Moderate severity

Rodent Ulcer (Eosinophilic Ulcer)

A raised, reddish-brown sore that most often appears on a cat's upper lip, part of a group of skin reactions known as the eosinophilic granuloma complex.

⚠️ Common symptoms

A well-defined ulcer or swelling on the upper lipA glistening, reddish-brown or ulcerated surfaceUsually not painful or itchy at firstOccasionally swelling that distorts the lipMay recur in the same spot

The name rodent ulcer is a historical misnomer — the lesion has nothing to do with mice. It is one presentation of the feline eosinophilic granuloma complex, a family of inflammatory skin conditions in which a type of white blood cell called an eosinophil accumulates in the tissue, typically as part of an allergic response.

Classically the ulcer forms on the upper lip, often to one side, appearing as a shiny, reddish-brown eroded patch. It usually does not seem to bother the cat, which means owners may only notice it during grooming or a cuddle. Because the root cause is generally an underlying allergy, lasting improvement depends on identifying and controlling that trigger rather than treating the sore alone.

Most cases respond well once the allergy is managed, though some cats are prone to flare-ups. Since a small number of lip lesions can be something more serious, any sore that fails to heal or keeps returning deserves veterinary attention and sometimes a biopsy.

🔍 Causes

Most rodent ulcers are driven by an allergic or hypersensitivity reaction — commonly flea-bite allergy, food allergy or environmental (atopic) allergy — rather than by rodents at all, despite the traditional name.

🛡 Prevention

Consistent, year-round flea control is one of the most effective preventives, alongside identifying and avoiding food triggers under veterinary guidance for cats prone to recurrence.

💊 Treatment

Treatment focuses on finding and controlling the underlying trigger, which is usually an allergy. Vets may investigate and manage flea, food or environmental allergies, and use anti-inflammatory medication to settle the lesion. Because these ulcers can rarely become something more serious, a vet may take a biopsy if a lesion does not respond as expected.

When to see a vet: Have any lip sore or swelling checked, as a persistent or growing lesion should be examined to confirm the diagnosis and rule out other causes.

⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

← Back to Health & Ailments