JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

January 2025

I-B01

Signalment (JPC# 1313232): Less than 1 month old, female, white tailed deer fawn

HISTORY: This young fawn was found weak and emaciated in the summer on a farm in Connecticut. Due to concerns about appearance of skin, animal was euthanized.

HISTOPATHOLOGIC DESCRIPTION: Haired skin: Diffusely, the epidermis is covered by a 1.2 mm thick, laminated serocellular crust. The serocellular crust is composed of layers of alternating orthokeratotic and parakeratotic hyperkeratosis, serum, entrapped hair shafts, and linear bands and accumulations of necrotic neutrophils and eosinophilic cellular and karyorrhectic debris (intracorneal pustules). Multifocally, within the serocellular crust and infiltrating the follicular epithelium and lumina are innumerable 1-2 um, paired bacterial cocci (zoospores) haphazardly arranged in rows and forming long branching filaments. Diffusely, the epidermis is moderately hyperplastic. Keratinocytes, predominantly in the stratum spinosum, are often swollen with increased clear space (intracellular edema). Throughout the less affected epidermis there are transmigrating neutrophils, lymphocytes, and histiocytes. The epidermis is occasionally lost (ulceration) and replaced by cellular and karyorrhectic debris with subjacent fibroblasts and small caliber blood vessels (granulation tissue). Within the superficial dermis subjacent to the serocellular crust, follicles are distended and occasionally effaced by many intact and necrotic neutrophils admixed with eosinophilic cellular debris, necrotic follicular epithelium, and fragmented hair shafts (folliculitis and furunculosis). Multifocally, the follicular epithelium has one of the following changes: necrosis, attenuation, or loss. Multifocally, the superficial dermis is infiltrated by perifollicular and periadnexal aggregates of macrophages, lymphocytes and plasma cells with fewer neutrophils. Superficial dermal blood vessels are congested.

MORPHOLOGIC DIAGNOSIS: Haired skin: Epidermitis, dermatitis, and folliculitis, suppurative, diffuse, severe, with epithelial hyperplasia, marked parakeratotic and orthokeratotic hyperkeratosis, intracorneal pustules, furunculosis, ulceration, and coccoid zoospores with branching filaments, etiology consistent with Dermatophilus congolensis, white tail deer (Odocoileus virginianus), cervid.

ETIOLOGIC DIAGNOSIS: Cutaneous dermatophilosis

CAUSE: Dermatophilus congolensis

SYNONYMS: Cutaneous streptothricosis; rain scald; rain rot; lumpy wool; strawberry foot rot; cutaneous actinomycosis; Kirchi; Gasin-Gishu; Senkobo disease; Drodo-Boka; Savi; Ambarr-Madow; (“mycotic dermatitis” is a misnomer)

GENERAL DISCUSSION:

• Gram-positive, filamentous, branching, facultative anaerobic bacteria (order Actinomycetales) that cause an acute, subacute, or chronic superficial exudative dermatitis
• Unusual life cycle where coccoid bodies germinate to produce branching filaments (may be > 5 um wide) 🡪 undergo transverse and longitudinal septation to produce parallel rows or columns of coccoid bodies (“train track” or “stacked coin” appearance); coccoid bodies are dormant until wet conditions, then become motile zoospores
• Most common in cattle, goats, sheep; occasional in horses; rare in pigs and small animals
• Most common in hot humid areas with heavy prolonged rainfall
• Usually a secondary invader; can co-infect poxviral lesions; damage to the hide may predispose to myiasis and secondary bacterial infections
  • Dermatophilosis cannot be reproduced experimentally to resemble natural disease regardless of dose
• Zoospores survive in crusts for approximately 42 months

PATHOGENESIS:

• Multiple factors involved in pathogenesis, which is poorly understood (not able to be reproduced experimentally)
• Two critical factors:
• Trauma to skin: Ectoparasites, shearing, thorny pastures, etc.; zoospores cannot penetrate hair, stratum corneum, etc.; require breaks in the skin
• Prolonged moisture: Needed for the activation, proliferation, and spread of the zoospores
• Other implicated factors include genetics, skin color (light-skinned areas more susceptible), concurrent diseases, and infection with the Amblyomma variegatum or Boophilus decoloratus tick
• Once in the epidermis: Zoospores germinate to filaments > invade viable epidermis and outer root sheaths > keratinocytes at sites of invasion cornify > neutrophils accumulate below and migrate into epidermis > separates epidermis from dermis > neutrophils inhibit further invasion > new epidermis formed > new epidermis invaded > cycle begins again, forming thick laminar and parakeratotic crusts

TYPICAL CLINICAL FINDINGS:

• Acute lesions are painful but rarely pruritic
• Begin as patches of erythema progressing to papules and pustules and then coalescing large, oval, domed scabs
• Incubation period averages 2 weeks, but can range from 1 day to a month

TYPICAL GROSS FINDINGS:

• Proliferative, exudative dermatitis producing raised, alopecic, erythematous, and sometimes papillomatous lesions covered by a thick, yellow-brown, keratinized crust / scab on any area of the body
• Early lesions are erythematous patches, followed by papules and pustules
• Coalescing exudative pustules that mat large groups of hairs together (“paintbrush” lesions)
• Underneath the removed scales there is little or no pus, and the epidermis is often moist and erythematous
• Cattle: Lesions begin on the dorsal midline and extend laterally to flanks, thoracic wall, shoulders, neck, axilla, and inguinal areas
• Subcutaneous and lymph node granulomas can occur; oral lesions rarely develop

