Signalment (JPC #21474-15): A young dog
HISTORY: This dog presented with a generalized alopecic and pruritic skin condition.
HISTOPATHOLOGIC DESCRIPTION: Haired skin: Diffusely, the epidermis is thickened up to 0.5 mm with acanthosis, prominent rete ridge formation, spongiosis, and multifocal orthokeratotic and parakeratotic hyperkeratosis with embedded adult arthropods and eggs. Occupying the stratum corneum, occasionally within tunnels, adult arthropods are ovoid, 200-300 x 100-150 um in diameter, and possess a spiny chitinous exoskeleton, jointed appendages, striated muscle, a body cavity (hemocoel), and intestinal and reproductive structures. The eggs are oval, thin-shelled, and 70 x 40 um in diameter. Multifocally there are intracorneal pustules containing degenerate neutrophils, necrotic cellular debris and proteinaceous fluid. Diffusely the superficial dermis is expanded by clear space (edema) and few lymphocytes, plasma cells, neutrophils and eosinophils. There are few ectatic hair follicles.
MORPHOLOGIC DIAGNOSIS: Haired skin: Epidermal hyperplasia and hyperkeratosis, diffuse, marked, with mild eosinophilic dermatitis and numerous intracorneal mites, etiology consistent with Sarcoptes scabiei, breed unspecified, canine.
ETIOLOGIC DIAGNOSIS: Sarcoptic dermatitis
CAUSE: Sarcoptes scabiei var. canis
CONDITION: Sarcoptic mange
CONDITION SYNONYM: Canine scabies
- Common in dogs, pigs and red foxes; uncommon in cattle and goats (psoroptic mange is more important in these species); rare in horses, sheep, and cats
- Highly contagious, intensely pruritic, nonseasonal skin disease that is relatively host specific but can cause transient disease in other species, including humans
- Young dogs are most often affected with no sex predilection; Labrador retriever, Cocker spaniel and Doberman pinscher are overrepresented
- Occurs most often with poor animal husbandry and overcrowding
- Reportable disease of food animals in most countries
- In the natural host, the parasite completes its life cycle in tunnels burrowed into and under the stratum corneum
- Burrowing female mites directly damage the epidermis (non‑pruritic incubation period is usually 6 days to 3 weeks) > mite secretions and excreta incite local irritation > the host becomes sensitized to mite or their products > allergic reaction (most important) > intense pruritus > self‑trauma, secondary infection
- The proteins released by the mites create an immediate (type I) and delayed (type IV) hypersensitivity which results in the skin disease presented.
- Poorly-nourished or immunosuppressed animals > develop massive mite infestations associated with specific syndromes:
- Norwegian-type scabies in immunosuppressed dogs (e.g., due to long-term corticosteroid therapy)
- Hyperkeratotic mange in breeding adult pigs (see comparative pathology)
- Reinfestation results in a shorter incubation time before clinical scabies
- Presumably, both a humoral and cell‑mediated hypersensitivity reaction occur, with formation of circulating immune complexes
- Entire 10‑25 day life cycle is spent on the host
- Transmission is by direct contact or via fomites; highly contagious
- Female mite burrows into superficial layers of the epidermis > deposits 15‑50 eggs in the freshly burrowed tunnel at the rate of 1‑5 per day > eggs hatch in 3‑5 days to produce the six‑legged larvae > larvae form molting pockets in the stratum corneum > larvae molt to become eight‑legged nymphs > after molting through two nymphal stages, they reach adulthood > mating & fertilization occur on the skin surface or in molting pockets
- Adult females probably don"t live longer than 3‑4 weeks and generally die after depositing the eggs; male mites die after copulation
TYPICAL CLINICAL FINDINGS:
- Severe pruritus and alopecia; pruritus often more pronounced at night and in warm environments
- Self‑trauma; secondary bacterial infections; generalized lymphadenopathy
- Immunosuppressed (e.g. long-term corticosteroid therapy in dogs) may exhibit "Norwegian-type scabies" so named after the similar disease of immunosuppressed humans; pruritus in these animals may be less evident than in animals affected by the common form of the disease
TYPICAL GROSS FINDINGS:
- Lesions often concentrated in sparsely haired regions of body; especially ears (lateral margins of the pinna), lateral elbows, hocks, ventral thorax and abdomen
- Primary parasite-related lesions: Erythematous maculopapular eruptions, crusting (crusts are often thick and yellowish white) and alopecia
- Secondary lesions: Bleeding, hemorrhagic crusts, self‑induced traumatic excoriations and lesions due to secondary bacterial infections
- Poorly-nourished or immunosuppressed animals: Chronic infection with lichenification, marked alopecia, hyperpigmentation, and fissuring
