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The Gross and Histologic Pathology of Common Skin Diseases in Domestic Animals
Elizabeth A. Mauldin, DACVP
Dermatology Resident
University of Pennsylvania
Review of Gross Path terms
Acanthosis- epidermal hyperplasia- a response of the skin to chronic injury; does not
necessarily mean chronicity as the skin can often turn over rapidly; “chronic hyperplastic
perivascular dermatitis” is often not very helpful to the clinician
Things to note- hyperkeratosis (ortho or parakeratosis); organisms in crusts (yeast or
bacteria), inflammatory infiltrate
Examples: Acral lick dermatitis; Atopy
Alopecia- gross term for hair loss; pattern and character of alopecia important for gross
diagnosis; symmetrical thinning alopecia often accompanied by atrophic striae in
hyperadrenocorticism; moth-eaten alopecia seen in follicular disorders; pattern baldness
(post auricular regions, ventral neck, ventrum and caudomedial thighs); focal alopecia
secondary to rabies injection site reaction; cyclical flank alopecia
Callous- a thickened, roughened, hyperpigmented, lichenified plaque; usually present at
pressure points; may be biopsied when the source of chronic/recurrent pyoderma
Comedone- a keratin filled dilated infundibulum that may also contain sebaceous
material and microorganisms
Examples: Feline chin acne; Muzzle folliculitis and furunculosis, Demodicosis;
Endocrinopathies (hyperadrenocorticism)
Crust- a collection of inspissated serous debris that contains degenerate leukocytes,
erythrocytes, keratin and often microorganisms; can often find old acantholytic
keratinocytes in crusts of Pemphigus foliaceus; often formed from a ruptured pustule or
vesicle, or bulla; character of the crusting give clues to clinical diagnosis (robust crusting
is seen in Pemphigus foliaceus; hemorrhagic crusts may indicate deeper infection or
vasculitis; yellow crusts commonly seen with scabies)
Epidermal Atrophy/Dermal atrophy- decreased thickness of the epidermis or dermis
most often due to endocrinopathy; epidermal atrophy can also be seen in developmental
disorders such as cutaneous asthenia
Pitfalls—regions examined, the skin of dogs and cats if often 1-2 cell layers thick; is
there concurrent sebaceous gland atrophy, and folliclular atrophy?
Examples: Feline hyperadrenocorticism versus Canine hyperadrenocorticism
Epidermal collarette- special type of scale arranged in a circular pattern of loose
keratin; usually caused by the roof of a vesicle, bulla, pustule or papule; or hyperkeratosis
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secondary to a point source of inflammation (pustule or papule); histologically rete pegs
are seen on the margins of the collarettes
Examples: Superficial spreading pyoderma; Folliculitis; unusual presentation pemphigus
folicaceus
Fissure- a linear defect in the skin that extends from the surface to the dermis; occur at
sites where the skin is thick and inelastic;
Examples: Pemphigus foliaceus, nasal and footpad hyperkeratosis
Follicular casts- an accumulation of keratin that tightly adheres to the hair shaft above
the follicular ostia; may be a primary lesion of vitamin A responsive dermatosis,
sebaceous adenitis, and idiopathic seborrhea; also seen in folliculitis due to demodicosis,
dermatophytosis, and staph.
Example: Sebaceous adenitis
Lichenification: thickening with accentuation of the normal markings of the skin, often
hyperpigmented; associated with chronic inflammation; may have bacterial or yeast
component
Example: Acanthosis nigricans
Macule- a primary flat discoloration up to 1 cm diameter; may occur secondary to
hemorrhage, hyperpigmentation or depigmentation Patch- a macular lesion greater than 1
cm diameter
Example: Feline lentigenes
Papule- solid elevation in the skin less than 1 cm diameter; often erythematous; can be
follicular or nonfollicular; if follicular often secondary to bacterial infection of hair
follicles; other causes nonfollicular papules include scabies and flea allergy dermatitis
Plaque- lesion that rises slightly above the skin surface and is larger than 1 cm diameter
Examples: Feline eosinophilic plaque; Canine eosinophilic plaque; Well’s syndrome*
*Holm KS. Morris DO. Gomez SM. Peikes H. Byrne KP. Goldschmidt MH. Eosinophilic
dermatitis with edema in nine dogs, compared with eosinophilic cellulitis in humans.
