Causes of Solid-Appearing Lumps & Bumps on the Skin in Cats

Nodules on a cat caused by sporotrichosis

Cats can develop small bumps (papules) or larger lumps (nodules) on their skin. The term 'tumor' means an abnormal growth or swelling, and is often used to designate cancer. Often the word 'lump' also brings the word 'cancer' to mind. There are, however, many other causes of lumps and bumps. The following table includes most of the conditions which result in solid lumps and bumps. The list is rather extensive, so you can understand why a quick diagnosis may be difficult to make and various diagnostic tests, such as biopsies, may need to be performed. The most common causes of solid lumps and bumps are color-coded gray in the table (some may be more common in certain geographical areas).

Condition	Description	Symptoms	Diagnosis	Treatment
Abscesses	Accumulation of pus; may or may not be caused by an infection; in cats, often due to bite wounds	These may appear as firm, fluid-filled nodules of varying shapes and sizes, with a small crusty area at puncture site; if due to infection, cat may have fever, loss of appetite, depression; may open and drain	History, physical exam, needle aspirate	Surgically open, drain and flush; if infected, administer appropriate antibiotics.
Allergic and irritant contact dermatitis	An allergic reaction following exposure to antibiotics applied to the skin; metals such as nickel; materials such as rubber, wool, and plastic; and chemicals such as dyes and carpet deodorizers; or inflammation caused by irritating substances such as poison ivy. Generally requires multiple exposures.	Red skin and small bumps or blisters on the areas of skin that are sparsely haired and directly exposed to the offending substance, itching; hair loss in chronic conditions	Patch test, exclusion trials	Restrict exposure to the allergen or contact irritant in the cat's environment; steroids, antihistamines
Apocrine sweat gland cyst	Rare in cats	Single, round, smooth nodules with no hair; may appear bluish; usually filled with a watery liquid; most common on head, neck, and limbs	Physical exam; biopsy	Surgical removal is optional
Basal cell tumors	Most common skin tumor in cats; benign tumors which may become cancerous, slow-growing tumors which rarely metastasize	Single, sometimes fluid-filled nodules which may ulcerate; usually on the head, neck, and chest; may be hyperpigmented	Biopsy	Surgical removal (if benign, surgical removal is optional)
Bee, wasp, hornet stings	Skin reactions can vary dramatically in severity	Immediately after the bite, see swelling, redness, pain, possibly itching; subsequently may develop extensive ulcers with draining; may develop hives or anaphylaxis	History, physical exam	Antihistamines, steroids; wet dressings if ulcerated; protect the area from self-inflicted trauma
Benign tumors	See specific type, e.g., Fibromas, Lipomas, Basal cell tumors
Chiggers (harvest mites)	Seasonal disease caused by larvae of the chigger	Itching, bumps usually on feet, abdomen, folds at base of ears	Visualization of mite larvae or microscopic examination of skin scraping	Pyrethrin
Coccidioidomycosis	Caused by the fungus Coccidioides immitis found in the soil in the Southwestern U.S.	Draining nodules, fever, weight loss; unlike dogs, no respiratory signs	Microscopic examination of drainage; blood test	Ketoconazole, itraconazole
Cryptococcosis	Fungal infection often transmitted through bird droppings; more common in cats with suppressed immune systems	Nodules often over the nose which may ulcerate; many other signs depending on what other body systems are infected	Microscopic exam of discharge, blood tests, culture, biopsy; look for underlying cause of immunosuppression	Itraconazole
Cutaneous horn	Benign growths of hard tissue which look like small horns; cause unknown, though may be associated with some underlying disease such as cancers, follicular cysts, or FeLV	½ to 2 inch hard horn-like growths; may be single or multiple; in cats may occur on foot pads	Clinical appearance; look for underlying cause	Surgical removal
*Cuterebra*	Caused by the 1-1½ inch larva of the Cuterebra fly; usually seen in late summer	Nodule forms around the larva; usually found on the head and neck; nodule has a small opening through which the larva breathes and will eventually escape	Clinical signs; opening the nodule and finding the larva	Surgically open the nodule and remove the larva; do NOT squeeze the nodule or break up the larva or a severe allergic reaction may occur
Drug or injection reaction	Rare skin reaction to a drug which is inhaled, given orally or applied topically; more common with penicillins, sulfonamides, and cephalosporins; usually occurs within 2 weeks of giving the drug	Can vary widely and may include itching, hair loss, redness, swelling, papules, crusts, ulcers, and draining wounds	History of being treated with a drug, symptoms, biopsy	Discontinue offending drug; treat symptomatically
Epitheliotropic lymphoma (mycosis fungoides)	Rare cancer of T lymphocytes seen in older cats; may be associated with FeLV	Redness, itching, scales, ulcerated nodules	Needle or other biopsy	Poor response to treatments which include chemotherapy, surgical removal, retinoids, fatty acids
