The sore on this cat’s lip (see arrow) is called a rodent ulcer. It used to be thought that cats got these from eating wild rats and mice, but now it is known to be an autoimmune condition.

Dear Dr. Doug,

Can cats get cold sores? I took my cat to the vet. He called it a rodent ulcer and gave her a steroid shot. It went away but came back about a month later. She never goes outside and I don’t have rats in the house. How could she be getting these?

Stacey, KW

Dear Stacey,

A “rodent ulcer” is not an actual disease. It is a term used to describe sores around the lip area on a cat. It is classified as part of an autoimmune skin condition called the eosinophillic granuloma complex. There are three manifestations to this complex, the “rodent ulcer,” also technically known as the eosinophillic ulcer, is the most common. The other two are eosinophillic plaque and linear granulomas.

The name “rodent ulcer” came about years ago when it was believed that cats got these “sores” on their lips when they caught and ate a wild mouse or rat, the thought being that rodents carried diseases that caused the lesions on the cat’s lip. After it was realized that indoor cats with no exposure to rodents, just like your cat, also developed these same sores, the rodent-related diagnosis was dropped, but the name “rodent ulcer” still used by many veterinarians today.

The ulcers have also been occasionally reported in dogs, but are most commonly associated with cats, and are usually seen on their upper lips, but can be anywhere around the mouth.

The inciting causes of these ulcers have be attributed to potential factors such as allergies, parasites, ringworm, bacterial and viral disease and even skin cancer. The chance of successfully treating these sores increases with the ability to determine and successfully identify the cause.

Treatment varies, obviously, determined by the initial cause of the ulcer. For instance, if the cat has ringworm, treating the ringworm should eradicate the sore. If the actual cause cannot be determined, then treating symptomatically is appropriate.

I suspect your veterinarian gave your cat a long-acting corticosteroid drug called Depomedrol. Corticosteroid therapy, in one of many forms, when the actual cause cannot be determined, is still the treatment of choice for rodent ulcers. There are long-acting injections, short-acting injections and pills that can be administered. The choice should depend on the recommendations of your veterinarian based on his or her clinical findings. These long-acting steroid injections can cause diabetes mellitus if repeated too many times — be sure to see your doctor for proper follow-up examinations.

One last note: Many of these rodent ulcers will spontaneously resolve in two to four weeks, even without the benefit of treatment. That said, in some cases the ulcer may get infected and can cause significant pain, enough that it will prevent your cat from eating.

If your cat’s new sore lip does not resolve on its own in the next few weeks I would recommend that you take it back to your veterinarian for a thorough physical examination, including possible blood testing, and in some cases, tissue biopsies, to see if the actual cause can be determined.

Dr. Doug Mader is an ABVP board-certified veterinary specialist practicing in the Keys. Send your questions to

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