here - Australian College of Veterinary Scientists

Recommendations:
Allergens should be selected on the basis of positive intradermal test responses,
incorporating all those that appear to be relevant in view of the history and
environmental exposure. In multiply allergic horses, more detailed information may
be needed regarding likely presence of allergens in that individual's environment, the
allergens know to be important in that geographical region, and the duration of the
presence of the allergen. Cross-reactivity within groups of allergens may be utilised
in making selections; cross-reactivity is likely in plants of the same genus and family.
In general grasses are the most cross-reactive and weeds less so. Mould and
arthropod allergens contain proteases that may have adverse effects on other
allergens when used in combination, which may or may not affect the success of
hyposensitisation.
Beneficial effects may be seen as early as 2-3 months into treatment, but may take as
long as a year to achieve benefit. Intervals between maintenance injections can be
tailored to the individual, according to the reappearance of clinical signs, varying
between weekly to monthly. Frequency of injections may very at different times of
the year. In general, lifelong administration of vaccine is required. Adverse reactions
are uncommon and usually mild. Reactions may include intensification of clinical
signs for a few hours to a few days after injection, local injection site reactions
(swelling +/- pain) and urticaria. Serious, anaphylactic reactions are extremely rare.
Reactions are treated symptomatically. Exacerbation of clinical signs is an indication
to reduce to dose of vaccine. Severely affected horses may need additional
glucocorticoid therapy to control their symptoms, but low-dose alternate day
prednisolone can be used in conjunction with ASIT, just as in small animal patients,
and immunotherapy still be successful at controlling the disease, with lower doses of
steroids than were required before or even subsequent withdrawal of steroids.
References:
Anderson GS, Belton P, Jahren E, Lange H & Kleider N (1996) Immunotherapy
trial for horses in British Columbia with Culicoides (Diptera: Ceratopogonidae)
hypersensitivity. Journal of Medical Entomology 33, 458-66
Barbet JL, Bevier D & Greiner EC. (1990) Specific immunotherapy in the treatment
of Culicoides hypersensitive horses: a double blind study. Equine Veterinary Journal 22,
232-5
Fadok VA (1996) Hyposensitisation of Equids with allergic skin/pulmonary disease.
Proceedings of the Annual Meeting of the American Academy and College of Veterinary
Dermatology , Las Vegas, Nevada p47
Rees CA (2001) Response to immunotherapy in six related horses with urticaria
secondary to atopy. Journal of the American Veterinary Medical Association 218, 753-5
Rosenkrantz WS (1992) Therapy of equine pruritus. Proceedings of the 2 nd World Congress
of Veterinary Dermatology Montreal, Canada p 435
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ACVSC Proceedings Dermatology Chapter Science Week 2005