Feline acne is a problem seen in cats primarily involving the formation of blackheads accompanied by inflammation on the cat's chin and surrounding areas that can cause lesions, alopecia, and crusty sores. In many cases symptoms are mild and the disease does not require treatment. Mild cases will look like the cat has dirt on its chin, but the dirt will not brush off. More severe cases, however, may respond slowly to treatment and seriously detract from the health and appearance of the cat. Feline acne can affect cats of any age, sex or breed, although Persian cats are also likely to develop acne on the face and in the skin folds. This problem can happen once, be reoccurring, or even persistent throughout the cat's life.

Sebaceous glands are skin glands that produce oil and are mostly found in the skin of the chin, at the base of the tail, and in the eyelids, lips, prepuce, and scrotum. They are connected to hair follicles. In acne, the follicles become clogged with black sebaceous material, forming comedones (also known as blackheads). Comedones can become irritated, swollen, infected, and ultimately pustules. These may elicit itching and discomfort due to swelling and bacterial growth inside infected glands. Cats may continue to scratch and reopen wounds, allowing bacterial infections to grow worse. Bacterial folliculitis occurs when follicules become infected with "Staphylococcus aureus", and commonly associated with moderate-to-severe feline acne. Secondary fungal infections (species "malassezia") may also occur.

Other conditions that can cause similar-appearing conditions include skin mites, ringworm, yeast infection, or autoimmune diseases such as eosinophilic granuloma complex ("rodent ulcers"). These can be ruled out by a simple biopsy of affected cells.

Feline acne is one of the top five most common skin conditions that veterinarians treat.

A pimple, zit or spot is a kind of comedo and one of the many results of excess oil getting trapped in the pores. Some of the varieties are pustules or papules. Pimples can be treated by various acne medications prescribed by a physician, or purchased at a pharmacy with a wide variety of treatments.

A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne. The word comedo comes from the Latin "comedere", meaning "to eat up", and was historically used to describe parasitic worms; in modern medical terminology, it is used to suggest the worm-like appearance of the expressed material.

The chronic inflammatory condition that usually includes both comedones and inflamed papules and pustules (pimples) is called acne. Infection causes inflammation and the development of pus. Whether a skin condition classifies as acne depends on the amount of comedones and infection. Comedones should not be confused with sebaceous filaments.

Comedo-type ductal carcinoma in situ (DCIS) is not related to the skin conditions discussed here. DCIS is a non-invasive form of breast cancer, but comedo-type DCIS may be more aggressive and so may be more likely to become invasive.

Although the exact cause of feline acne is unknown, some causes include:

- Hyperactive sebaceous glands

- Poor hygiene

- Stress

- Developing secondary to fungal, viral, and bacterial infections

- Reaction to medication

- Drinking from plastic containers to which the cat is allergic

- Demodicosis or mange, causing itchiness and hair loss

- Suppressed immune system

- Hair follicles that don't function properly

- Rubbing the chin (to display affection or mark territory) on non-sanitized household items

- Hormonal imbalance

- Contracting the infection from other cats in the same household

Inside the pore are sebaceous glands which produce sebum. When the outer layers of skin shed (as they do continuously), the dead skin cells left behind may become "glued" together by the sebum. This causes a blockage in the pore, especially when the skin becomes thicker at puberty. The sebaceous glands produce more sebum which builds up behind the blockage, and this sebum allows bacteria to grow, including the species "Staphylococcus aureus" and "Propionibacterium acnes", which causes inflammation and infection.

Typical features of acne include increased secretion of oily sebum by the skin, microcomedones, comedones, papules, nodules (large papules), pustules, and often results in scarring. The appearance of acne varies with skin color. It may result in psychological and social problems.

The severity of acne vulgaris (Gr. ἀκµή, "point" + L. vulgaris, "common") can be classified as mild, moderate, or severe as this helps to determine an appropriate treatment regimen. Mild acne is classically defined by the presence of clogged skin follicles (known as comedones) limited to the face with occasional inflammatory lesions. Moderate severity acne is said to occur when a higher number of inflammatory papules and pustules occur on the face compared to mild cases of acne and are found on the trunk of the body. Severe acne is said to occur when nodules (the painful 'bumps' lying under the skin) are the characteristic facial lesions and involvement of the trunk is extensive.

Large nodules were previously referred to as cysts, and the term "nodulocystic" has been used in the medical literature to describe severe cases of inflammatory acne. True cysts are in fact rare in those with acne and the term "severe nodular acne" is now the preferred terminology.

"Acne inversa" (L. invertō, "upside down") and "acne rosacea" (rosa, "rose-colored" + -āceus, "forming") are not true forms of acne and respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea. Although HS shares certain common features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the defining features of acne and is therefore considered a distinct skin disorder.

Acneiform eruptions are a group of dermatoses including acne vulgaris, rosacea, folliculitis, and perioral dermatitis. Restated, acneiform eruptions are follicular eruptions characterized by papules and pustules resembling acne.

