Introduction
Contact dermatitis is an inflammatory skin disease caused by contact of the skin with a chemical.
There are thought to be two types of contact dermatitis; irritant contact dermatitis, which is caused by a direct chemical contact on the skin. This is rare. And secondly, allergic contact dermatitis, which is a type IV hypersensitivity reaction to an antigen.
Chemicals such as acids, alkali, detergents, irritant plants, medications and home furnishings can all cause the condition. In cats and dogs lesions usually occur on the skin of the ventral abdomen, feet, chest, eyelids and axillae. In horses lesions are most prevalent on the muzzle, lower limbs and under tack.
Signalment
This condition can appear in most domestic species, but is rare in small animals and rare (probably due to undiagnosis) in cattle too. There is no sex, breed or age predilection to the disease.
Clinical Signs
The skin will appear erythematous with patches and papules and occasionally vesicles. Many lesions are also caused by self-trauma and include ulcers and crusts.
Lesions will appear in typical sites such as those described above and relating to the causes e.g. if the allergy is to the carpet then a dog will have the signs on his abdomen and scrotum.
Diagnosis
History and clinical signs may be indicative of the disease.
Contact restriction is a way of diagnosing the condition, but is often difficult to perform. Re-exposure to the allergen and re-emergence of clinical signs would then be diagnostic.
Patch tests are rarely performed on animals, but include applying substances to an area for 48 hours and then removing and assessing for papules and plaques at these sites.
Pathology
Grossly, there will be erythema, papules, +/- vesicles and exudation forming crusts in later stages. If the irritation is chronic, lichenification, hyperpigmentation and alopecia may also be noticed.
Microscopically, one will find spongiotic superficial perivascular dermatitis and mononuclear cells. If the disease is chronic, then epidermal hyperplasia will also be present. Eosinophils may also be involved if the aetiology is allergic.
Treatment and Control
It is important to avoid the offending substance wherever possible.
Anti-inflammatory therapy may be required topically, such as glucocorticoids. If secondary bacterial or yeast infection has occurred then appropriate treatment for these is also required.
References
Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition), Elsevier Science.
Bond, R. (2008) Dermatology Study Guide, Royal Veterinary College.
Foster, A, and Foll, C. (2003) BSAVA small animal dermatology (second edition), British Small Animal Veterinary Association
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