Feline acne	Skin condition of unknown cause which may occur as single episode or continue as chronic condition; most commonly seen on the chin; can develop into a more serious, deep infection if not treated	Comedones (black heads) on lips and chin, later developing pustules and small nodules; may itch - especially in chronic cases; chin may become swollen; can become secondarily infected	Physical exam; tests to rule out underlying causes or diseases with similar symptoms; skin biopsy	Mild: antiseborrheic shampoos, antibacterial creams, topical Vitamin A; Severe: antibiotics, fatty acids, retinoids (use with caution, can be irritating)
Feline leprosy	Caused by the bacteria 'Mycobacterium'; usually seen in young cats	Single or multiple nodules which may drain; usually on head or neck; nonpainful; cat does not show other signs of disease	Biopsy, culture	Surgical removal, anti-mycobacterial drugs such as rifampin or clofazimine
Feline pox	Viral disease; outdoor cats more commonly affected; presumably transmitted by bite wounds	Nodule at the site of a previous bite wound; progresses to multiple nodules which may ulcerate and have crusts; may itch	History, physical exam; biopsy; specialized testing to identify the virus	Lesions usually resolve in 3-4 weeks; antibiotics for any secondary infection; antihistamines for itching; no steroids; in some cats, lesions progress and do not respond to treatment
Fibroma	Uncommon benign tumor	Single nodule with a pedicle, usually on legs, groin, or sides	Biopsy	Surgical removal is optional
Fibrosarcoma	Rapidly growing, invasive tumor; may occur at the site of a vaccination or injection, especially in cats; may be induced by a form of FeLV	Irregular-shaped, firm nodule; may ulcerate	Biopsy	Surgical removal, however, since tumor is invasive need to remove large area around tumor, sometimes including large masses of muscle and bone; if tumor is on a leg, amputation of the leg is commonly recommended; surgery may be combined with chemotherapy and radiation
Follicular cyst	Most common cyst; may be called 'sebaceous cysts' by some veterinarians	Single, round nodules on or underneath the skin; may appear bluish; may contain a thick yellowish to gray material; usually found on the head, neck, and trunk	Biopsy	Surgical removal optional; do NOT squeeze these cysts since a severe skin reaction will occur
Granulomas	May be due to infections; the body's reaction to foreign material such as plant material (e.g., foxtail) and suture material; other constant irritation; or unknown causes	Solid, firm nodules of varying sizes; those due to foreign bodies often have draining tracts; may develop hair loss, ulcers, and secondary infections	History, clinical signs, biopsy, surgical exploratory	Surgical removal of the foreign body (in the case of plant material, tracts may be extensive and require major surgery); antibiotics if infected; treat any other underlying cause
Hemangiosarcoma	Malignant, invasive tumor more common on sun-damaged skin	Blue to reddish black nodule; usually on ears, head, legs, groin, and axillae; often ulcerate	Biopsy	Surgical removal; need to remove large area around the tumor; if tumor is on a leg, amputation of the leg is commonly recommended
Hematoma	Localized collection of blood that has leaked out of blood vessels, e.g., bruise	These may appear as firm, fluid-filled nodules of varying shapes and sizes	Needle aspirate	Depending on location and size may resolve on their own, or need drainage
Histoplasmosis	Fungal infection which can rarely cause skin lesions	Ulcerated and draining nodules; most commonly see respiratory and gastrointestinal symptoms	Needle aspirate or biopsy	Ketoconazole, itraconazole
Hookworms	Infection with the larvae (immature forms) of hookworms	Red bumps, usually on feet, rough foot pads, abnormal nail growth, itching	Physical exam, history of poor sanitation	Treat for intestinal infection; move cat to different environment
Lichenoid dermatosis	Often a response to other underlying disease such as fleas or bacterial infections	Small, flat nodules with thick surfaces	Biopsy, look for underlying disease	Treat underlying cause; this reaction usually resolves on its own
Lipoma	Uncommon benign fatty tumor	Usually single, soft, domed nodule; can become very large	Fine needle biopsy	Surgical removal if large or interferes with movement
Lymphoma	Type of cancer; may occur in other organs without skin involvement	Nodules with ulcers; redness	Biopsy	Surgery, chemotherapy, radiation; lymphoma of the skin does not usually respond to treatment as well as other lymphomas
Mammary cancer	Most common in unspayed females; in cats, 85% are malignant	Single or multiple nodules under the skin, of varying sizes, often irregular in shape; may ulcerate and drain	Biopsy	Surgical removal
Mast cell tumor	Common cancer which is graded from 1-4: Grade 1 is one slow-growing tumor, and Grade 4 is rapidly growing malignant tumors with metastasis; in cats, most are Grade 1	Tumors may be of various sizes, appearances, and numbers	Biopsy to grade the tumors which determines treatment and prognosis	Depends upon grade; surgical removal, taking large area around tumor; chemotherapy; prednisone; radiation
Melanoma	Malignant tumor; uncommon in cats	Usually single, dark-colored nodule which often ulcerates	Biopsy	Surgical removal, taking large area around tumor
