INTEGUMENTARY SYSTEM

Callus

Thick, firm, hyperkeratotic, hairless plaque with increased skin folds, wrinkles, or fissures caused by chronic pressure or trauma over bony prominences, commonly affects the elbow, sternum, and side of digit, may develop follicular plugging and comedones.

Comedo (Comedones)

Plug of stratum corneum and sebum within a hair follicle causing follicular distention due to follicular plugging, associated with Canine Solar (Actinic) Dermatosis, Chin Acne, Schnauzer Comedo Syndrome, Hyperadrenocorticism, and Palmar/Plantar Interdigital Cysts.

Crust

Dried exudate composed of fluid, blood, pustular debris, scale, or microorganisms on the skin surface caused by drying of inflammatory exudate, associated with Chronic Pustular Disease, Staphylococcal Infection, and Pemphigus Foliaceus.

Cyst

Epithelium-lined cavity filled with liquid or semisolid material in the dermis or subcutis caused by development of an epithelial-lined cavity, may communicate with the skin surface, associated with Follicular Cyst, Dermoid Cyst, and Apocrine Gland Cyst.

Epidermal Collarette

Thin layer of scale expanding peripherally to form a ring caused by epidermal peeling after superficial lesions, associated with Superficial Bacterial Infection, Insect Bite, and Fungal Infection.

Erosion

Partial-thickness loss of epidermis producing a shallow, moist depression caused by ruptured vesicle, ruptured pustule, or superficial trauma, heals without scarring.

Excoriation

Shallow linear break in the epidermis caused by abrasion or scratching, represents a superficial skin injury.

Fissure

Deep linear cleft extending from epidermis into dermis caused by skin cracking, commonly affects pawpads, associated with Pemphigus Foliaceus, Superficial Necrolytic Dermatitis, and Digital Hyperkeratosis.

Lichenification

Thickened, rough epidermis due to chronic rubbing or scratching, may be hyperpigmented, caused by chronic irritation or pruritus, hallmark of chronic dermatitis.

Macule

Flat, nonpalpable color change in skin <1 cm caused by pigment or vascular change, associated with Hemorrhage, Lentigo, and Vitiligo.

Neoplasm

Abnormal tissue mass with uncontrolled growth that persists after the initiating stimulus is removed due to neoplastic proliferation, examples include Lipoma, Mast Cell Tumor, and Squamous Cell Carcinoma.

Nodule

Elevated, firm, solid palpable lesion greater than ≥1 cm usually in the dermis or subcutis caused by inflammatory or neoplastic processes, associated with Bacterial/Fungal Infection and Infectious/Sterile Granuloma.

Papule

Elevated, firm, palpable lesion less than <1 cm that may coalesce into plaques caused by a localized epidermal or dermal lesion, associated with Insect Bite, Papilloma, and Superficial Folliculitis.

Plaque

Elevated, flat-topped palpable lesion ≥1 cm that may merge into larger lesions due to coalescence of papules or localized dermal reaction, associated with Calcinosis Cutis, Reactive Histiocytosis, and Eosinophilic Plaque.

Scar

Fibrous tissue replacing normal skin after dermal injury due to healing of dermal damage, associated with Healed Wound and Surgical Scar, hair may not regrow.

Ulcer

Full-thickness loss of epidermis and basement membrane extending into the dermis or deeper tissues caused by severe tissue injury, ischemia, or infection, associated with Vasculitis, Indolent Ulcer, Feline Herpesvirus Dermatitis, and Feline Ulcerative Dermatitis Syndrome, heals with scarring.

Vesicle

Fluid-filled lesion less than <1 cm caused by accumulation of serous or hemorrhagic fluid, associated with Burns, Viral Infections, and Immune-Mediated Disease (Bullous Pemphigoid).

Bulla

Fluid-filled lesion greater than ≥1 cm.

Wheal

Transient, elevated dermal edema often pale centrally with an erythematous rim caused by hypersensitivity or allergic reactions, associated with Insect Bites, Urticaria, and Allergic Reactions.

