Dr Vivienne Duggan explaines urticaria or equine hives

URTICARIA, more commonly known as hives, are a frequent occurrence in horses, particularly during the late spring and summer months.

Hives are only a symptom of disease, however, and not actually a disease condition. While some horses develop hives in response to changes in their environment such as extremes of cold, heat, pressure on the skin, sunlight or stress, hives are often a sign that the horse’s immune system is reacting to something to which it has been exposed.

This allergen triggers cells in the skin called mast cells. When these cells are triggered, they release their contents into the surrounding tissues which irritate local blood vessels, causing them to leak fluid. This results in a raised steep-walled, flat-topped and often quite circular lesion that retains the shape of your finger when you apply sustained pressure to it.

The lesions can be seen over the head, neck, trunk and limbs. The lesions don’t lose their hair cover and are not usually hot, painful or particularly itchy. Usually they bother the owner by their appearance more than they bother the horse. The lesions usually resolve spontaneously within 24-48 hours; however, if the horse continues to be exposed to the trigger, new lesions will appear to take the place of the resolving ones.

Flies, midges and mites are known to elicit this response in horse skin. Horses may also react to something that has changed in the feed or the bedding, a new pasture, a new shampoo, a new detergent used to wash their rug, tack or brushes.

Not uncommonly, the allergen is a drug they have been administered; vaccines, de-wormers and antibiotics such as penicillin have all been implicated in the development of horse hives, as have various vitamin injections, supplements and other medications. Even fly spray can cause susceptible horses to “break out” in hives.

Some horses repeatedly break out in hives at the same time of year every year supporting the involvement of environmental triggers such as pollen or weeds.

Given the array of possible triggers, and the fact that the response can take several hours to appear, it is often difficult to pinpoint exactly what the actual trigger was. Having your veterinary surgeon take a biopsy of the skin will only confirm that it is a hyper-sensitivity, or allergic, reaction, but it will not tell you what the allergen is.

MANAGEMENT

Reviewing recent changes in management may help. If you have recently tried a new shampoo, or an herbal supplement, that might be the culprit. Likewise, if the horse was recently vaccinated or de-wormed.

The distribution of the lesions on the horse might also provide a clue to the allergen; for example, if only the areas that are in contact with a rug are affected and you have recently changed your detergent that might be a relevant finding.

So what can you do if your horse or pony develops hives?

Removing the animal from the source of the reaction is, of course, advisable, if that source can be identified. If not, changes in feed, environment and/or management may assist in controlling the condition. If you suspect that it is a reaction to something in the feed, you could restrict the diet back to grass hay and water for two-four weeks until the reaction is under control and then gradually reintroduce feedstuffs at two week intervals while noting the response to each addition.

However, this is a very time-consuming process and other factors which may be contributing to the condition, such as environmental triggers, can affect the interpretation. All drugs and supplements should be discontinued and likewise, gradually reintroduced if necessary. Fly sheets and fly masks may reduce the exposure to irritant flies and midges; likewise, stabling at dawn and dusk can help reduce the incidence of break outs, if they are insect-related. Using antihistamines can have quite disappointing results although some horses may respond. Topical treatments such as hypo-allergenic shampoos can also have variable results.

Your veterinary surgeon can prescribe an injection of corticosteroids which is a symptomatic treatment that will very effectively dampen the immune response and ameliorate the reaction. Most urticarial lesions will respond rapidly, within 24 hours, to this treatment; however, there are significant adverse, career-threatening side effects, such as laminitis, and competition regulation considerations that should be discussed with your veterinary surgeon before treatment with corticosteroids is undertaken.

SKIN TESTS

In recurrent cases of hives, intra-dermal skin testing can be performed by your veterinary surgeon to help to narrow down the list of allergens to which the horse may be sensitive. Intra-dermal skin testing involves injecting a miniscule amount of a range of potential allergens into pre-determined spots in the skin, usually under the mane, along with negative (saline) and positive (histamine) control injections.

The reaction in the skin to the different allergens and the positive and negative control spots is measured over the following four hours. Interpreting this test can be challenging in horses, with many false negative and false positive responses; therefore many consider intradermal testing in horses somewhat unreliable.

Blood tests to look for antibody responses to specific allergens have also not been standardised for use in horses. However, if specific allergens are reliably identified by intra-dermal skin testing, then hypo-sensitisation (allergen-specific immunotherapy) has been reported to successfully reduce the clinical signs of recurrent urticarial lesions in some horses. Unfortunately, this procedure is costly and time-consuming and is reported to have quite variable results.

Dr Vivienne Duggan MVB PhD Dipl. ACIM & ECEIM is President of Veterinary Ireland Equine Group and is a lecturer in Equine Medicine at the UCD School of Veterinary Medicine

Email: hq@vetireland.ie

Telephone: 01-4577976