In its relatively brief time on earth (typically ranging from a few weeks to a year or so), the teeny, brown wingless cat flea (Ctenocephalides felis felis) asks little from life. All it wants are a few basics—a comfortable place to live, adequate nourishment, and a hospitable environment in which to raise a family. The warm, moist, furry hide of a cat can provide all of these amenities—and that’s why fleas, if given the opportunity, will swiftly hop on an unwary animal’s back and take up residence there.

Living in such comfortable surroundings may be just fine for the flea, but the situation can cause no end of itchy torment for its host—especially to a cat that happens to be allergic to flea saliva, which the little insects deposit in their hosts’ skin when they bite. And they do have a tendency to bite.

“An allergy,” explains William Miller Jr., a professor of dermatology at Cornell University’s College of Veterinary Medicine, “is an exaggerated response to a stimulus, and this overreaction can manifest itself in an animal’s skin. All cats can be affected to some extent by flea bites, but an allergic cat will react with disproportionate severity. Where it would take dozens of flea bites to significantly harm a normal cat’s skin, the same amount of damage to the skin of an allergic will result from just a few bites.”    

This hypersensitivity, Dr. Miller points out, is a physiologic aberration whereby a cat’s immune system mistakenly recognizes a nontoxic foreign substance that has entered its body as harmful. In an effort to combat the substance—called an allergen—the animal’s immune system releases a chemical compound called histamine. The itching that characteristically signals the presence of a flea bite allergy is caused by the eruption of small, pale, fluid-filled lumps on the skin, which form in response to the allergen’s presence.

According to Dr. Miller, cats tend to be bitten mostly on the back of the neck and the top of the tail head. “Cats are grooming animals,” he points out, “and the fleas quickly figure out that a cat can’t get at those areas. So the cat starts scratching, and because cats have very sharp claws, they can get very severe skin lesions very quickly.” However, he adds, cats with flea allergy dermatitis are apt to show distressful signs—reddish, crusty bumps, for example—even in areas that have not been savagely scratched as well as those that are obviously itching. The lower back, thighs, abdomen, head, and neck are among the areas most commonly affected. As aggravating as the itchy welts may be, Dr. Miller notes, they do not, in themselves, pose a serious health problem. But the incessant scratching that they prompt may cause secondary skin wounds and, consequently, vulnerability to severe bacterial infections.

Obsessive scratching is the clearest indication that a cat is infested with fleas—especially if the weather is warm and muggy. On close inspection, the insects can be seen with the naked eye, usually on the animal’s belly, back, or near the tail. Fleas may also be detected by having the animal stand on a large piece of damp white paper or a pillowcase and running a fine-toothed comb through its coat. The fleas, along with small black specks (their feces) will be visible on the white background.

Suspected infestation can be readily confirmed by means of veterinary diagnosis involving the animal’s medical history, a physical examination, and possible skin testing. If a definitive diagnosis of flea allergy is established, the veterinarian may prescribe any of several available systemic medications, such as: an antibiotic (a substance that can inhibit or destroy the growth of invasive microorganisms); a corticosteroid (a hormone often used to moderate an immune response and reduce inflammation); or an antihistamine (which inhibits the action of the chemical agent whose production causes the release of fluids into the tissues of an affected cat that can lead to inflammation and itching). No such medications, of course, should ever be used without the specific recommendation of a veterinarian.

At the same time, Dr. Miller advises, measures must be taken to remove all fleas from a vulnerable cat’s environment. “You have to treat the animal,” he notes, “and you also have to protect against flea infestation in its environment.” Your veterinarian can help you choose the best topical and/or oral flea medications for your cat, and offer advice on how to treat the environment.