Congenital Alopecia/Hypotrichosis

Congenital absence or reduced amount of hair due to congenital follicular dysplasia, affects Mexican Hairless Pig, Chinese Crested Dog, Mexican Hairless Dog, and Sphynx Cat, may be an inherited trait.

Collagen Dysplasia

Defect in collagen causing fragile, hyperextensible, loose skin due to enzyme defects affecting collagen synthesis or processing, includes Cutaneous Asthenia, Hyperelastosis Cutis, Dermatosparaxis, and Ehlers-Danlos-Like Syndrome, lesions include seromas, hematomas, frequent skin tears, and numerous scars.

Epitheliogenesis Imperfecta (Aplasia Cutis)

Incomplete development of the epidermis, skin adnexa, or oral mucosa due to congenital failure of epithelial development, presents as sharply demarcated skin defects exposing red moist dermis, affects the face, extremities, and mucous membranes, severe cases may cause infection, dehydration, and electrolyte imbalance.

Solar (Actinic) Injury

Skin damage caused by ultraviolet light due to acute or chronic UV exposure, associated with Sunburn, Solar Dermatosis, Neoplasia, Early Erythema, and Warm Tender Skin, chronic UVB exposure may cause epidermal neoplasia.

Photosensitization

Photodynamic skin injury after UVA exposure in the presence of a photodynamic agent causing ROS/free radicals, primarily affects nonpigmented sun-exposed skin, common sites include Horse, Face, Nose, Distal Limbs, Cattle, White-Haired Areas, Teats, Udder, Perineum, Nose, Sheep, Ears, Eyelids, Face, Nose, Coronary Band, and Shorn Sheep, Back and Exposed Areas.

Allergic Contact Dermatitis

Immune-mediated skin inflammation after allergen exposure caused by Type IV hypersensitivity, requires prior sensitization and develops after re-exposure.

Irritant Contact Dermatitis

Skin inflammation from direct chemical irritation due to nonimmune skin injury, caused by acids, alkalis, soaps, detergents, urine, feces, wound exudates, plants, and topical medications.

Injection Site Granuloma

Localized granulomatous inflammation at an injection site caused by a chronic immune response to injected material, presents as a palpable nodule.

Injection Site Eosinophilic Granuloma (Horse)

Local eosinophilic granuloma after injection caused by delayed Type IV hypersensitivity, often from silicone-coated needles, develops 1–3 days after injection with central necrosis.

Feline Injection-Site Sarcoma (FISS)

Malignant mesenchymal tumor developing months to years after injections caused by chronic inflammation leading to fibroblast proliferation, genetic mutation, and sarcoma, affects cats, includes Fibrosarcoma most commonly, Myxosarcoma, Osteosarcoma, Rhabdomyosarcoma, Chondrosarcoma, and Histiocytic Sarcoma, triggered by vaccines, injectable drugs, microchips, nonabsorbable sutures, and trauma.

Rabies Vaccine-Associated Alopecia

Localized alopecia following rabies vaccination caused by immune-mediated vasculitis, ischemia, and hair follicle atrophy, common in small soft-coated dogs, especially Poodles, with hyperpigmentation at the injection site.

Spider Envenomation

Dermal injury from venomous spider bite caused by lipase, hyaluronidase, and sphingomyelinase-D, commonly due to Brown Recluse (Loxosceles reclusa) affecting the face and limbs, progresses from blister to pale halo with erythema, necrosis, eschar, and ulceration, heals by fibrous scar, dogs may develop eosinophilic folliculitis and furunculosis.

Snake Envenomation

Local and systemic effects following snakebite caused by venom containing enzymes, proteins, peptides, and kinins, affects Dogs and Horses more than Cats, commonly involves the head and limbs, causes local pain, edema, erythema, necrosis, skin sloughing, systemic paralysis, coagulopathy, shock, myocardial damage, rhabdomyolysis, and acute renal failure, Crotaline snakes are highly destructive.

