The triggering factor for urticaria can come from a multitude of sources, including insect bites, chemicals, food products, contact allergens or medications applied, ingested, or injected. Often the offending agent is never identified, with the horse recovering and the skin condition never occurring again.
In a small minority of horses, the offending agent remains in the surrounding environment or in the diet and the horse continues to get episodes of, or does not recover from, urticaria, despite appropriate therapy. It can affect horses of any age, sex or breed.
The condition arises from hypersensitivity, intolerance or an allergy to an offending agent, or it can be brought on by other physical factors such as pressure, heat, cold or exercise. Once triggered, a cascade of cellular events follows, resulting in the release of inflammatory mediators (the more commonly known one being histamine) from cells. These mediators act on small blood vessels and capillaries, allowing fluid to leak out into the dermis (underlying skin layer), causing oedema (fluid filling) and the formation of plaques and small lumps. The overlying skin, the epidermis, is usually not affected so there is no injury to the visible skin or hair loss. The reaction usually occurs quickly but can take days to weeks to resolve if left untreated.
Clinically, urticaria is seen as oedematous swellings or wheals that can either be localised to one region of the horse or generalised, affecting the entire body of the horse. The oedema is what is known as “pitting oedema”, meaning there will be an indentation produced when finger pressure is applied to the areas. It can occur in varying degrees of severity and the reaction can occur quite quickly. I have known of several occasions where the horse was “normal” but when seen 15 minutes later is swollen up and almost unrecognisable. Sometimes, the horse is found with several small nodules, which do not appear to worry the horse and if left untreated often resolve on their own. Owners will often refer to these skin changes as heat lumps or protein bumps.
In some circumstances the horse can become itchy, however, in most cases the lumps do not appear to worry the horse. For those cases where the horse becomes pruritic or itchy, the initial skin picture and diagnosis can become complicated by self-mutilation — where the horse bites or rubs itself and causes large wounds to the skin.
In extreme cases where there is severe generalised swelling, the horse can show signs of difficulty breathing because of the oedema in their nasal passages. If this occurs, treatment should be sought quickly.
As mentioned above, there are many agents that can trigger urticaria and trying to identify the primary offender can be complicated. A thorough history regarding the horse and its environment is key to helping solve the puzzle, but even then, with the best of histories available, an answer is not always obtained. During the warmer months, insect bites are a common cause of urticaria and assessing the environment for biting flies, ants, spiders and mosquitos is warranted. Many drugs, regardless of how they are administered, can cause the reaction, so a close look at recent administration of antibiotics, vaccinations, vitamins, anti-inflammatory medications, or worm pastes should occur. A history of moving into a new paddock that contains different grasses or weeds, or a change in stable bedding, may offer clues, as does a history of purchasing and using new rugs or gear. Other things to look for include the diet and any other supplements used, and any changes to these in the preceding months.