Introduction

• Rare, delayed or cell-mediated hypersensitivity reaction to contacted allergens in sensitized individuals.
• Cause: various allergens.
• Signs: alopecia, erythema, papular eruption.
• Diagnosis: history, clinical signs (resolution with avoidance/confinement), histopathology, patch test.
• Treatment: glucocorticoids, bathing, and removal of allergen.
• Prognosis: good if allergen identified.

Pathogenesis

Etiology

• Compounds are simple, acting as haptens to link epidermal proteins.
• Plant resins and oils.
• Medications, eg neomycin in otic preparations, for containing shampoos.
• Carpet mordants and dyes.
• Chlorine in water.
• Flea collars.
• Insecticides.
• Plastic feed bowls.
• Plants (Commelinanceaefamily).

Pathophysiology

• Chemicals act as haptens to bind to proteins, forming a conjugate which binds to Langerhans' cells in the skin. These altered cells trigger a T-cell response.
• Following exposure to antigen in a sensitized animal, mononuclear cells infiltrate the skin and then are removed by sensitized lymphocytes leaving intra-epidermal vesicles.
• This type IV hypersensitivity reaction leads to pruritus.

• Low molecular weight haptens link epidermal proteins to form antigens. Antigen is trapped, processed and presented by Langerhans' cells. They produce cytokines which attract T-cells → activation of keratinocytes to release pro-inflammatory cytokines. They also travel to local lymph nodes to amplify the immune response.

Timecourse

• Sensitization takes a minimum of 6 months.
• After contact with an allergen, clinical signs will take 48 h to develop in a sensitized dog. This is a delayed allergic reaction.

Diagnosis

Treatment

Prevention

Outcomes