Page 855 - The Veterinary Care of the Horse

When a horse is exposed to an allergen for the first time, it produces antibodies (IgE) against

it. These bind to mast cells and basophils in the tissues. If exposed to the allergen for a
VetBooks.ir second time, the allergen binds to the IgE and this causes the mast cells to release histamine
and other chemical mediators which increase the permeability of the blood vessels, allowing

them to leak and form the wheals. This reaction may be almost immediate and occur in
minutes, or it may take several hours to develop.

Diagnosis

The diagnosis is usually made on the history and the clinical signs. However, if the lesions
are chronic, oozing or infected, skin scrapes may be taken to rule out the presence of

parasites. A skin biopsy may also be helpful on occasions. Intradermal skin testing may be
helpful. The side of the neck is clipped and a number of substances to which the horse might

be allergic are injected into the skin. The reaction to the injected allergen is read after 15
minutes, 4–6 hours and 24 hours and compared to control sites injected with saline and
histamine.

This test will only give meaningful results if medicines such as corticosteroids,

antihistamines, phenylbutazone and acepromazine are withdrawn at least 2 weeks earlier as
these interfere with the results. Your vet will advise you on this.

When to call the vet

Call your vet if:

• the horse is distressed
• the eyelids and muzzle are swollen

• the horse is having difficulty breathing or swallowing

• serum is leaking through the skin

• the rash develops while the horse is being treated with medication

• there is no improvement after 2 days and the condition is preventing the horse from
taking part in its usual activities.

Treatment

The aim of treatment is to reduce the horse’s exposure to the allergen and restore the skin to
normal.

MEDICATION