Inflammation of the skin can be produced by numerous agents, including external irritants, burns, allergens, trauma, and infection (bacterial, viral, parasitic, or fungal). It can be associated with concurrent internal or systemic disease; hereditary factors also may be involved. Allergies form an important group of etiologic factors, especially in small animals.
The skin’s response to insult is generically called dermatitis and manifests as any combination of pruritus, scaling, erythema, thickening or lichenification of the skin, hyperpigmentation, oily seborrhea, odor, and hair loss. The usual progression of a skin disease involves an underlying trigger (disease syndrome) that causes primary lesions such as papules, pustules, and vesicles. Pruritus is a common clinical sign in many diseases, and in conditions that are not inherently pruritic, it is often present because of secondary infections or as a result of production of inflammatory mediators. As the inflammatory changes progress, crusting and scaling develop. If the process involves the deeper dermis, exudation, pain, and sloughing of the skin may occur. Secondary bacterial and yeast infections commonly develop as a result of skin inflammation. As dermatitis becomes chronic, acute signs of inflammation (eg, erythema) subside and primary lesions become obscured by signs of chronic inflammation (thickening of the skin, hyperpigmentation, scaling, seborrhea). Often, the skin becomes drier; if pruritus is not a component of the underlying trigger, it will often develop at this stage. Resolution of dermatitis requires identification of the underlying cause and treatment of secondary infections or other complications.