Miliary dermatitis in cats	Part of the common eosinophilic allergic syndrome in cats which includes eosinophilic granulomas, eosinophilic plaques, and rodent ulcers; may also be associated with infections, autoimmune diseases, hormonal disorders, and nutritional deficiencies	Multiple, small, crusty bumps, usually over hips, neck, and back of thighs; moderate to severe itching	Microscopic examination of swab from lesion, biopsy, CBC (find increased eosinophils); look for underlying cause, e.g., parasites, food allergy, atopy	Treat underlying cause if found; corticosteroids; fatty acid supplements
Mosquito bite hypersensitivity	Severe allergic reaction to mosquito bites; lesions most common on nose and ear tips - also footpads, lips, and chin	Acute lesions are red, raised, and oozing; with time develop hair loss, scales, nodules, and pigment changes; some cats develop fever and swollen lymph nodes	History of exposure to mosquitoes; lesions resolve when cat is hospitalized or otherwise restricted from exposure to mosquitoes	Restrict exposure to mosquitoes, insect repellents, prednisone
Nevi	Usually benign lesions; some types may indicate the presence of an underlying disease	Well-delineated, firm nodules, often multiple and on the head and neck	Biopsy	Surgical removal, although recurrence is common; depending upon the type, look for underlying disease
Nocardia	Bacterial infection usually acquired from a puncture wound	Usually see respiratory signs; skin lesions include draining nodules	Bacterial culture, microscopic examination of drainage	Poor prognosis; antibiotics
Panniculitis	May be caused by trauma, foreign bodies, infections, autoimmune diseases, or unknown causes	Deep-seated nodules, often ulcerated and draining; usually on the body vs. the head or limbs; may see loss of appetite, depression	Microscopic exam of drainage; biopsy; tests to rule out other causes	Surgical removal; if multiple lesions, prednisone and Vitamin E; may need long-term treatment
Phaeohyphomycosis	Caused by wound contamination with a fungus	A single nodule on the legs or multiple ulcerated and draining nodules over the body	Microscopic examination of drainage, culture, biopsy	Surgical removal though often recur; possible antifungal medications
Pyoderma-deep	Bacterial infections of skin and underlying tissue often secondary to another skin disease such as self-inflicted trauma, wounds, acral lick granulomas, allergies, seborrhea	Ulcerated pustules or nodules, draining tracts, crusts, thickened skin	Skin scrapings, biopsy, culture	Clip and cleanse area; antibiotics, prevent self-trauma (licking, scratching), NO steroids
Pythiosis	Caused by an aquatic mold	Ulcerated draining nodules on the legs, head, and base of tail which may itch; often see other signs of illness due to infection of the gastrointestinal tract	Microscopic examination of drainage; biopsy	Often fatal; surgical removal
Sebaceous gland cyst	Extremely rare	Firm nodules, usually less than ½ inch in diameter	Biopsy	Surgical removal
Sebaceous gland tumor	Rare in cats; rarely spreads or recurs; several types	Nodules which may ulcerate; usually on the head and legs	Biopsy	Surgical removal if invasive; if a benign lesion, removal is optional
Skin cancer	See specific type, e.g., Fibrosarcoma, Melanoma, Squamous cell carcinoma, Mast cell tumor, Lymphoma
Spider bites/eosinophilic folliculitis	Bites from some spiders and caterpillars contain strong toxins; usually appear on the nose of dogs and paws of cats	Immediately after the bite, swelling, redness, pain; subsequently may develop extensive ulcers with draining	History, biopsy	Corticosteroids, wet dressings, protect the area from self-inflicted trauma; may develop permanent loss of hair and scarring
Sporotrichosis	Caused by the fungus Sporothrix schenckii which generally enters through a puncture wound	Raised nodules with multiple draining tracts; cats may develop fever, depression, and loss of appetite	Microscopic exam of drainage; culture; fluorescent antibody test	Potassium iodide, ketoconazole, itraconazole
Squamous cell carcinoma	Common malignant tumor; may occur more commonly in sun-damaged or chronically irritated skin	Two forms: cauliflower-like lesions, often ulcerated more common on ears; crusted ulcers on head or feet (around claws)	Biopsy	Surgical removal, radiation, hyperthermia
Tick bites	Ticks cause a local inflammation in the skin, even when the entire tick is removed	Nodule and redness at site of the bite; may itch and develop crusts; may last several months	History	Remove the tick; use a tick preventative; allow nodule to resolve on its own
Urticaria (hives)	Reaction, often allergic, to insect bite, drug, vaccine, sunlight, etc.	Multiple swellings, with hair standing up over swellings; may itch	History, physical exam	Often resolves on its own; in the case of allergic reactions, antihistamines, epinephrine, or corticosteroids depending upon severity
Xanthoma	Result of abnormality in lipid metabolism; can be a result of diabetes mellitus or due to hereditary affecting lipid metabolism (hyperlipidaemia)	White or yellow nodules, usually on head, extremities, and bony prominences; often painful and itchy	Physical exam and history; biopsy;	Treat underlying disease; change diet; lesions often recur if surgically removed
Zygomycosis	Uncommon fungal disease	Draining nodules; may also see pneumonia, vomiting, or jaundice depending upon the body organs involved	Microscopic examination of the drainage; biopsy	Often fatal; surgical removal of nodules followed by amphotericin B, benzimidazoles, or potassium iodide