The hybrid term "acneiform", literally, refers to an appearance similar to acne.

The terminology used in this field can be complex, and occasionally contradictory. Some sources consider acne vulgaris part of the differential diagnosis for an acneiform eruption. Other sources classified acne vulgaris under acneiform eruption. MeSH explicitly excludes perioral dermatitis from the category of "acneiform eruptions", though it does group acneiform eruptions and perioral dermatitis together under "facial dermatoses".

Acne fulminans begins as pain and inflammation in the joints. It eventually progresses into a swelling of the lymph nodes located at the base of the neck, causing inflexibility in the neck within weeks after the nodes swell. This swelling will eventually decrease, but this decrease will be accompanied by an increased inflammation and swelling of the joints, as well as a complete loss of appetite, though these symptoms are often ignored. After some time, the disease will cause an extreme loss of weight and atrophy of the muscles, leading to the decline of physical abilities.

Excoriated acne (also known as Picker's acne or Acne excoriée des jeunes filles) is a mild acne accompanied by extensive excoriations.

Steroid acne is an adverse reaction to corticosteroids, and presents as small, firm follicular papules on the forehead, cheeks, and chest. Steroid acne presents with monomorphous pink paupules, as well as comedones, which may be indistinguishable from those of acne vulgaris. Steroid acne is commonly associated with endogenous or exogenous sources of androgen, drug therapy, or diabetes and is less commonly associated with HIV infection or Hodgkin's disease.

Oil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in adolescents. Acne is also found premenstrually and in women with polycystic ovarian syndrome. Smoking may worsen acne.

Oxidation rather than poor hygiene or dirt causes blackheads to be black. Washing or scrubbing the skin too much could make it worse, by irritating the skin. Touching and picking at comedones might cause irritation and spread infection. It is not clear what effect shaving has on the development of comedones or acne.

Some, but not all, skin products might increase comedones by blocking pores, and greasy hair products (like pomades) can worsen acne. Skin products that claim to not clog pores may be labeled noncomedogenic or non-acnegenic. Make-up and skin products that are oil-free and water-based may be less likely to cause acne. It is not known whether dietary factors or sun exposure make comedones better, worse or have no effect.

A hair that does not emerge normally can also block the pore and cause a bulge or lead to infection (causing inflammation and pus).

Genes may play a role in the chances of developing acne. Comedones may be more common in some ethnic groups. People of recent African descent may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.

A papulopustular condition is a condition composed of both papule and pustules.

Examples of papulopustular conditions include:

- Papulopustular rosacea

- Papulopustular acne

- Sycosis

Oil acne is an occupational skin condition caused by exposure to oils used in industry.

Tropical acne is unusually severe acne occurring in the tropics during seasons when the weather is hot and humid.

Acne keloidalis nuchae (also known as "Acne keloidalis", "Dermatitis papillaris capillitii", "Folliculitis keloidalis", "Folliculitis keloidis nuchae", and "Nuchal keloid acne") is a destructive scarring folliculitis that occurs almost exclusively on the occipital scalp of people of African descent, primarily men. This is mainly because men often cut their hair very low as opposed to women, allowing the hair to prick the occipital scalp and upset it. Acne keloidalis nuchae most commonly presents itself in individuals aged 13 to 25. The disease is closely related to pseudofolliculitis barbae and both occur frequently in black men in the military, where it is so common that services often have widely known protocols for management.

Acne conglobata presents with blackheads appearing around the face, neck, chest, upper arms and buttocks in groups of two or three. Pimples form around the blackheads; they are large and engorged with fluid and may be sensitive to touch. They remain for a while and continue to grow and fill with pus until they eventually rupture. After the lesion has drained, it fills up again. After they rupture, several nodules can fuse together to form larger shapes. The lesions remain for a long time. They form a scab in the center but they continue to spread outwards. When the lesions do eventually heal, they leave scars that can be the usual type of acne scar (atrophic) or a raised bump like those normally left behind by a burn or a cut (keloidal).

Hidradenitis suppurativa is a chronic inflammatory skin condition, considered a member of the acne family of disorders. It is sometimes called acne inversa. The first signs of HS are small bumps on the skin that resemble pimples, cysts, boils, or folliculitis. As the disease progresses and abscesses reoccur, they become larger and more painful; eventually tunnels of scar tissue connect the lesions. These lesions may open up if they become too enlarged and drain bloodstained pus. One risk factor is age; HS usually first appears during the 20s and early 30s. The condition is much more common in women than in men but is usually more serious and debilitating in men. Other associated conditions include obesity, diabetes, metabolic syndrome, arthritis, acne, and other inflammatory disorders. Early diagnosis of this disease is very important to decrease the number of flares, pain, and discomfort.

Hidradenitis suppurativa (HS), also known as acne inversa, is a long term skin disease characterized by the occurrence of inflamed and swollen lumps. These are typically painful and break open releasing fluid or pus. The areas most commonly affected are the underarms, under the breasts, and groin. After healing scar tissue remains. Self consciousness or depression may result.

