South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Equine Allergies: what can we do about those itchy horses?
Annette Petersen*
Associate Professor of Dermatology
Department of Small Animal Clinical Sciences, Michigan State University, East Lansing,
Michigan, 48824-1314,
Email: [email protected]
Insect hypersensitivity is the most common cause of pruritus in horses. A hypersensitivity reaction to bites
by various Culicoides spp. is the best documented cause of the syndrome, but allergic reactions to bites by
black flies, mosquitoes and other flies are also recognized. Equine insect hypersensitivity is a problem with
a worldwide distribution as is reflected by the many different names for the syndrome. Nearly a 1000
Culicoides spp. are found worldwide and common names for these insects include gnats, no-see-ums, biting
midges, sand flies, and punkies. They are tiny winged insects 1-2 mm in length that breed in standing water
(ponds, lakes, irrigation canals, marshes, swamps, and watering troughs). Their numbers are greatest under
warm and humid conditions.
Clinical signs of insect hypersensitivity can be observed in any breed or age of horse but several breeds
appear to have a hereditary predisposition: Icelandic, German Shire, Arabian, Connemara, Swiss
Warmblood, Quarter Horse, and ponies. Signs may start between 2 and 4 years of age and often become
more severe as horses age. Typically, pruritus starts in the spring, becomes worse in the summer, and
regresses in the fall in temperate regions or is year round in tropical regions. Pruritus frequently becomes
more severe and of longer duration each subsequent year, especially when concurrent environmental
allergies (atopic dermatitis) are present. With hypersensitivity to Culicoides spp. pruritus is most commonly
observed along the dorsum including small papules and secondary excoriation along the mane, rump and
tail base. With progression, lesions may extend to the neck and shoulder regions, ears, and face. Less
commonly, a more ventral distribution of skin lesions develops, dependant on the feeding sites of the
offending insects. With chronicity and continued self-trauma, “buzzed-off manes” and “rat tails” may
develop along with lichenification and in severe cases, skin folds or rugae in the base of the mane and tail,
behaviour changes (e.g., restlessness and irritability making the horse unsuitable for riding) and even
weight loss.
Hypersensitivity to other fly bites other than from Culicoides spp. tends to produce pruritus and multiple
papules starting over the neck and shoulders. The mane and tail are often unaffected with fly bite
hypersensitivity but lesions can extend to the head and may even become generalized. More severely
affected horses can also develop secondary bacterial infection of the skin (folliculitis): signs of pyoderma
range from erected hairs over small papules, with or without crusts, to deeper lesions with nodules and
draining tracts. Urticaria is an uncommon manifestation of insect hypersensitivity but common with atopy.
The diagnosis of insect hypersen sitivity is established by recognition of a warm-weather seasonal
occurrence of mane and tail pruritus and by ruling out other causes (e.g., other ectoparasites such as ticks
and mites or atopic dermatitis). Improvement with insect control provides additional support for a
diagnosis. Intradermal testing with insect antigen preparations is also available at many referral practices
but interpretation of the results requires experience because clinically normal horses frequently have
positive reactions. For these reasons, intradermal testing with insect allergens should not primarily be
pursued to establish the diagnosis but rather to confirm the clinical diagnosis and to select extracts for
immunotherapy. Skin testing requires clipping the hair, usually in the upper neck region, and possible
sedation.
Withdrawal period from antihistamine and corticosteroid medications in horses can be short (7 days) prior
to intradermal testing. Serum samples can also be submitted to commercial laboratories for ELISA
measurement of IgE concentrations against various insect allergens. The test is costly and results appear to
be less reliable than intradermal testing results. Nevertheless, when referral for skin testing is not possible
and serological testing results appear to compare well with the patient‟s history, the results can be used to
pursue immunotherapy.
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