In a Type I reaction, the animal has been previously exposed to an antigen and produces an excess of antibodies. If this antigen appears in the blood, the reaction can be either body-wide (such as anaphylactic shock) or localized (such as itchy patches on the skin). If the antigen enters through the skin or mucous membranes, a more localized reaction is typical.
Anaphylactic shock is a rare, life-threatening, immediate allergic reaction to food, an injection, or an insect sting. The most common signs occur within seconds to minutes after exposure to the antigen.
The horse’s gums are very pale, and the limbs feel cold. The heart rate is generally very fast, but the pulse is weak. Facial swelling does not usually occur, but there may be itchiness around the face and head.
Anaphylaxis is an extreme emergency. If you think that your horse is having an anaphylactic reaction, seek emergency veterinary assistance immediately. A veterinarian can give intravenous injections of epinephrine and other medications to counteract the reaction. Treatment for related problems, such as respiratory distress, may also be needed.
Hives (urticaria) and areas of swelling are caused by allergic reactions to drugs (both topical and body-wide), irritating substances (for example, stinging nettle), chemicals, something eaten or inhaled, or insect bites or stings. Allergic reactions generally develop within 20 minutes of being exposed to the allergen (antigen) and disappear within 24 hours.
Hives are common in horses and are the least severe type of anaphylactic reaction. Small bumps occur on the skin. Often, the hair stands up over these swellings and sometimes they itch. Horses often become excited or restless. Hives can develop on any part of the body but are seen mainly on the back, flanks, neck, eyelids, and legs. In advanced cases, they may be found on the gums, nose, around the eyes (conjunctiva), rectum, and vagina. HIves can also occur due to other causes unrelated to the immune system, including pressure, heat, exercise, stress, or sunlight. Ringworm infections, autoimmune disease, and inflammation of blood vessels (vasculitis) can also cause hives in horses.
Swelling occurs due to fluid accumulation under the skin. It can be widespread, affecting the face (especially on the lips, the muzzle, and around the eyes), body, genital region, or legs. Swelling is more serious than hives. If it extends to the airways, the condition can become life-threatening.
Hives are usually not life threatening and typically go away by themselves once the cause of the allergic reaction is removed or passes through the body. Veterinarians often treat hives and swelling by providing corticosteroids or other drugs. Your veterinarian will make treatment decisions based on your horse’s individual situation.
In some mares, a milk allergy develops when the pressure inside the mammary glands increases enough that some stored milk components (usually the protein casein) are forced into the mare’s circulatory system. The mare’s immune system reacts to these “foreign” proteins in her blood. This results in a hypersensitivity reaction that may be localized (hives and/or swelling involving only a small part of the mare’s body) or generalized and severe (anaphylactic shock). Recovery is usually prompt once the mare’s mammary gland is emptied.
Heaves (recurrent airway obstruction) is a common respiratory disease of horses. It is thought to occur because of an allergic reaction caused by longterm exposure to allergens (such as mold or dusts in hay) in poorly ventilated barns. The allergic reaction causes inflammation and constriction of the lower airways. Signs include longterm coughing, discharge from the nose, and difficulty breathing. Reducing the horse's exposure to allergens is paramount in treating the condition. Your veterinarian will likely also prescribe medications to improve breathing.
Sweet itch is a skin allergy in horses that is usually seen in the warm summer months. It is associated with some insect bites, especially night-feeding Culicoides. These insects include midges (“no-see-ums”) and a member of the black fly family. Sweet itch is characterized by intensely itchy patches that appear along the back of the horse from the ears to the tail and near the anus. Sweet itch is diagnosed by history, signs, skin tests, and identifying the insect. Treatment includes keeping the horse away from the biting insects and providing medication to control the itching and allergic reaction. Preventive measures include destroying the flies’ breeding grounds, spraying stable areas with an approved pesticide, and using a fan to move the air around the horses.
Also see professional content regarding anaphylactic reactions.