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Intradermal Testing (IDT) in Horses:
The UK Perspective
Janet Littlewood
Horses with a history of recurrent pruritus and /or urticaria represent the largest
proportion of equine dermatology referral cases in the author's practice. Referred
cases usually have a history of recurrent problems of at least 2 months duration.
Ideally an elimination diet is undertaken before referral – although the almost
complete absence of well-defined cases of adverse cutaneous food reaction in horses
makes it a rare diagnosis, and so this is not considered essential, either prior to or
after referral, although it is advised. Clients are often reluctant to limit their horses to
strict, narrow elimination diets, particularly if they want to continue to work the
horse. History and clinical examination are undertaken to rule out other possible
causes of urticaria and pruritus, and if the signs are consistent with hypersensitivity,
intradermal testing is undertaken. The author requests that horses be off
antihistamines for a minimum of 2 weeks and have received no steroids for a month
(ideally), although withdrawal of low dose alternate day steroids may not need to be
for this period. Food additives, particularly aloe vera extract, should also be
withdrawn – this advice is given on the basis of negative intradermal test results
being obtained in a horse currently receiving aloe vera, with positive results obtained
at retesting 3 weeks after withdrawal.
Horses are sedated for intradermal testing, with detomidine hydrochloride (0.06-
0.12mg/kg) and butorphanol 0.075-0.15mg/kg) intravenously. An area on the lateral
aspect of the shoulder is clipped with a number 40 blade and injection sites marked
with a permanent marker pen at 2 cm intervals. Allergens for testing are obtained
from Greer Laboratories Inc., North Carolina, USA with additional forage mites
supplied by Artuvetrin Biologicals nv, Lelystad, Netherlands. In the past allergens
have also been supplied by Stallervet, France. The selection of allergens used in
intradermal testing in dogs in the UK was originally based on advice obtained from
colleagues in human allergy medicine and using information from publications and
texts. Over the years the author has changed and refined the test kit, in conjunction
with colleagues and in the light of frequency of negative and positive reactions. In
fact, apart from the Culicoides antigen, which is restricted for testing of horses, the
author currently uses the same allergens in dogs and horses. Allergens supplied from
Greer are stored in concentrated form and diluted to testing strength, whereas the
European allergens are supplied at testing strength. The testing strengths used are
selected on the basis of advice from the allergen companies and from published data.
The restrictions on experimentation in the UK mean that very little work has been
done on optimising of allergen concentrations, apart from work done on Culicoides
ACVSC Proceedings Dermatology Chapter Science Week 2005 31