Growing Lump Under Skin in Cats

A growing lump under your cat’s skin is a symptom, not a diagnosis. Some masses are caused by infection or inflammation, such as an abscess after a bite wound. Others are benign growths like cysts or lipomas. Some are tumors that may be locally invasive or cancerous. Because many lumps feel similar at home, size, texture, and location alone usually cannot tell you what the mass is.

Cats deserve a prompt exam when a lump is new or changing. Skin and soft tissue tumors are among the most commonly seen tumors in cats, and Merck notes that a fine-needle aspirate or biopsy is often needed to identify what a mass is. Rapid growth, firmness, ulceration, pain, or a lump at a prior injection site raise concern and should move the visit up sooner. Early evaluation often gives your vet more options, whether the problem turns out to be an abscess, a cyst, or a tumor.

It also helps to remember that some serious masses are hidden deeper in the tissue. Merck notes that tumors arising in fat or other tissues under the skin may not be obvious until a hands-on exam is done. That is one reason home monitoring is useful, but it should not replace veterinary testing when a lump is enlarging.

One common cause is an abscess. Cats that go outdoors or have contact with other cats can develop a bite wound that seals over, trapping infection and pus under the skin. These lumps are often painful, warm, and swollen, and some cats also run a fever or seem tired. VCA notes that bite wounds commonly lead to abscesses under the skin, while PetMD describes abscesses as pockets of infection that may need drainage, cleaning, and antibiotics.

Other possibilities include cysts, granulomas, and benign fatty masses. PetMD describes cysts as benign masses that may contain fluid or thicker material and can become inflamed or infected. Granulomas are areas of chronic inflammation that can feel firm or gritty. Lipomas are benign fatty growths that are usually soft and movable, though Merck notes they should not be ignored because they can enlarge and may be hard to distinguish from more serious fatty tumors without testing.

Tumors are another important category. Merck describes mast cell tumors as one of the more common skin tumors in cats, often appearing as solitary hairless lumps, while fibrosarcomas and other soft tissue sarcomas can feel firm and deep under the skin. Injection-site sarcomas are a special concern in cats because they can develop where vaccines or other injections were given. VCA notes these are usually single, firm lumps under the skin, often over the flank or shoulder area, and Merck notes that injection-site sarcomas are now recognized after some non-vaccine injections as well.

Schedule a veterinary visit soon for any lump that is growing, newly noticed, or different from the surrounding tissue. Even a soft lump can need testing. VCA advises prompt attention for masses that appear suddenly or grow rapidly, because those features can be associated with malignant skin tumors. If your cat is eating less, hiding, acting painful, or grooming the area constantly, that also raises the urgency.

See your vet immediately if the lump is painful, hot, draining, bleeding, ulcerated, or your cat has fever, lethargy, trouble breathing, or trouble walking. These signs can point to infection, tissue damage, or a mass affecting deeper structures. A rapidly enlarging swelling after a fight or puncture wound can become an abscess quickly.

Injection-site lumps deserve special attention. A commonly used rule is to have your vet evaluate a post-injection lump if it is still present 3 months later, is larger than 2 centimeters, or continues to enlarge 1 month after the injection. This does not mean the lump is cancer, but it does mean your vet should decide whether sampling or biopsy is needed.

Your vet will start with a full history and hands-on exam. They will ask when you first noticed the lump, whether it has changed size, whether your cat goes outdoors, has had recent vaccines or injections, or may have been in a fight. Merck notes that a precise diagnosis of skin disease and masses depends on history, physical examination, and appropriate testing.

A fine-needle aspirate is often the first test. This uses a small needle to collect cells from the lump for microscopic review. Merck states that cytologic examination of fine-needle aspirates of skin and soft tissue tumors is commonly used to help determine tumor type and plan next steps. However, cytology does not always give a final answer. Merck also notes that histology from a biopsy is generally required for a definitive diagnosis and for grading some tumors.

Depending on the lump, your vet may recommend biopsy, culture, bloodwork, or imaging such as X-rays or ultrasound. Abscesses may need fluid sampling or culture. Deep or invasive masses may need imaging before surgery to understand their extent. If cancer is suspected, your vet may also discuss staging tests and referral options so you can choose a conservative, standard, or advanced path that fits your cat and your goals.

Do not squeeze, lance, or try to drain a lump at home. That can worsen pain, spread infection, or make later testing harder. Instead, keep your cat indoors, prevent licking or scratching, and watch for changes in size, redness, heat, drainage, or discomfort. VCA recommends preventing rubbing, scratching, licking, or biting of skin tumors because irritation can lead to inflammation, ulceration, infection, and bleeding.

A helpful home routine is to measure the lump with a soft tape measure or compare it with a coin, then take a clear photo every 1 to 2 weeks in the same lighting. Write down the date, location, and whether the lump feels soft, firm, movable, or attached. Also track appetite, energy, grooming, and litter box habits. This record helps your vet see whether the mass is stable or changing.

If your cat has already seen your vet, follow the plan exactly. Give medications as directed, use an e-collar if recommended, and go to rechecks even if the lump looks better. Contact your vet sooner if the mass grows quickly, opens, bleeds, smells bad, or your cat seems painful or unwell.