Ask a Vet: What Causes Mouth Ulcers in Cats?


In recent months I have seen an upsurge in questions submitted through my website and Facebook Page that are similar to this one:

My cat has quite severe mouth ulcers on his upper lip and a bit inside his mouth. We’ve taken him to a vet in China and did some tests. A test showed that the possible causes of my cat’s condition are that his infected area on the lip contains a large number of inflammation cells. Also, a blood test showed that his infected upper lip has two types of bacteria. The vet prescribed antibiotics and steroids to him based on the examination, but these pills don’t seem to be working. We’re very hopeless seeing him suffer from the condition. Could you please help him with his condition?
Thank you! –Cate

Medicine is a constantly evolving field. I think back to some medical practices that have been en vogue over the course of history and cringe at the crudeness and ignorance that they betrayed. Bleeding to balance yellow and black bile was once the standard of care. Ignatz Semmelweis was ridiculed when he suggested that hand washing might prevent disease. And back in the day, vets thought ulcers such as the ones described by the reader were caused by rodent bites.

Photo of Dr. Eric Barchas by Liz Acosta
Photo of Dr. Eric Barchas by Liz Acosta

At first thought, it makes sense. If a cat is bitten by a prey animal, the bite might become infected and inflamed, leading to the lesions that subsequently were called rodent ulcers. However, even if one ignores the fact that, despite the antics of Tom and Jerry, cats are designed to catch rodents without injury, one cannot escape the modern reality that rodents have nothing to do with the lesions.

Nobody knows how our understanding of the lesions described by Cate will evolve in the future. But here is what we know now.

The ulcers described by Cate almost certainly are manifestations of feline eosinophilic disease, formerly known as eosinophilic granuloma complex, formerly known as rodent ulcers. Allergy appears to be the trigger for feline eosinophilic disease. There probably also is a hereditary component. Secondary infections can occur and exacerbate the lesions.

Feline eosinophilic disease most commonly manifests as ulcers on the lips. In my experience the upper lip near the nose most frequently is affected, and in severe cases the ulcers can erode significant portions of the lips. Other parts of the body also may be affected. It is not uncommon for cats to develop rashes on their abdomen or on the backs of their thighs.


The oral ulcers appear to be painful. I have known cats who were very fractious — which is to say, hard to work with at the vet’s office — who became sweet and docile once their eosinophilic disease was controlled. Ulcers on the lips, if severe, can lead to permanent disfigurement.

I believe I know why I have been getting so many questions about eosinophilic disease in the past few months. The lesions are caused by allergies, and the No. 1 allergen in cats is more prevalent in the summer. I’m talking about fleas.

Because feline eosinophilic disease is linked to allergies, the first step in treating it is eliminating allergens. Any cat with the condition should be on very good flea control. Special hypoallergenic diets should be considered. Affected cats should be kept indoors to reduce exposure to environmental allergens and mosquitoes. (Mosquitoes are common triggers for feline allergies.)

Treatment with antibiotics usually helps because secondary infections are common and can exacerbate the lesions. However, antibiotics alone are not likely to cure the lesions.

Vet examines a cat’s teeth by Shutterstock
Vet examines a cat’s teeth by Shutterstock

Allergies are a manifestation of aberrant immune system function. They occur when the immune system overreacts to allergens. Therefore, most cats with feline eosinophilic disease require medications to alter the function of the immune system in order to bring the condition under control. Steroids such as prednisolone are the agents used most frequently. Another medication, cyclosporine, also commonly is used.

Cate, it sounds like your veterinarian is on the right track for the most part. When treating feline eosinophilic disease it can take weeks or months for significant progress to occur. Patience is necessary. However, you might want to confirm the dose of steroids that your cat is taking; it might need to be adjusted up for a while, then tapered once you see a response. However, to truly tackle the problem you will need to be aggressive about allergen avoidance.

Cate, your cat will probably require steroids until the lesions have resolved fully. After that I hope that allergen avoidance will be sufficient to keep him comfortable and happy. Meanwhile, pain killers should be considered to help him feel better.

Feline eosinophilic disease is yet another reason why all cats should be on modern, effective flea preventatives.

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