here - Australian College of Veterinary Scientists

Clinical features:
Whilst summer seasonal recurrent dermatitis has been long-recognised in the horse
and the aetiopathological role of hypersensitivity responses to the bites of insects,
particularly Culicoides species was elucidated as long ago as the early 1970's, the
concept of equine atopy was later in becoming recognised, being featured in
textbooks by the end of the 1980's. Although now universally recognised, the
prevalence of equine atopic dermatitis is not known. Scott and Miller (2003) report
atopy as the seventh most common equine dermatosis diagnosed at the Cornell
University Clinic, accounting for 3.89% of all skin conditions, but some or all of an
additional 2.11% with a final diagnosis of idiopathic pruritus and 2.44% with
idiopathic urticaria might also be considered as atopic – giving a possible total of
8.44% of horses with skin disease.
Clinical signs most commonly begin in young horses, 1.5-6 years of age. There is no
sex predilection, but some reports suggest breed predisposition, but there is not
usually any comparison to a reference population to validate the observation(s).
Signs may be seasonal or non-seasonal, depending upon the allergens involved, and
may initially be seasonal and progressing to be perennial in nature. The major clinical
signs in equine atopic dermatitis are pruritus and/or urticaria. Early in the course of
the disease affected horses may present with pruritus without obvious
dermatological lesions. Rubbing, biting, stamping, tail-flicking and, occasionally, head
-shaking may be reported. Subsequent self-inflicted damage resulting in the presence
of excoriations, partial to complete alopecia, lichenification, thickening and folding
of the skin, hyperpigmentation and secondary bacterial infections. Affected areas
included the head, ears, neck, trunk, and legs; in some cases there is also involvement
of the regions classically affected in IBH – the dorsolateral neck, mane, dorsal
midline lumbar area and tail base. Urticaria is also a common reaction pattern in
atopic horses, sometimes seen in conjunction with pruritus, but in some cases being
non-pruritic. Some atopic horses present with tufted papules or nodules, which may
become crusted and alopecic. Biopsy of these lesions reveals the presence of
eosinophilic folliculitis (sterile) and/or eosinophilic granulomata.
Horses with atopic dermatitis are only rarely reported to show signs of concurrent
allergic conjunctivitis, rhinitis and chronic obstructive pulmonary disease (COPD).
Whilst there is evidence that COPD is an atopic disease (genetically driven,
hypersensitivity response to environmental allergens causing clinical disease), reports
of concurrent atopic dermatitis and COPD are rare.
In a series of 24 horses referred to the author whilst at the Animal Health Trust,
Newmarket, for investigation of clinical signs suggestive of an allergic aetiology, but
not consistent with classical summer seasonal recurrent dermatitis due to insect-bite
hypersensitivity, the history and presenting signs were analysed (Littlewood et al
1998). Seven of the horses were thoroughbreds (TB) and a further eight were TBcrosses,
with five Welsh cob, unspecified cobs or cob-crosses, one Arab, one Arabcross,
one Belgian warmblood and one pony. Age at diagnosis ranged from 2-30
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ACVSC Proceedings Dermatology Chapter Science Week 2005