What Is a Rodent Ulcer?

You’re worried about your cat Henry. About a week ago, you noticed that a small, hairless, red sore had developed mysteriously on his upper lip, and it isn’t going away. Perhaps you have nothing to worry about. Maybe this angry-looking eruption will simply disappear within a few days.

But you’ve never before observed anything like this, so you wisely decide to take Henry to see your veterinarian. In an effort to reassure you, the vet gives your cat a thorough exam and, regarding the mean-looking sore on Henry’s lip, tells you not to be overly concerned — it’s simply a case of “rodent ulcer.” What? Does this mean that Henry has been bitten by a rat? Or that he’s been somehow interacting with the mice that occasionally scamper around in your garage?

Not at all, says the vet reassuringly. In fact, he explains, Henry’s problem, despite its name, has nothing to do with rats or mice — not to mention the squirrels, gophers, gerbils, prairie dogs or any of the other creatures that fall into the rodent category. Then why in the world, you ask, is the sore on Henry’s lip regarded as a clinical sign of something called rodent ulcer?

Where the name comes from

“The term comes from an old, mistaken belief that these sores were caused by a mouse or rat bite,” explains Michael Stone, DVM, clinical professor of small animal medicine at Tufts, “and even though the veterinary community now knows that this is not the case, the term ‘rodent ulcer’ has stuck.” Indeed, that particular name for the disorder continues to be accepted although several other terms — lick granuloma and indolent ulcer, for example — have also come into use by veterinarians when discussing feline skin sores resembling the unsightly one on Henry’s upper lip. Currently, the sores are considered to be a form of a condition referred to as eosinophilic granuloma complex (EGC).

This disease category is named after certain white blood cells (eosinophils) that form in a cat’s bone marrow and rush to counter inflammation (granuloma) in an area of a cat’s body that has been irritated or traumatized. In addition to rodent ulcers, the EGC category includes two other types of feline skin disorders: eosinophilic plaques, which are moist, itchy, red sores that are usually found on a cat’s inner thighs and abdomen; and eosinophilic granulomas, pale yellow or pink lesions that tend to appear on a cat’s thighs or face.

Rodent ulcers, which form on a cat’s upper lip adjacent to one of the animal’s canine teeth, are typically well defined, reddish-brown in color, and shallow, with thickened edges. “Their characteristic appearance,” says Dr. Stone, “is a flat or concave lesion on the edge of the upper lip, located directly below the nose or slightly to one side. In most cases, the lesion does not appear to bother the cat, to cause pain, or to affect the cat’s appetite or behavior. But it looks pretty ugly.”

In most cases, the precise cause or causes of these sores remain unknown. A wide range of possibilities exists, however, including bacterial infection, flea or food allergies, parasitic infestation, autoimmune disease, ringworm infection, traumatic damage, feline leukemia virus infection and mosquito bite hypersensitivity. Rodent ulcers are seen in cats of all breeds, with females and very young cats at elevated risk. Indeed, the clinical signs are observed three times as often in females as males. And, says Dr. Stone, “The disorder is most commonly diagnosed in cats who are six years of age or younger. This may be due to the fact that allergies, which are thought to play a role in the development of these lesions, are most common in this age group.”

Can become uncomfortable

Most rodent ulcers cause little discomfort and will disappear spontaneously over a period of weeks or months. Some lesions, however, may persist, causing chronic pain. Because of this discomfort, an affected animal’s appetite may decline, potentially resulting in weight loss and physical weakness. According to Dr. Stone, if an untreated lesion does not eventually resolve spontaneously, its appearance may remain the same or it may progress to involve a larger portion of an affected cat’s upper lip.

Regarding diagnosis of a suspected case of rodent ulcer, Dr. Stone says: “In most instances, a visual examination will be sufficient, although a tissue sample may be needed in some cases in order to rule out other possible problems, such as a malignancy, or to confirm the findings of the visual examination.” Another diagnostic measure could be analysis of a blood sample, which might reveal increased numbers of eosinophils.

How they’re treated

Treatment of a rodent ulcer is aimed at getting rid of an existing growth and preventing a future recurrence. At present, the therapy of choice for EGC is the aggressive use of glucocorticoids. Ulcers often disappear, for example, when treated with daily doses of prednisone, an anti-inflammatory medication that works by suppressing the immune system. Surgery to remove a rodent ulcer is never a treatment option, notes Dr. Stone, although it may be used to obtain a biopsy.

Some cats develop lip ulcers repeatedly throughout their lives and therefore require repeated courses of treatment. Other cats may simply outgrow their inclination to develop rodent ulcers as they mature. In most cases, however, once a rodent ulcer is successfully treated, the chances of recurrence are slim. Dr. Stone urges owners to be vigilant. “Be on the lookout for potential skin ulcer causes, such as an allergy to fleas and to certain foods,” he advises. “Watch for any signs of a rodent ulcer, and contact your veterinarian if you spot any lesions on your cat’s lips.” — Tom Ewing