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Palisading crust of thick, laminar, alternating layers of orthokeratotic-parakeratotic hyperkeratosis, serous fluid, degenerate inflammatory cells, and necrotic debris with varying degrees of perifolliculitis-folliculitis, intraepidermal pustular dermatitis, or hyperplastic superficial perivascular dermatitis
• Narrow, branching filaments with transverse and longitudinal septation that form parallel rows of cocci in superficial layers of the skin, within crusts, and within hair follicles
• Cytology: Elongated branching structures of cocci arranged in paired rows (“railroad tracks”) or randomly scattered individual cocci in the background, neutrophils, +/- eosinophils and macrophages

ADDITIONAL DIAGNOSTIC TESTS:

• Histopathology is characteristic
• Culture is difficult
• Cytologic exam of direct smears reveals parallel chains of cocci in “railroad track” configurations
• RT-qPCR assays have recently been validated for the detection of D. congolensis

DIFFERENTIAL DIAGNOSIS:

Proliferative/Hyperkeratotic Skin Lesions in Cattle:

• Dermatophytosis (Trichophyton spp., Microsporum spp.): Chained or disassociated arthrospores and hyphae can be located in the stratum corneum, hair follicles, and hair shafts; "domino effect"- serially damaged hair follicles damaged by necrotizing and pyogranulomatous folliculitis suggests dermatophytosis
• Ectoparasitism: Demodex spp., Psoroptes spp.- Elongate mites, measuring 40 x 250-300 um (shorter and longer forms exist), and keratinous debris plugging follicles; variable numbers of mites
• Pustular dermatitis (impetigo; subcorneal pustular dermatosis; pemphigus foliaceus): Pustules present with varying inflammatory cells, zoospores not present
• Viral dermatitis: Intracytoplasmic, eosinophilic inclusion bodies, located within epithelial cells
  • Contagious pustular dermatitis (parapoxvirus)
  • Lumpy skin disease (capripoxvirus):
• Photosensitization: Parakeratotic hyperkeratosis, acanthosis, apoptotic cells scattered through the dermis, with dermal hyperemia, edema, and perivascular mononuclear cellular infiltrate
• Zinc-responsive dermatosis: Parakeratotic hyperkeratosis extending into hyperplastic follicular infundibula is characteristic

COMPARATIVE PATHOLOGY:

• Sheep: Variable location; scale crusts in woolly skin are often pyramidal because the lesion spreads laterally as the crust is formed and may be up to 3 cm thick (“lumpy wool disease”); on the extremities known as “strawberry foot rot”; concurrent infection with contagious ecthyma and pox may occur
• Horses: Lesions are located on the dorsal aspect of the body and look as if large drops of liquid have scalded the skin (“rain scald”)
• Cats: All reported lesions have been subcutaneous or extracutaneous infections and consisted of draining nodules involving the area of the popliteal lymph node, subcutaneous tissue of the paw, and serosal surface of the bladder
• Goats: Head, dorsal midline, legs (grease heel), and ventral abdomen, often seen in conjunction with parapoxvirus (contagious ecthyma)
• Camels: affects rump, neck, flanks, and lower abdomen (legs are generally spared). Hyalomma spp. tick may play an important role in the pathogenesis in camelids
• Tortoises, turtles: D. chelona (now named Austwickia chelonae)
• American alligators, estuarine crocodiles, and freshwater crocodiles: Dermatophilus spp.
• Captive bearded dragons: Associated with Ranavirus infection
• Deer: Face, ears and distal limbs
• NHPs: reported in owl monkeys and titi monkeys; presented as well-circumscribed areas of alopecia and exudative dermatitis that progressed to hairless papillomatous masses.

References:

1. Abee CR, Mansfield K, Tardif S, Morris T. Nonhuman Primates in Biomedical Research: Volume 2: Diseases. 2^nd ed. San Diego, CA: Elsevier; 2012, 110 - 111.
2. Conley KJ, Shilton CM. Crocodilia. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:860-861.
3. Faccin M, Wiener DJ, Rech RR, Santoro D, Rodrigues Hoffmann A. Common superficial and deep cutaneous bacterial infections in domestic animals: A review. Vet Pathol. 2023;60(6):796-811.
4. Howerth EW, Nemeth NM, Ryser-Begiorgis MP. Cervidae. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:165.
5. Lane LV, Yang PJ, Cowell RL. Selected Infectious Agents. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:45.
6. Mauldin EA, Peters-Kennedy J. Integumentary System. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Vol. 1. St. Louis, MO: Elsevier; 2016:632-634.
7. Origgi FC. Lacertilia. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:884-885
8. Raskin RE, Conrado FO. Chapter 3: Integumentary System. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:46.
9. Rodriguez CE, Duque AMH, Steinberg J, Woodburn DB. Chelonia. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:845.
10. Welle MM, Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1167-1168.
11. Wilcock BP, Njaa BL. Special Senses. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:503.

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