Journal of the American Veterinary Medical Association. 215(5):649-53, 1999 Sep 1. 99
Pustule- small circumscribed elevation filled with pus; can be located in the epidermis,
hair follicle or subepidermal; color may vary from white yellow to red or green; usually
composed of neutrophils but may contain eosinophils (pemphigus folicaceus, sterile
eosinophilic pustulosis, allergic or parasitic disorders); green pustules may indicate a
gram negative infection; large pustules need to rule-out immunosuppression and PF
Things to note: presence or absence of bacteria or acantholytic cells and inflammatory
cell types; Is it follicular or nonfollicular?
Examples: Muzzle folliculitis and furunclosis; impetigo
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Scale- a collection of loose collection of stratum corneum
Can be prominent on gross examination and subtle on histopath due to processing;
character of scaling varies with disorder; varies from greasy to dry; silvery white scale
seen in cutaneous lymphoma and sebaceous adenitis
Seborrhea- clinical term for increased scaling of the skin; with or without increased
sebum production
Tumor- solid or cystic elevation in the skin 2 cm or more in diameter.
Vesicles and Bulla- a noncystic transparent, fluid-filled elevation in the skin; vesicles are
less than 1 cm diameter; bullae are greater than 1 cm diameter; caused by intra or
intercellular edema, acantholysis, hydropic degeneration of basal cells, subepidermal
edema or antibody to component of basement membrane; can be very difficult to submit
intact bulla on biopsy
Example: *Epidermolysis Bullosa Aquisita
*Olivry T., et. al. Canine epidermolysis bullosa acquisita: circulating
autoantibodies target the aminoterminal noncollagenous (NC1) domain of collagen VII in
anchoring fibrils. Veterinary Dermatology. 1998, 9,19-31.
Hemorrhagic bulla- seen in deep pyoderma; usually consists of large collections of
suppurative exudate and erythrocytes; ulcerate and form draining tracts
Examples: German shepherd pyoderma; pedal folliculitis and furunculosis
Wheal- firm, circumscribed, raised elevation in the skin composed of edema; often
erythematous with a central area of pallor; short-lived (urticaria)
Examples: Canine urticaria
Common Follicular Disorders of the Dog and Cat
I. Disorders characterized predominately by follicular inflammation
Folliculitis- inflammation of the lumen of the hair follicle
Perifolliculitis- inflammation around the hair follicle with exocytosis of cell through the
outer root sheath
Mural folliculitis- inflammation of the wall of the hair follicle
Furunculosis- perforating folliculitis
A. Superficial bacterial folliculitis- the most common bacterial skin infection in the
dog
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B. Canine Demodicosis- interface mural folliculitis, suppurative folliculitis and
furunculosis and nodular dermatitis, eosinophils rare; follicular melanosis and
pigmentary incontinence
* Caswell, JL. Yager, JA, et al. Canine Demodicosis: A Re-examination of the
Histopathologic Lesions and Description of the Immunophenotype of Infiltrating
Cells. Veterinary Dermatology, Vol 6, No.1, p9-19, 1995.
C. Feline Demodicosis- caused by two different demodex mites: one which infects hair
follicles (Demodex cati) and a variety referred to as “stumpy demodex” and now
officially named Demodex gatoi which dwells in the stratum corneum; clinical signs
of stumpy demodex include self-induced symmetrical alopecia, erythema,
excoriations on the abdomen and thorax; Demodex cati more commonly associated
with ceruminous otitis, alopecia and crusting on the head with occasional truncal and
generalized lesions.
*Morris, DO, Beale KM. Feline Demodicosis. Current Veterinary Therapy XIII.
D. Dermatophytosis- suppurative mural folliculitis; pyogranulomatous furunculosis;
arthrospores, hyphae in hair shafts; rarely- M. pericolor, hyphae in stratum corneum
only; severity depends on host adaptation
*Pemphigus foliaceus associated with Dermatophytosis
*Poisson L, Olivry T, and C. Lemmons. Subcorneal neutrophilic acantholytic pustular
dermatitis: an unusual manifestation of dermatophytosis resembling canine
pemphigus foliaceus. Advances in Vet Derm Vol 3. Proceedings from Third World
Congress of Vet Derm, 1996, Edinborough, Scotland, 1998, 456-457.
