Atopic Dermatitis

Atopic dermatitis, the second most common hypersensitivity in dogs and cats, is a common, genetically programmed, pruritic disease of dogs and cats. The sensitizing antigen is generally an inhaled environmental allergen.

- Atopy is a Type I hypersensitivty reaction because it involves inflammatory cells.

SIGNALMENT:

Canines: Females tend to more affected than males. Though any breed can develop atopy, several breeds have a marked predisposition: Cairn terriers, West Highland white terriers, Shar peis, Scottish terriers, Lhasa apsos, wire-hared fox terriers, Dalmations, Pugs, Irish setters, Boston terriers, Golden retrievers, Boxers, English setters, Laborador retrievers, and minature schnauzers.

The onset usually occurs 1-3 years of age, with the exception of the Shar pei which can begin as young as 3 months.

Felines show no breed or sex predilection.

PRESENTATION: Pruritis is the outstanding clinical sign in canine atopy. Primary skin lesions, in the absence of secondary infections, are not commonlhy seen. Skin lesions seen in atopic animals are usually associated with self-trauma, secondary pyoderma, and secondary seborrheic skin disease. The lesions are generally found on the face, feet, and abdomen. Chronic inflammation and pruritis produces variable degrees of hyperpigmentation, lichenification, alopecia, and salivary staining.

These clinical signs initially seasonal or non-seasonal depending on allergen. Most animals with seasonal allergies tend to develop non-seasonal allergies.

The cutaneous syndromes associated with feline atopy include (1) Facial pruritis with or without lesions (2) pruritic ears with or without lesions (3) widespread pruritic papulocrustous dermatitis (miliary dermatitis) (4) indolent ulcer, eosinophilic plaque, or eosinophilic granuloma (5) symmetric alopecia and (6) generalized pruritis with or without lesions.

DIFFERENTIALS:

Canines: flea bite hypersensitivity, food hypersensitivity, contact dermatitics, scabies, intestinal parasite hypersensitivity, folliculitis, pelodera dermatitis, and hookworm dermatitis.

Felines: various causes of papulocrutous dermatitis, symmetric alopecia, indolent ulcer, eosinophilic plaque, and eosinophilic granuloma

DIAGNOSTICS: Elimination trials can be used to determine food allergies.

Skin tests can be used to determine the allergens to which the animal is hypersensitive. It is essential to understand that a positive skin test means only that the patient has skin sensitizing antibodies, not neccessarily a clinical allergy. False results can often occur.
Reasons for false positives include: (1) irritant test allergens (2) bacterial or fungal contaminated test allergens (3) skin sensitizing antibody only (4) poor technique (5) substance causes non-immunogenic histamine release (6) "irritable"skin and (7) dermatographism.
False negative can occur due to (1) subcutaneous injection (2) too little antigen (3) drug interference (4) anergy or tested at the peak of hypersensitive reaction (5) estrous, pseudopregnancy, or sever stress (6) parasitism (7) off season testing and (8) histamine hyposensitivity.

Histopathology:
Canine: variable degrees of superficial perivascular dermatitis with mononuclear clls or neutrophils usually predominating Commonly, find characteristics consistent with secondary pyoderma.
Feline: variable degrees of superficial perivascular dermatitis with eosinophils usually predominating.

CLINICAL MANAGEMENT: Avoidance of allergens, hyposensitization, systemic antipruritic drugs.


-There is controversy concerning the route of allergen exposure in AD.  Did you note the clinical distributions of Buzz's lesions?  Dogs with AD develop lesions on ventral hairless areas, or frictional sites such as the axilla (sites for rubbing of allergen into the skin).  In addition, eosinophils can be present in the epidermis - similar to what is found in atopy patch testing (allergen affixed topically for 24 hours).  Traditionally, atopy has been referred to as allergic inhalant dermatitis.  This is largely anecdotal as few studies have proven topical (skin) reactions in dogs sensitized to aeroallergens.  Dogs with naturally occurring AD do not have asthma.

    

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