The exact cause is usually unclear but believed to involve a combination of genetic and environmental factors. About a third of people have an affected family member. Other risk factors include obesity and smoking. The condition is not infectious, related to hygiene, or to the use of deodorant. The underlying mechanism is believed to involve either a dysfunction of apocrine sweat glands or hair follicles. Diagnosis is based on the symptoms.

There is no known cure. Warm baths may be tried in those with mild disease. Cutting open the lesions to allow them to drain does not result in significant benefit. While antibiotics are commonly used evidence for their use is poor. Immunosuppressive medication may also be tried. In those with more severe disease laser therapy or surgery to remove the affected skin may be carried out. Rarely a skin lesion may develop into skin cancer.

It is estimated to affect between 1% and 4% of people. Females have the condition about three times more often than males. Onset is typically in young adulthood and may become less common after 50 years old. It was first described some time between 1833 and 1839 by Alfred Velpeau.

The cause of HS remains unknown and experts disagree over proposed causes. The condition probably results from a combination of genetic and environmental factors.

Lesions occur in any body areas with hair follicles, although areas such as the axilla, groin, and perianal region are more commonly involved. This theory includes most of the following potential indicators:

- Postpubescent individuals are more likely to exhibit HS.

- Plugged apocrine (sweat) gland or hair follicle

- Excessive sweating

- Androgen dysfunction

- Genetic disorders that alter cell structure

- Patients with more advanced cases may find exercise intolerably painful, which may increase the rate of obesity among sufferers.

The historical understanding of the disease suggests dysfunctional apocrine glands or dysfunctional hair follicles, possibly triggered by a blocked gland, create inflammation, pain, and a swollen lesion.

Acne aestivalis (also known as Mallorca acne) is a special form of mono-morphic light eruption. It is a monomorphous eruption consisting of multiple, uniform, red, papular lesions, reported to occur after sun exposure.

Tar acne is an occupational skin condition caused by exposure to tars used in industry.

Cats with eosinophilic granuloma complex (EGC) may have one or more of four patterns of skin disease.

The most frequent form is "eosinophilic plaque". This is a rash comprising raised red to salmon-colored and flat-topped, moist bumps scattered on the skin surface. The most common location is on the ventral abdomen and inner thigh.

Another form of EGC is the "lip ulcer". This is a painless, shallow ulcer with raised and thickened edges that forms on the upper lip adjacent to the upper canine tooth. It is often found on both sides of the upper lips.

The third form of the EGC is the "collagenolytic granuloma". This is a firm swelling that may be ulcerated. The lesions may form on the skin, especially of the face, in the mouth, or on the feet, or may form linear flat-topped raised hairless lesions on the back of the hind legs, also called "linear granuloma".

The least common form of EGC is "atypical eosinophilic dermatitis". It is unique in that it is caused by mosquito bite allergy and the lesions form on the parts of the body with the least hair affording easy access to feeding mosquitoes. This includes the bridge of the nose, the outer tips of the ears and the skin around the pads of the feet. The lesions are red bumps, shallow ulcers and crusts.

Atrophodermia vermiculata is also known as "Acne vermoulante", "Acne vermoulanti", "Atrophoderma reticulata symmetrica faciei", "Atrophoderma reticulatum", "Atrophoderma vermiculata", "Atrophoderma vermiculatum", "Atrophodermia reticulata symmetrica faciei", "Atrophodermia ulerythematosa", "Atrophodermie vermiculée des joues avec kératoses folliculaires", "Folliculitis ulerythema reticulata", "Folliculitis ulerythematous reticulata", "Folliculitis ulerythemosa", "Honeycomb atrophy", "Ulerythema acneforme" and "Ulerythema acneiforme"

Fiddler’s neck usually involves highly localized lichenification, mild hyperpigmentation, and erythema where the chin rest or instrument body presses against the skin of the neck. Other signs and symptoms include scale buildup, cyst and scar formation, papules and pustules related to local infection, and focal edema. In Blum & Ritter’s study in West Germany (1990), they found that 27% of their population had only minor issues, 72% had a palpable mass at the site, and 23% reported pain and other signs of inflammation such as hyperthermia, pulsation, and cystic, pustular, or papular lesions. Size of masses were an average of 2 cm in diameter ranging up to 4 cm, some being associated with purulent drainage, continuous discharge, and crusting. Dystrophic calcinosis cutis has also been reported. Other serious sequelae include sialolithiasis of the submandibular gland and adenolymphoma of the parotid gland.

The histopathology of fiddler’s neck frequently shows hyperkeratosis and acanthosis, along with plugging of follicles. Histiocytic infiltration with granulomas to foreign body and follicular cysts are also common. Foreign body granulomas are thought to derive from abrasion of the wooden surface of the chin rest and its absorption into the superficial dermis. The location and complex mechanism of causation for fiddler’s neck give rise to a wider spectrum of skin changes when compared to contact dermatitis from more common irritants. Fiddler’s neck can be differentiated from rosacea and sarcoid reaction with granulomas.