A "rodent ulcer" is a non-painful, well-defined erosion on a cat's upper lip — typically the midline of the philtrum — and is the most recognizable form of the feline eosinophilic granuloma complex. It is not caused by rodents and not actually an ulcer in the surgical sense; it's a localized hypersensitivity reaction, most commonly to fleas or to environmental and food allergens (Buckley & Nuttall, 2012, JFMS). Strict flea control plus addressing the underlying allergy resolves most cases within 4 to 8 weeks.
Last reviewed: June 2026
What Rodent Ulcer Actually Is
Rodent ulcer is the lay term for an indolent or "eosinophilic" ulcer, one of three skin lesions making up the feline eosinophilic granuloma complex along with eosinophilic plaque and linear granuloma. The lesion is a sharply demarcated, glistening, copper-pink erosion of the upper lip, usually 0.5 to 2 cm across and centered just above the philtrum. Despite its dramatic appearance, the cat is rarely bothered by it — most cats eat, groom, and behave normally. The condition is not infectious and not a true ulcer in the dermatopathologic sense. It is a reactive inflammatory lesion driven by underlying hypersensitivity.
The Three Main Underlying Causes
The single most common underlying cause is flea-bite hypersensitivity. Even in apparently flea-free indoor cats, careful skin scraping, flea-comb checks, and trial flea control resolve a large fraction of cases. The second is environmental allergy (atopy) to dust mites, pollens, mold, and other inhaled allergens. The third is food allergy, usually to a protein the cat has been eating for months or years. A small minority of cases are associated with viral infection, including feline herpesvirus 1 (FeHV-1) which can produce indolent-appearing lip lesions. As described in Greene's Infectious Diseases of the Dog and Cat, FeHV-1 should be considered in cats with concurrent ocular or upper respiratory signs.
How Vets Confirm the Diagnosis
A typical rodent ulcer is recognizable on exam. The workup confirms it is an eosinophilic lesion and rules out neoplasia such as squamous cell carcinoma, which can mimic the appearance especially in older white-faced cats. Fine needle aspiration shows eosinophils in classic cases; biopsy with histopathology is the definitive test and is recommended whenever the lesion is atypical, ulcerative beyond the lip, large, or unresponsive to therapy. A complete blood count often shows peripheral eosinophilia. Skin scraping, fungal culture, and a flea comb examination round out the basic workup.
Treatment That Addresses the Cause
Symptomatic therapy alone — corticosteroids or cyclosporine — frequently induces remission in 2 to 4 weeks but the lesion returns when treatment is withdrawn. Durable resolution requires addressing the cause. Step one is strict year-round flea control on every animal in the household with a modern isoxazoline (fluralaner, lotilaner, sarolaner) or a topical product, continued for at least 2 to 3 months even if no fleas are seen. Whole-household, year-round parasite control is a core recommendation in the current feline life stage guidance (AAFP-AAHA Feline Life Stage Guidelines, 2021). Step two is a strict 8-week novel-protein or hydrolyzed elimination diet if food allergy is suspected. Step three is environmental allergy workup with intradermal testing or serology and allergen-specific immunotherapy. Antibiotics are used only if secondary bacterial infection is present.
When Steroids and Cyclosporine Help
Glucocorticoids — prednisolone 1 to 2 mg/kg orally once daily tapered over 4 to 6 weeks, or triamcinolone — are highly effective at shrinking active lesions and giving the cat comfort while underlying-cause workup is in progress. Cyclosporine 7 mg/kg orally once daily is an alternative for cats that cannot tolerate steroids and for chronic relapsing cases. Both medications are bridges, not cures. Cats started on steroids without parallel allergy workup typically relapse within weeks of taper. Aggressive parasite control plus diet trial plus a brief steroid course is the combination that produces durable remission.
When to Suspect Cancer Instead
Squamous cell carcinoma is the most important differential, especially in older cats, cats with white facial fur, or cats with significant outdoor sun exposure. SCC lesions tend to be more friable, bleed easily, are often unilateral rather than centered on the midline, and frequently are painful and slowly destructive of underlying tissue. Any lesion not responding to standard rodent-ulcer therapy within 4 to 6 weeks, or any lesion in a high-risk cat, deserves biopsy. Early SCC is potentially curable with surgical excision; advanced disease has a poor prognosis.
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When to See a Vet
Call your vet today if:
- A new well-defined sore on a cat's upper lip
- A lip lesion that grows beyond 1 cm or extends to the gums or hard palate
- A previously controlled rodent ulcer that returns after treatment is stopped
- A lip lesion plus weight loss, reduced grooming, or refusal to eat
- An older cat or white-faced cat with a lip lesion that bleeds easily
Go to the ER immediately if:
- Acute severe facial swelling and difficulty breathing (anaphylaxis)
- Active heavy bleeding from a lip or oral lesion that does not stop with gentle pressure in 10 minutes
- Cat with a lip ulcer plus sudden severe lethargy and pale gums
- Sudden inability to close the mouth or swallow
- Necrotic, foul-smelling lip tissue with systemic signs of sepsis
Frequently Asked Questions
Why is it called a "rodent ulcer" if rodents have nothing to do with it?
The name comes from outdated 19th-century European dermatology — the lesion was thought to resemble damage caused by gnawing rodents. The term stuck despite being misleading. Modern veterinary dermatology uses indolent ulcer or eosinophilic ulcer. There is no association with mice, rats, or any rodent contact in either causation or transmission.
How much does rodent ulcer workup and treatment cost?
Initial vet exam typically runs $50 to $150 in the US. Cytology or biopsy adds $80 to $250. Year-round isoxazoline flea control for an average cat runs $180 to $360 annually. A hydrolyzed protein diet trial costs $80 to $150 for 8 weeks. Allergy serology testing is $250 to $500. Allergen immunotherapy is $400 to $1,200 annually. A short course of prednisolone is $20 to $60. Cyclosporine is $80 to $200 per month. Identifying and treating the cause is much cheaper than years of steroid courses.
Can I use a topical antibiotic ointment on the lesion?
Topical antibiotic ointments rarely help because the lesion is not primarily infected — it is an inflammatory response to an underlying allergen. Cats also lick ointments off quickly, which limits any effect and can cause GI upset. A vet-prescribed steroid taper plus addressing the underlying cause is far more effective than topical care.
Is this contagious to my other cats or to me?
No. Rodent ulcer is not infectious. It is an immune-mediated reaction in one individual cat. Other cats in the household are at risk only if they share the same underlying allergen exposure — for example, fleas. Whole-household flea control is recommended whenever one cat has a confirmed eosinophilic lesion.
Will my cat need lifelong medication?
Most cats need lifelong cause-management — typically year-round flea control plus avoidance of a triggering food or, less commonly, allergen-specific immunotherapy for environmental allergens. They do not need lifelong steroids. The goal of workup is to identify the trigger so the cat can stay in remission on cause-control alone.
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