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 result can be either anaphylactic shock or more localized reactions, such as itchy patches or hives on the skin. If the antigen enters through the skin, 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. These signs include severe respiratory distress and the sudden onset of diarrhea, vomiting, excessive drooling, shock, seizures, coma, and death. 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 to counteract the reaction. Treatment for related problems, such as respiratory distress, may also be needed.
Hives and Swelling
Hives (urticaria) and areas of swelling are caused by allergic reactions to drugs, chemicals, something eaten, insect bites, or even sunlight. They generally develop within 20 minutes of being exposed to the allergen (antigen). Hives 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. Swelling is most often noticed on the face, especially on the lips, the muzzle, and around the eyes, but may also occur on the body or legs.
Hives and swelling 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 these reactions by providing corticosteroids or antihistamines. 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.
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 identified by skin tests. 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.
Last full review/revision July 2011 by Christine Andreoni; Kevin T. Schultz, DVM, PhD