Causes of Fluid-filled Bumps on the Skin of Cats

Small bumps on the skin which are filled with pus are called 'pustules.' Larger bumps filled with pus are abscesses. If small bumps contain a clear fluid, they are called 'vesicles.' There are many conditions which can cause these lesions, and most of them are included in the table below. The number of conditions is extensive, so you can understand why a quick diagnosis may be difficult to make and various diagnostic tests may need to be performed. The most common conditions causing fluid-filled 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 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
Bacterial infection (pyoderma) *See Folliculitis, Pyoderma-deep*	Often occurs as a result of another condition such as a parasite, allergic, or hormonal condition
Erythema multiforme	Hypersensitivity reaction to infections or drugs; may also be caused by cancer or other diseases	Hair loss, 'bull's eye' lesions and vesicles often around mouth, ears, groin, and axilla; in some instances, ulcers develop; depression, fever	History, clinical signs, rule out other diseases causing similar signs; skin biopsy	Treat or remove underlying cause
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)
Folliculitis	Infection of the hair follicles; symptoms usually appear on face, neck, and head	Pustules develop in the hair follicles and open and form crusts; may itch and develop hair loss	Skin scraping; culture; biopsy; look for underlying condition such as allergy or FIV	Antibiotics, usually for 3-4 weeks; treat any underlying condition
Hematoma	Localized collection of blood that has leaked out of blood vessels, e.g., bruise	These may appear as firm or fluid-filled nodules of varying shapes and sizes	Needle aspirate	Depending on location and size may resolve on their own, or need drainage
Lupus erythematosus	Autoimmune disease affecting many body systems including joints, kidneys, muscles, nervous system	Skin lesions may include thickening or ulcers of the foot pads, scaling, and recurring bacterial infections with pustules	Special blood tests (LE test); biopsy	Prednisone and other immunosuppressive drugs; treat underlying infections
Pemphigus erythematosus	Less severe form of pemphigus foliaceus, an autoimmune disease; exposure to sunlight may play a role in its development	Pustules, drainage, and crusts; usually on face and ears	History, physical exam, skin scraping and biopsy	Topical and oral steroids; other immunosuppressives
Pemphigus foliaceus	The most common form of pemphigus in the cat; an autoimmune disease	Often affects feet and head; starts with pustules and progresses to severe crusting; depigmentation of the nose is common; itching may occur; if footpads and nails affected often see lameness; symptoms wax and wane; severely affected cats may have fever and loss of appetite	History, physical exam, skin scraping and biopsy	Corticosteroids, other immunosuppressive therapy, gold injections
Pemphigus vulgaris	A rare form of pemphigus, an autoimmune disease	Large vesicles which break open, ulcerate, and develop thick crusts; lesions often found in the mouth	Biopsy	Poor prognosis; prednisolone and other immunosuppressants
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
Pyoderma-superficial *See Folliculitis*
Stud tail (tail gland hyperplasia)	A sebaceous gland (on the top of the tail near its base) enlarges; most often occurs in confined, unneutered males	Oily area, hair loss, and crusts on area over gland; may become hyperpigmented	Clinical signs	Castration usually does not resolve the condition; antiseborrheic shampoos, retinoids; if confined, allow cat more freedom
Toxic epidermal necrolysis	Severe immune reaction to infections or drugs; may also be caused by cancer or other diseases	Vesicles, erosions, ulcers, crusts over large areas of the body, especially mouth and feet; may look like a severe burn	History, clinical signs, skin biopsy	Prognosis is poor; treat underlying condition; give supportive care, corticosteroids may be helpful