Contact Dermatitis: Is It Really a Rare Disease?

Contact dermatitis is typically described in the old literature as a rare disease due to the fact that animals have fur and that should protect them from getting contact with offending allergens. As a consequence of this dogma, contact allergy is not frequently considered as differential diagnosis for many pruritic cases and wrongly lumped under atopic dermatitis. Many cases have been therefore undiagnosed and considered as really bad atopic dermatitis cases not responsive to traditional therapy. Although the prevalence of contact dermatitis, particularly the allergic type, may vary according to the geographical location, such condition is actually quite common as complicating allergy in atopic individuals.

Contact allergy, a type IV hypersensitivity, indeed seems to be more common in patients that already suffer from atopic dermatitis and have disrupted skin barrier due to either self-trauma or primary skin barrier defect. Recent studies have shown that the skin of dogs with atopic dermatitis is more permeable and less able to protect from water loss. This impaired skin barrier is therefore more prone to absorb little allergens such as the haptens responsible for contact allergy and this is noticeable clinically in patients that develop contact allergy on top of an existing atopic dermatitis. Short coated breeds of dogs are predisposed and Pit Bulls, Boston Terriers, Dachshund, Bulldogs are commonly diagnosed in tropical climates with this condition.

Contact allergy can develop against a variety of allergens from medications to carpet fibers but tropical climates also have a high prevalence of plants like the Commelinenciae which are very allergenic to dogs. Many plants in this family have been linked to contact allergy in dogs (e.g., Tradescantia fluminensis, Commelina diffusa) but they do not seem to cause problems to people. These plants thrive after heavy rains and are ground covering weeds which are quite difficult to eliminate with regular weed killers. Commonly used topical medications that can cause contact allergy are neomycin and propylene glycol, which are frequently included in otic preparations. Usually animals are exposed to the responsible allergen for months before developing clinical signs. This is because a period of sensitization is necessary. This ranges from 6 months to several years in some cases. Once sensitization has occurred clinical signs manifest 1-2 days after exposure to the offending allergen. Clinical signs include intense pruritus and a papular dermatitis affecting the plantar surface of all four feet, ventral abdomen, muzzle, concave surface of the pinnae, ventral thorax, axillae, groin and even perineal area in patients that sit on the offending allergen. Due to the intense pruritus excoriations and ulcerations are common due to self-trauma. Additionally, patients develop secondary infections, both bacterial and yeast, which further contribute to the level of pruritus.  […]