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, a localized reaction is more common.
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. Dogs differ from other domestic animals in that the major organ affected by anaphylactic shock is the liver, rather than the lungs. Therefore, gastrointestinal signs are the major signs of anaphylactic shock rather than respiratory signs. These signs include sudden onset of diarrhea, excessive drooling, vomiting, shock, seizures, coma, and death. The dog’s gums may be pale, and the limbs may feel cold. The heart rate is generally very fast, but the pulse is weak.
Anaphylaxis is an extreme emergency. If you think that your dog is having an anaphylactic reaction, seek emergency veterinary assistance immediately. A veterinarian can give intravenous injections of epinephrine (adrenalin) to counteract the reaction. Treatment for other associated problems, such as difficulty breathing, may also be needed.
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 exposure to the allergen (antigen). Hives are the least severe type of anaphylactic reaction and cause small bumps to appear 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. The swelling can be so severe that the dog cannot open its eyes.
Hives and swelling are usually not life threatening and typically go away by themselves if the source of the allergic reaction is removed or passes through the body. Veterinarians treat these reactions by giving appropriate antihistamines.
Like humans, dogs can also have seasonal allergies (usually caused by pollen exposure). They can cause a watery nasal discharge and sneezing called allergic rhinitis. Nonseasonal rhinitis may be due to exposure to such allergens as molds, dander, bedding, or feeds. The condition is diagnosed by the presence of eosinophils (a type of white blood cell) in the nasal discharge, a favorable response after treatment with antihistamines, the disappearance of signs when the offending antigen is removed, and its seasonal nature. Although skin tests can diagnose the allergic reaction in people, skin testing is not presently an accurate means to diagnose nasal allergies in animals.
Chronic allergic bronchitis is characterized by a dry, harsh, hacking cough that is easily brought on by physical activity. The disease may be seasonal or may occur year-round. The condition is treated with bronchodilators and expectorants, which aid in the removal of thick, sticky mucus. Your veterinarian may prescribe additional medications (such as steroids) to help control this type of immune-mediated bronchitis. It is usually not possible to avoid the antigen causing the reaction.
Eosinophilic Bronchopneumopathy (formerly known as PIE Syndrome [Pulmonary Infiltration with Eosinophilia])
Infiltration of the lungs with a thick fluid and white blood cells, called eosinophilic bronchopneumopathy, is caused by allergens, viruses, and parasites in dogs. Pets with the disorder generally become lethargic and have difficulty breathing with normal exercise. It is usually not possible to determine the antigen causing the reaction. Veterinarians can prescribe steroids to help control the signs.
Food allergies occur in pets as well as people. They can lead to inflammation of the small intestine. Dogs exhibit vomiting within 1 to 2 hours of eating and may have intermittent soft stools. The dog is usually otherwise healthy, although there can be weight loss and poor coat condition in severe cases. Food allergies often develop following an intestinal infection.
Both the diagnosis and treatment of food allergies are done by strictly controlling the diet at the direction of a veterinarian. Dogs should be fed an elimination diet that does not contain any ingredients that have been previously fed. Veterinarians can recommend a complete and balanced home-cooked meal or a commercial prescription diet. In case the signs do not disappear after the dietary changes, prescription medications may provide relief for affected dogs.
Skin allergy, also called atopy, occurs when a dog’s skin overreacts to certain allergens in the environment. It has been estimated that 10% of all dogs have these allergies, which are commonly due to inhaled substances, such as dust mites, pollen, mold, or dander. Certain breeds of dogs, including terriers, Dalmatians, and retrievers, are predisposed to developing skin allergies. The most commonly affected areas are the feet, face, ears, elbows, armpits, and abdomen. The affected areas are very red, itch, and may have small bumps.
The condition is diagnosed by history and physical examination. Although allergy tests cannot diagnose skin allergies, they can help determine the source of the allergy and help direct treatment. In one such test, the dog is injected with small amounts of possible allergens into the skin. If the dog is allergic to the injected substance, a swelling will occur immediately at the injection site.
Skin allergies cannot by cured and require lifelong management and routine veterinary assessments. Avoiding the allergens that trigger the allergy is ideal but difficult to do. Relief from itching can be achieved through the use of prescription drugs, therapeutic shampoos, avoiding flare-inducing triggers (such as fleas, allergens, or infections), and "allergy shots." This last treatment, called allergen-specific immunotherapy, consists of an extended series of injections of the offending allergen under the skin until improvement is noted. This type of treatment is effective in 60% of dogs. If your veterinarian prescribes a medication to control your pet’s allergic reactions, be sure to follow the directions carefully and fully, including any restrictions regarding exposure to carpets, chemicals, or other potential hazards. (See also Allergies in Dogs : Airborne Allergies (Atopy or Canine Atopic Dermatitis).)
Also see professional content regarding Type I reactions.