Horses, just like humans, can and do get allergies.
Although allergies in horses are not fully understood, we do know that the root cause lies in the immune system. The body becomes hypersensitive and appears to “overreact” to seemingly innocuous substances called allergens.
Histamine, a naturally occurring hormone in the body, reaches overly high levels, sensitizing the body to specific triggers. Once a horse has had an allergic reaction to a substance, the severity of the body’s response tends to increase in severity with repeated exposures.
The symptoms of allergies are typically expressed either in the skin and/or the respiratory systems. When allergic reactions affect the skin they cause itching, recurrent hives or small, flat-topped bumps called wheals, patchy hair loss, or crusting and other seemingly “strange” skin reactions.
Allergies affecting the respiratory system generally manifest as a nasal discharge, a mild cough, and/or slight exercise intolerance. As the allergic condition advances the coughing becomes progressively more frequent and deeper. ‘Runny noses’ and ‘watery eyes’ are often early signs of allergies. Elevated systemic histamines sensitize mucous membranes increasing the permeability of little blood vessels. This causes fluids to weep from the capillaries into the tissues, such as the eyes, nose, and skin. Horses will often snort in response to the irritation.
Allergen triggers generally fall into one of five categories. The most commonly recognized equine allergen is the saliva from insect bites. Although tiny biting midges tend to cause the most severe reaction, other biting insects include mosquitos, horseflies, black flies and deer flies, that can also trigger localized itchy swellings of the skin.
Just about anything the horse’s skin comes in contact with can become an allergen and cause an allergic reaction. Generally the allergic agent can be identified, in the case of topical allergens, as the skin lesions appear only on or near the area of the body where the allergen was applied.
Common offenders are fly sprays, shampoos, and grooming products. It is also not uncommon for some horses to develop skin reactions to the synthetic materials used to construct saddle blankets/pads, bits, girths and foot/wrap protection products.
Airborne agents such as mould, dust, and pollens are also known to cause signs of respiratory irritation or distress upon inhalation. Clinical signs include nasal drainage, cough, and/or laboured breathing.
This condition known by its many names as heaves, recurrent airway obstruction (RAO), or chronic obstructive pulmonary disease (COPD), falls into this category. Runny eyes, listlessness, and head shaking can be other non-specific signs of such environmental sensitivities.
The rarest form of allergens in horses are food related. When food allergies occur they usually manifest as hives/wheals, non-specific itchiness, or the “strange” skin conditions mentioned above. The most common offenders in this category are preservatives from processed feeds, supplementation products, particular types of plants and weeds, or the overuse of supplements, treats, and alfalfa.
Horses may experience adverse allergic reactions associated with medications and vaccinations as well. Some reactions are truly allergic in nature, while others are an inflammatory response to local irritation at the injection site. Irritation at the injection site presents as a localized area of swelling, inflammation and pain. Those horses with mild allergic reactions often develop varying degrees of generalized hives.
In very rare incidences, the most severe form of an allergic reaction, an anaphylactic reaction, may occur with systemic shock, collapse and death. It is important to note any adverse events related to medications or vaccinations as the reactions become increasingly more serious when the insult is repeated. Procaine penicillin has a reputation as an injectable drug that can cause a reaction.
Although some horses are indeed allergic to penicillin, it is more common for horses to react to the procaine carrier, violently so if it enters the bloodstream. This type of reaction is immediate, causing stimulation of the central nervous system. Horses usually show signs of excitement, occasionally falling down and/or thrashing. Although the situation is alarming, it is usually temporary and self-resolving.
Allergic responses can cause the horse discomfort and produce strange symptoms. The initial response to the reaction might be to “make the symptoms go away” as soon as possible.
Antihistamines and corticosteroids are current treatments which address these symptoms. However, these treatments will only be temporary if the horse continues to be exposed to the offending agent, whether from the environment, by ingestion, or topically. Seeking out the offending agents may seem like a fishing expedition, yet ultimately best management strategies for allergies in horses is to identify and avoid the allergen.