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 on the skin). If the antigen enters through the skin, the 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 cat'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 cat is having an anaphylactic reaction, seek emergency veterinary assistance immediately. A veterinarian can give intravenous injections of epinephrine to counteract the reaction. Treatment for other problems, such as difficulty breathing, 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 nose, and around the eyes. The swelling can be so severe that the cat cannot open its eyes.
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 antihistamines. Your veterinarian will make treatment decisions based on your pet's circumstances.
Coughing and wheezing are the most common signs of allergic bronchiolitis, which is an inflammation of the lower portion (bronchioles) of the airway. This disease may be mistaken for other conditions such as asthma or lungworm disease. The early signs of the disease can easily disappear with common medications. If the disease increases in severity, more powerful medication may be required. Your veterinarian can adjust the prescribed medication based on your cat's reaction. It is usually not possible to identify the antigen causing the allergic reaction.
Allergic asthma is more often found in cats than in other animals; however, it is still less common than in humans. It occurs more frequently in summer and after going outdoors. Asthma attacks can be moderate or lengthy and severe. The signs are shortness of breath and frantic attempts to inhale. The condition occurs as a result of constriction of the breathing passages triggered by the release of compounds, such as histamines, that combat allergens. Corticosteroids may be recommended to alleviate severe signs, but they do not treat the underlying cause of the asthma. Determining the allergic trigger can be difficult.
PIE Syndrome (Pulmonary Inf iltration with Eosinophilia)
Infiltration of the lungs with a thick fluid and white blood cells, called PIE syndrome, is caused by allergens, viruses, and parasites. It is uncommon in cats. Animals with PIE syndrome generally become lethargic and have difficulty breathing with normal exercise. It is usually not possible to determine the antigen causing the reaction. Medications can help control the signs of the disorder.
Food allergies occur in cats as well as people. They often develop following an intestinal infection with a virus, bacteria, or protozoan and can lead to inflammation of the lining of the stomach and intestines. In cats, the first (and sometimes only) sign is vomiting that occurs within 1 to 2 hours of eating. Weight loss, diarrhea or soft feces, and poor coat condition may also occur. Feces usually are normal in amount and frequency, but consistency varies from semi-solid to watery. They may be extremely odorous. Severe cases of food allergies are characterized by diarrhea and sometimes by bloody feces.
Both the diagnosis and treatment of food allergies are done by strictly controlling the diet at the direction of a veterinarian. Your veterinarian is likely to recommend a basic diet that includes a protein source the cat has not eaten before, such as ground, cooked turkey or lamb. Follow the recommended diet carefully to help identify the food that causes your pet's allergic reaction. Once signs have disappeared (usually after 1 to 2 weeks), additional foods can be introduced 1 at a time until the problem food is identified. Commercial prescription diets are also available. Kittens with food allergies often grow out of them. Older animals may need special and restricted diets for the rest of their lives.
Skin Allergies (Atopy)
Skin allergy, also called atopy, occurs when a cat's skin overreacts to certain allergens in the environment. In cats, food allergies are probably a more common cause of skin allergies than inhaled allergens (such as pollen). Veterinarians diagnose skin allergies by a medical history, physical examination, and various tests including exclusion trials (where potential allergens are removed from the environment and then reintroduced) and skin tests.
The key to managing this condition is removing or restricting exposure to the allergen or contact irritant in the cat's environment. Treatment consists of an extended series of injections of the possible allergen under the skin until improvement is noted. Several medications are available to help control the skin allergy. Your veterinarian will select a treatment program that is appropriate for your cat and its specific allergy (see Skin Disorders of Cats: Allergies of Cats).
Last full review/revision July 2011 by Christine Andreoni; Kevin T. Schultz, DVM, PhD