A variety of ominous skin eruptions that can appear just about anywhere on a cat’s body may signal the emergence of a troublesome condition referred to broadly as eosinophilic granuloma complex (EGC). According to William H. Miller Jr., VMD, a professor of dermatology at Cornell University’s College of Veterinary Medicine, the eruptions can include “oozing masses, yellowish-pink ulcerations, or big tumor-like bumps,” depending on their specific source.

Most such lesions emerge when certain immune system cells (eosinophils) mistakenly release inflammatory chemicals to counter the invasion of parasites when, indeed, no such invasion has occurred. In any case, the chemicals will produce the clinical signs of allergy, such as itching, swelling, and inflammation. Rather than parasitic invasion, Dr. Miller points out, the causes of these lesions are most frequently allergic reactions to flea, mosquito, or mite bites—or the bites of “anything else that injects saliva into an animal’s system.” Other possible causes include systemic reactions to substances including antibiotics, heart medications, allergy-inducing foods, and a wide variety of airborne pollens.

EGC comprises three distinct phenomena: eosinophilic granuloma; eosinophilic plaque; and indolent ulcer. Some cats may experience only one of these dermatologic problems, while others may simultaneously be assailed by two or three of them. Each of the three has its own distinctive characteristics. Indeed, Dr. Miller notes, their grouping is questionable, since their most salient commonalities lie in the grief they cause an affected animal and in the diagnostic challenges they present. The grouping was devised many years ago, he points out, when the three conditions were first described. However, he notes, eosinophilic granuloma is the only one of the three that actually presents with a granuloma (a superficial, spherical mass of immune system cells). “But they deserve to remain as a grouping,” he notes, “because all three show the activity of eosinophils and have an apparent allergic condition as the triggering event.”

Among the three manifestations of the complex, eosinophilic granuloma is, by far, the most frequently diagnosed. The lesions are typically raised, linear, clearly defined, and yellowish-pink in color, but there are wide variations in their clinical appearance. “It can be a bump,” says Dr. Miller, “or a great big ulcer that looks like a tumor.” These lesions can emerge anywhere on a cat’s body, he points out, but appear most frequently on the rear legs and in the mouth.

Less frequently observed are eosinophilic plaque lesions, which, he says, look like “red, angry hives” and appear most commonly on an animal’s abdomen or thighs. And he describes an indolent ulcer as a pustulant sore typically observed on one or both sides of a cat’s upper lip.

In many cases, the lesion will spontaneously vanish over time. However, if the itching it causes persists, an affected cat may keep scratching at it, thereby opening it and subjecting the lesion to secondary infection. “And this can become a prime concern,” says Dr. Miller. “If the lesion is on the back of the head, a cat’s nonstop scratching can open the skin within hours—and the wound can then get very nasty very quickly. On the other hand, some lesions can exist for months on end, and if a cat doesn’t pay attention to it, it can just disappear without consequence.”

Diagnosis will typically entail a review of the affected cat’s medical history, a thorough physical exam, and a search of the animal’s skin and hair coat for external parasites. Sometimes a skin biopsy will be required. Also, says Dr. Miller, the cat’s behavioral habits must be considered. For example, he says, “If the lesion appears during the summer, and the cat is in the habit of going outdoors late at night when there are a lot of mosquitoes around, you can pretty well guess what the cause is.” Treatment with steroids will usually make individual lesions vanish, he points out, unless the wound has become infected, in which case antibiotic therapy will probably be necessary.

Most cats don’t ever develop EGC lesions, Dr. Miller points out, observing that, in general, they are quite uncommon. Nevertheless, he advises owners to reduce the chances of the affliction through such measures as keeping an animal indoors when mosquitoes and other biting creatures are in the environment, using appropriate insect repellents, and strictly avoiding the use of food to which a cat is known to be allergic.