here - Australian College of Veterinary Scientists

Affected horses usually scratch and chew themselves and rub against environmental
objects (stalls, fences, doorways, posts, trees etc) Reflex nibbling movements can
often be elicited by manipulation of the skin of the mane. Horses usually show one
of three patterns of skin disease i.e dorsal, ventral or some combination thereof and
these differences in lesion distribution probably reflect the different preferential
feeding sites of the various insect species.
Dorsal insect hypersensitivity is characterised by pruritus with or without crusted
papules, usually beginning at the mane, rump and base of tail. The condition then
extends to involve the face, pinna, neck, shoulder and dorsal thorax. Self trauma and
chronicity lead to excoriations, variable hypotrichosis and alopecia, lichenification
and pigmentary disturbances especially melanoderma and melanotrichia.
Ventral insect hypersensitivity is characterised by pruritus with or without crusted
papules, beginning on the ventral thorax and abdomen, axillae and groin. The legs
and intermandibular space are often involved. Secondary changes may occur as
above. Horses with insect bite hypersensitivity rarely have involvement of the flanks.
In addition, horses with insect hypersensitivity uncommonly develop urticaria and
when they do it is typically papular.
Secondary bacterial folliculitis is not uncommon. Affected horses may suffer
behavioural changes (anxious, nervous, restless, aggressive) and be unfit for riding,
showing or working. Some horses will lose weight due to the constant irritation.
Some horses have concurrent atopic disease or adverse food reactions which can
greatly complicate the diagnostic work-up and therapeutic management.
Diagnosis
The differential diagnosis will vary according to the distribution of lesions. The
definitive diagnosis is based on history, physical examination, ruling out other
conditions and response to insect control.
Intradermal testing with insect extracts, especially Culicoides is usually positive with
insect hypersensitivity. Reactions may be present at 30 minutes (immediate), 4-6 hrs
(late phase) and 24-48hrs (delayed) or combinations of these. Normal horses can
have positive reactions and the prevalence of positive reactions in normal horses
increases with increasing age.
Antibodies against Culicoides and Stomoxys have been detected in the serum of
affected horses by ELISA. Other investigators have found no difference in ELISA
scores using Culicoides extract between normal horses and horses with insect
hypersensitivity. The diagnostic value of ELISA results is presently undocumented.
ACVSC Proceedings Dermatology Chapter Science Week 2005 91