E. Eosinophilic folliculitis and furunculosis- Mosquito bite hypersensitivity in the cat;
Arthropod bites; Pemphigus foliaceus; Eosinophilic folliculitis, allergic
F. Canine juvenile cellulitis- should really be included with nodular or diffuse
dermatitis; infection of sebaceous glands occurs early in course of the disease; folliculitis
develops as lesions progress and is followed by deep nodular pyogranulomatous
dermatitis and cellulitis
II. Disorders characterized by prominent follicular keratosis
Hyperkeratosis of the follicular infundibulum is seen in many disorders; must combine
other lesions and clinical presentation for diagnosis
A. Seborrhea is a clinical term used to describe excessive scaling of the skin; most
often caused by increased epidermal turnover; classically divided into seborrhea
oleosa (greasy) and seborrhea sicca (dry); seborrhea can be a primary disorder of
keratinization or secondary to almost anything (atopy, hypothyroidism, food allergy,
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etc); clinician must rule-out other causes before making a diagnosis of primary
seborrhea; biopsies often performed to help in diagnosis, particularly Vitamin A
responsive dermatosis
Primary seborrhea; Vitamin A responsive dermatosis - loose epidermal and
marked follicular orthokeratosis with parakeratotic caps; can be difficult to
differentiate but Vit A more pronounced follicular keratosis
B. Canine Zinc responsive dermatosis- epidermal and follicular parakeratotic
hyperkeratosis; marked follicular parakeratosis; Artic breeds; look for characteristic
clinical presentation: excessive scale and crusting around eyes, mouth and footpads
*Colombini S. Dunstan RW. Zinc-responsive dermatosis in northern-breed dogs: 17
cases (1990-1996). Journal of the American Veterinary Medical
Association. 211(4):451-3, 1997.
C. Feline chin acne- idiopathic disorder of keratinization affecting the chin and lips of
cats; develop classic comedones; papules and pustules; if develop chin swelling may
be biopsied to rule-out eosinophilic granuloma complex
Canine muzzle folliculitis and furunculosis- disorder affecting short-coated breeds,
lips and chin, muzzle; papules and comedones; secondary infection; suppurative
furunculosis
D. Sebaceous adenitis- follicular orthokeratosis often seen in disease in standard
poodles; difference in clinical lesions and histopathology in short-coated versus longcoated breeds
III.
Disorders characterized by atrophy of hair follicles
Follicular atrophy- atrophic changes beyond normal telogen phase; “faded follicles”;
pronounced connective tissue sheath surrounding hair follicle
Canine Hyperadrenocorticism- telogen follicles; follicular keratosis and plugging;
sebaceous gland atrophy; calcinosis cutis
Traction alopecia- atrophic faded follicles; normal follicles often at periphery; may have
atrophic epidermis with pigmentary incontinence; vacuolated basal cells and few civatte
bodies
Rabies vaccine-induced vasculitis and panniculitis- nodular lymphocytic panniculitis
with faded follicles; subtle to overt vascultis; hydropic degeneration of basal
keratinocytes; pigmentary incontinence
Follicular atrophy with excessive trichilemmal keratinization- this bright eosinophilic
keratin anchors the base of the hair shaft to the root sheath after anagen phase; in some
disorders this keratin becomes much more pronounced; hair follicles arrest in telogen or
late catagen phase (flame follicles); includes a controversial group of disorders
Endocrine dermatoses
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Growth hormone and sex hormone responsive dermatosis
Post clipping alopecia
 Histopath must be combined with clinical presentation and functional testing to make
accurate diagnosis
IV. Disorders characterized by follicular dystrophy/dysplasia
Follicular dystrophy- abnormal hair shafts, stain with eosin (normal hair shafts are
refractile and nonstaining), often contain pigmentary abnormalities; follicular
orthokeratosis; need historical findings, breed, coat color
Color mutant alopecia/Black hair follicular dysplasia -dystrophic follicles with
pigmentary abnormalities
Cyclical flank alopecia- not a true follicular dystrophy, telogen follicles with deformed
profiles referred to as club foot or witches foot; may see anagen bulbs;
hyperpigmentation
References:
Scott DW, Miller WH, Griffin CE: Muller and Kirk’s Small Animal Dermatology,5th Ed.
W.B. Saunders Co., Philadelphia.
Jager, JA and Wilcock BP: Color Atlas and Text of Surgical Pathology of the Dog and
Cat. Vol 1., Mosby, Spain, 1994.
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