Section: Veterinary Care In-depth
Diagnosis In-depth Diagnosis is based on clinical signs, distribution (variable depending on the type of substance involved), resolution of clinical signs with avoidance, and recurrence of clinical signs with re-exposure. Implicating a specific allergen is difficult and requires cooperation with the owner to identify possible sources.
Contact allergy can be differentiated from inhalant allergy (atopy) by the persistence of a primary papular eruption (red bumps) after appropriate antibiotic therapy. In addition, contact allergy completely resolves with confinement whereas atopy does not.
Boarding the animal at the hospital for 1 to 2 weeks is recommended to accomplish avoidance. Animals should be shampooed before beginning confinement to remove possible residual material from the skin.
Patch testing can also be used to diagnose contact allergy. Patch testing is difficult and time-consuming but allows identification of the specific substance responsible for clinical signs. The skin on the chest wall is clipped one to two days before the test and potential allergens are applied in close contact with the skin for 48 hours. Thorough bandaging is the best way to achieve contact with the allergen (closed patch test). An open patch test consists of applying the potential allergen to the inner part of the ear and observing the skin for 48 hours.
Specialized devices called Finn chambers may be used to apply the suspected allergen. A positive reaction is indicated by the appearance of edema (soft swelling) and a papular eruption (red bumps) 24 to 48 hours after application of the allergen.
Microscopic pathology. Skin biopsies show inflammation around blood vessels and swelling of cells in the epidermis.
Treatment In-depth
The main therapy of allergic contact dermatitis consists in allergen avoidance and topical or systemic corticosteroids. Anti-inflammatory doses of prednisone (0.5-1 mg/kg for 5 to 7 days and then on alternate day regimen as needed) are usually sufficient to control clinical signs. Topical steroids that may be beneficial include Resicort® or FS shampoo®.
Hyposensitization has been ineffective.
Antihistamines and essential fatty acids are not effective for contact allergy.
Secondary bacterial infections should be addressed with a course of systemic antibiotics (e.g. cephalexin 10 to 15 mg/lb twice daily orally for a minimum of 3 weeks)
Follow-up
Complete resolution of clinical signs usually is obtained after 10 to 14 days of avoidance of the offending substance. No prevention is possible unless the offending substance has previously been identified.
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