Urticaria

Sudden development of wheals due to dermal edema caused by immunologic hypersensitivity, transient, may be pruritic, often not pruritic in horses, serum may ooze causing matting of hair.

Angioedema

Swelling involving deeper dermal and subcutaneous tissues, may become life-threatening if the upper respiratory tract is involved.

Atopic Dermatitis (Atopy, Allergic Inhalant Dermatitis)

Genetically predisposed chronic pruritic allergic dermatitis associated with IgE, caused by environmental allergens penetrating a defective epidermal barrier leading to IgE-mediated mast cell activation, characterized by percutaneous allergen exposure, pruritus, and chronic inflammation.

Sebaceous Gland Adenoma

Benign sebaceous gland tumor commonly protruding above the epidermal surface due to sebaceous gland neoplasia, affects dogs, appears hairless, greasy, and shiny because of sebaceous secretion.

Squamous Cell Carcinoma (Skin)

Malignant epidermal tumor of squamous cells caused by solar UV-induced neoplasia, affects dogs, occurs on nonpigmented sparsely haired skin, commonly the abdomen, with multiple ulcerated lesions.

Trichoblastoma

Benign hair follicle tumor that is circumscribed and raised due to hair follicle trichoblastic neoplasia, affects cats, commonly on the upper eyelid dorsal to the eye, appears sparsely haired, freely movable, and noninvasive.

Infundibular Keratinizing Acanthoma (Intracutaneous Cornifying Epithelioma/Keratoacanthoma)

Benign follicular tumor with keratin accumulation due to hair follicle infundibulum neoplasia, affects dogs, may form a cutaneous horn, also associated with Viral Papilloma, Solar Keratosis, Squamous Cell Carcinoma, and other adnexal lesions.

Cutaneous Lymphoma

Neoplastic proliferation of lymphocytes within the dermis forming nodules due to lymphoid neoplasia, affects horses, commonly on the neck and lateral thorax, dermal nodules elevate the overlying epidermis.

Cutaneous Histiocytoma

Benign histiocytic tumor forming a raised dermal nodule due to histiocyte neoplasia, affects dogs, commonly on the nose, appears as a circular raised alopecic tan nodule, frequently undergoes spontaneous regression.

Epitheliotropic Lymphoma

T-cell lymphoma with lymphocyte invasion of the epithelium, affects dogs, commonly involves the lip, oral mucosa, and mucocutaneous junctions, causes swelling, erythema, depigmentation, and erosions, may mimic systemic lupus erythematosus.

Mast Cell Tumor

Neoplasm of mast cells producing nodular skin masses due to mast cell neoplasia, affects dogs, commonly on the ventral thorax, presents as an irregular erythematous nodule, mast cell degranulation releases histamine, eosinophil and neutrophil chemotactic factors, prostaglandins, serine esterases, and TNF-α, which may mimic inflammation.

Hemangioma

Benign vascular tumor of endothelial cells due to endothelial cell neoplasia, affects dogs, commonly on the hind leg, appears as a raised red to dark red circumscribed mass, common in nonpigmented sparsely haired skin.

Hemangiosarcoma

Malignant vascular tumor of endothelial cells, affects dogs, presents as multiple raised red masses, common in nonpigmented sparsely haired skin, Whippets are predisposed.

Fibrosarcoma

Malignant fibroblast tumor invading surrounding tissues due to fibroblast neoplasia, affects cats, commonly on the leg, locally invasive, often ulcerated, difficult to excise completely, limb amputation may be required for distal limb tumors.

Melanocytoma

Benign melanocytic tumor forming a raised pigmented skin mass due to melanocyte neoplasia, affects dogs, appears as a raised brown to black hairless mass, dermis is diffusely infiltrated by pigmented melanocytes.

Melanoma

Malignant melanocytic tumor of the skin affecting dogs, junctional activity, clusters of melanocytes within the epidermis, is a hallmark feature of melanocytic tumors.