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Typical allergic form lacks mites and lesions are indistinguishable from other allergic dermatoses
- Epidermis: Moderate to severe acanthosis; variable spongiosis; hyperkeratosis and multifocal parakeratosis or serocellular crust
- Mites are difficult to demonstrate; a clue is the presence of focal areas of epidermal edema, exocytosis, degeneration, and necrosis; numerous mites may be present in lesions of immunocompromised animals
- Adult mites are 200-400 um, with males smaller than females and having a spiny chitinous exoskeleton, jointed appendages, and striated muscle; ovoid eggs are 100-150 um; both are usually seen in the keratin layer or superficial cell layers of the epidermis
- Dermis: Edema, superficial, often perivascular infiltration of eosinophils, mast cells, lymphocytes and macrophages; +/- neutrophils and plasma cells in traumatized lesions
- Immunosuppressed animals: Numerous mites in epidermal burrows; marked parakeratosis
ADDITIONAL DIAGNOSTIC TESTS:
- Response to appropriate therapy: the most useful diagnostic procedure
- Multiple, superficial skin scrapings (ear margin, lateral elbow); often negative
- ELISA test - 92% sensitive and 96% specific
- Histologic examination is useful but not conclusive unless characteristic mites are present
- Food hypersensitivity: Pruritic; face, feet and ears commonly affected with erythematous or crusted papules
- Flea allergy dermatitis: Flea debris or fleas observed; crusted papules; alopecia along dorsal midline and sacrum
- Cheyletiellosis: Mites do not burrow; live on epidermal surface; prominent hyperkeratosis not uncommon
- Cutaneous dirofilariasis: Often form nodular perivascular aggregates; segments of microfilaria within vessels or within extravascular inflammatory foci
- Zinc-responsive dermatosis: Primarily a diffuse parakeratotic hyperkeratosis; no organisms evident
- Hookworm dermatitis: Distal extremities and footpads; linear accumulations of neutrophils and eosinophils form “tracks” that extend into the dermis
- Food allergy dermatitis: Superficial perivascular infiltrates composed of lymphocytes, macrophages, eosinophils, and mast cells; eosinophils may extend to surround vessels of the deep dermis, if present, highly suggestive of food allergy
- Flea allergy dermatitis: Mild chronic epidermal lesions; dermal changes very similar to food allergy dermatitis and may be indistinguishable
- Pigs ( scabiei var suis): Most important ectoparasite of swine; most common site for primary lesions/mites is inner surface of pinna; papular lesions associated with allergic reactions and lesions secondary to self-trauma occur on rump, flank, and abdomen; 2 distinct clinical forms exist:
- Hypersensitivity form: Most common form; intense pruritus > self‑trauma and alopecia 7‑11 weeks after repeated infestation; seen in young growing pigs with intense pruritis
- Hyperkeratotic mange: Affects chiefly breeding adults; chronic infestation with thick, adherent scabs and lesions containing numerous mites; most severe on head, neck, and legs; seen in multiparous sows or debilitated animals; moderate pruritus
- Cat: scabiei rare in cats; Agent of “feline scabies” is Notoedres sp.
- Goats: scabiei var caprae; lesions on face, pinna, neck, and legs
- Cattle: scabiei var bovis; Predilection for head, neck; may become generalized
- Sheep: scabiei var ovis; dvspredilection for lips, nostrils, external pinna, +/- legs
- Horses: S scabiei var equi; rare; lesions on head, pinna, neck
- Wild canids (red fox, kit fox, grey wolf, coyote): Highly susceptible to sarcoptic mange; develop a strong type I hypersensitivity reaction; no resistance to re-infection; the most common disease reported in wild red foxes in the southeastern U.S; May be associated with increased human interaction
- Guinea pigs (Trixacarus caviae): Keratosis, crusting, alopecia over the neck, shoulders, inner thighs, and abdomen; mite burrows in stratum corneum
- Rabbits: One report of an outbreak of scabiei in Holland lop rabbits; Notoedres cati produces similar lesions in rabbits.
- Alpacas: scabiei var auchinae; weight loss and decreased fiber production.
- Iberian ibex: Sacoptic mange in this species is associated with seasonal wasting, anemia, and immune suppression in the winter months
- Human: Infestations of animal origin have been reported; however, mites stay on skin surface and do not complete life cycle
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- Jimenez MD, Bangs EE, Sime C, Asher VJ. Sarcoptic mange found in wolves in the Rocky Mountains in western United States. J Wildl Dis. 2010; 46(4):1120-1125.
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