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The Allergic Reaction

Pets, like people, have allergic responses when their immune system overreacts to certain substances. When they enter the body, the offending substances, called antigens or allergens, set off an alarm. This alarm stimulates the body to produce antibodies to defend itself against what it perceives as a threat — the allergen invaders. The antibodies attach themselves to immune cells, called "mast cells," within the skin and other body tissues. When the allergens penetrate these tissues' surfaces, they are captured by the antibodies, which then stimulate the mast cells to release powerful chemicals into the surrounding tissues. It is these chemicals, called histamines, that cause inflammation and itching.

The body's immune system is meant to protect against harmful substances, so why do some animals have a severe reaction to non-threatening substances? "We think it's very similar to the situation in human beings," says Daniel O. Morris, D.V.M., a board-certified veterinary dermatologist and chief of staff of the veterinary hospital at the University of Pennsylvania in Philadelphia. Some animals, like some individuals, "may carry genes that put them at risk for developing allergic reactions," he says.

Another explanation is the hygiene theory, says Morris, which suggests that if you allow children to be exposed to infectious organisms early in life, their immune response may better control infections and make them less likely to develop allergies. In societies that stress cleanliness and try to protect children from dirt and disease, a child's body may overreact when confronted with a foreign substance, even a harmless one. The hygiene theory may explain why allergies in children are on the rise in the developed world, and the same explanation could apply to allergies in pets, says Morris.

Diagnosing

There are many conditions that can make a pet itch or have hair loss, including endocrine, autoimmune, infectious, and parasitic skin diseases. It takes some detective work to identify the cause. A veterinarian may be able to diagnose the problem or may refer your pet to a veterinary dermatologist, a specialist in treating skin conditions in animals.

If allergies are suspected, the first thing a veterinarian will usually ask is if the pet is on a flea-control product, says Troutman. Flea allergies are the most common type of allergies and the easiest to control, she says. And just because a pet is kept indoors doesn't mean it can't have fleas. An owner might bring fleas into the house on a piece of clothing, and the fleas can jump onto the pet. Just a single fleabite can cause an allergic animal to itch severely for more than five days, according to the American Animal Hospital Association.

Pet owners have many options for flea control on pets and in their environment. Veterinarians can recommend an appropriate product. (See "Taking the Bite out of Fleas and Ticks," July-August 2001 FDA Consumer.)

Once flea allergies are ruled out and if the itch is non-seasonal, food allergies are checked next. Food allergies are not related to a season, while many atopic allergies start out as a seasonal problem, says Morris. Dogs and cats that develop atopic allergies usually show symptoms between 1 and 5 years of age, he says, but food allergies can crop up at any time. They are high on the list of suspects when a dog or cat first exhibits itchy skin at an age less than 6 months or over 5 years.

To test for food allergies, the pet is put on an "elimination diet" for at least 10 weeks, which means it is fed food that consists of a protein and carbohydrate that the pet has not eaten before, such as duck, venison, and potatoes. Veterinarians offer these special foods, and some may be found in retail stores. Or the owner may choose to feed the pet a homemade diet of foods recommended by the vet.

If the animal's itching subsides by at least half, the allergen is considered to be one or more food ingredients, says Jeffers. To confirm this, the owner can reintroduce the old food to see if the symptoms return. To find the specific ingredients that trigger the allergy, the owner should feed the special diet again and add one ingredient at a time from the old diet for at least a week until the itching increases, indicating that the last added ingredient is an allergen. Or the owner may choose to stay with the special food to avoid causing the pet discomfort each time an allergic ingredient is fed.

While the pet is being tested for food allergies, it should not be given treats, chewable medications, table scraps, or rawhide toys that may contain an allergen.

To check for atopic and contact allergies, veterinary dermatologists use an intradermal allergy test, or skin reaction test. The pet is mildly sedated, a postcard-sized area on the side of the pet is shaved, and small amounts of potential allergens are injected into the skin on the shaved area. If the pet is allergic to a particular substance, the skin will become inflamed at the area of the injection.

Jeffers tested Nora, a wire fox terrier, for 58 different allergens. The dog had been "scratching and biting herself all over, 24 hours a day," since it was 3 months old, says owner Katie Mathews of Bethesda, Md. "The scratching kept Nora up all night and kept the family up all night," she says. Before she was referred to Jeffers, Mathews had taken Nora to several veterinarians, who prescribed various antihistamines, shampoos, sprays, and a food elimination diet — none of which worked. "Steroids were successful," says Mathews, "but I didn't want to keep her on them because of the long-term side effects." Mathews also "wanted to get to the root of the problem" so that the allergic substances could be avoided if possible.

Through skin testing, Jeffers determined that Nora had atopic and contact allergies and was allergic to dozens of substances, including pollens, molds, dust mites, grass, cotton, and wool.

Tags: Pets

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