that a major role of hypersensitivity reactions in the pathogenesis of pyoderma may be via an effect on epidermal permeability, promoting penetration of staphylococcal antigens from the stratum corneum which then cause the lesions of pyoderma. Hypersensitivity reactions may also lead to changes in the skin surface microclimate leading to increased bacterial counts on the skin surface, so exacerbating the condition.