Allergic contact dermatitis

Allergic contact dermatitis (ACD) and irritant contact dermatitis (ICD) are two very similar conditions mediated by direct contact with environmental substances.

Aetiology and pathogenesis

ACD requires previous sensitisation. It is a type 4 (cell-mediated) hypersensitivity reaction to small, low-molecular-weight chemicals (haptens) that bind to host proteins. Haptenated proteins are phagocytosed, processed and presented by antigen- presenting cells, especially epidermal Langerhans’ cells, to T cells bearing the appropriate T-cell receptors. These recirculate to the skin and, on subsequent exposure to the hapten, trigger a cell-mediated immune response.ICD, in contrast, is not antigen specific and is directly triggered by noxious compounds that damage keratinocytes. The effector stages and inflammation in ACD and ICD share similar immunological pathways, resulting in almost identical clinical signs and histopathology.

Clinical features

The refractory period for ACD is reported to rarely be less than 2 years, so one would not expect it to appear in very young animals. However, the inquisitive nature of puppies and, perhaps, their juvenile pelage, might predispose them to exposure to irritants and, therefore, ICD. German Shepherd dogs comprised 50% of the dogs in one series of con-firmed ACD cases. ACD requires multiple exposures, whereas ICD will occur on first exposure.

Well-demarcated inflammation of the hairless skin of the axillae

Fig. 2.25 Well-demarcated inflammation of the
hairless skin of the axillae and groin in a German
Shepherd dog with a contact reaction to a cleaning fluid.

ACD usually affects individual animals, but ICD can affect all in-contact animals. Most cases of ACD and ICD are perennial, although it does depend on the timing of exposure, and seasonal examples will be met, typically to vegetative allergens/irritants.  Acute and severe ACD/ICD may result in erythema, oedema, vesicles, and even erosion or ulceration (Figs.  2.25, 2.26). Primary lesions include erythematous macules, papules and occasionally vesicles. Secondary lesions (e.g. excoriation, alopecia, lichenification and hyperpigmentation) tend to mask these primary lesions. There is usually a well-defined margin between affected and normal skin (Fig. 2.27). Pruritus is variable, but maybe intense.

marked erythema and exfoliations

Fig. 2.26 Marked erythema and exfoliation in
the axilla of a cross-bred dog after a reaction to a
shampoo.

The distribution of the lesions reflects the exposed contact areas and, therefore, hairless dogs and cats are at more risk. Clinical signs are usually confined to sparsely haired skin, but prolonged contact will result in extension to adjacent areas and, with time, the chin, ventral pinna, ventral neck and medial limbs, and the entire ventrum will be affected. Generalised reactions may be seen in cases of reactions to shampoos. Chronic otitis externa may result from sensitivity to topical neomycin or other potential irritants and sensitisers. Other potential substances include metals, plastics, fibres, leather, dyes, oils and cleaning fluids.

Severe suppurative otitis

Fig. 2.27 Severe suppurative otitis in a dog that
developed a reaction to commercial ear products.

Differential diagnoses

• Atopic dermatitis and/or food allergies.

• Sarcoptic mange.

• Demodicosis.

• Neotrombicula (harvest mite or berry bug) or chigger infestation.

• Bacterial or yeast dermatitis.

• Hookworm dermatitis.

Treatment

If the allergen or irritant can be identified, and if exposure can be restricted, then the prognosis is good. Failure to identify the cause or prevent access results in reliance on symptomatic therapy, usu-ally with systemic glucocorticoids. Topical therapy can be appropriate with localised lesions. In some individuals, complete control may be very hard to achieve without the side effects of glucocorticoid therapy becoming apparent. Ciclosporin or topical tacrolimus (not licensed for animals) can be effective and better tolerated. Pentoxifylline (10  mg/kg po q12 h) ameliorated lesions in three dogs sensitised to plants of the Commelinaceae family.10 Barrier creams, Lycra™ bodysuits and/or prompt washing can be used if some exposure is unavoidable.

Allergic contact dermatitis may be refractory to steroid therapy.