JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-P04

SIGNALMENT (JPC# 21474-19): 4-year-old castrated mixed breed dog

HISTORY: This case is from a raccoon hunting hound dog that developed skin nodules, primarily affecting the distal forelimbs. The nodules were pruritic, alopecic, and erythematous with minimal hemorrhagic exudate. Unsuccessful treatments included antibiotics, ivermectin, and lime sulfur dips. The dog is housed in an outdoor kennel with a concrete floor. There was no history of travel prior to lesion development

MICROSCOPIC DESCRIPTION: Haired skin: Diffusely, hair follicles are ectatic and follicular lumens are distended by lamellated and fragmented keratin and occasional nematode larvae. Nematode larva are 20-30 µm in diameter and have a rhabditiform esophagus that is composed of a corpus, isthmus, and bulb. There is diffuse, moderate, lymphoplasmacytic perivascular and perifollicular dermatitis. Inflammation also extends into the follicular wall and is rarely transmural extending to the follicular lumen (mural folliculitis). There is mild hyperplasia of the follicular epithelium and of the overlying epidermis. There is mild basket-weave orthokeratotic hyperkeratosis. Apocrine glands are diffusely ectatic and often contain basophilic inspissated secretory product.

MORPHOLOGIC DIAGNOSIS: Haired skin: Dermatitis and folliculitis, perivascular, perifollicular, and mural, lymphoplasmacytic, chronic, diffuse, moderate with intrafollicular nematode larvae, mixed breed, canine.

ETIOLOGIC DIAGNOSIS: Pelodera (rhabditic) dermatitis

CAUSE: Pelodera strongyloides (formerly Rhabditis strongyloides)

GENERAL:

• A free-living nematode of the order Rhabditida whose larva cause nonseasonal, pruritic dermatitis most often in dogs, occasionally in cattle; horses and sheep are rarely affected
• Lesions commonly occur in areas in contact with unsanitary, moist bedding (e.g. ventral trunk, feet, legs, perineum, tail)
• Life cycle is direct
• The free-living saprophytic adult nematode is found in longstanding, decaying organic debris, especially straw, rice hulls, or hay sorted in long term contact with the ground in damp conditions

PATHOGENESIS:

• Moist, unsanitary conditions predispose to infection; larvae enter via broken skin
• Larvae invade hair follicles; acute eosinophilic dermatitis develops, commonly with suppurative folliculitis
• Pruritus can result in self-induced trauma that may lead to secondary bacterial infections
• Infection can spontaneously clear when the animal is removed from unsanitary conditions

TYPICAL CLINICAL FINDINGS:

• Moderate to marked pruritus in most species (absent in cattle); localized alopecia; dermatitis of the ventral feet, legs, thorax and abdomen, perineum, and tail (areas in contact with unsanitary, moist bedding)
• Most reported cases have been in short-coated dogs

TYPICAL GROSS FINDINGS:

• Erythematous, alopecic, papular, crusting dermatitis +/- pustules (especially in dogs)

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Eosinophilic and pyogranulomatous inflammation surrounding nematodes in the lumina of the hair follicles or dermis
• Eosinophilic and lymphoplasmacytic perivascular and perifollicular dermatitis, folliculitis (often mural), and furunculosis, with hyperkeratosis and nematode segments
  • Epidermal hyperplasia characterized by irregular acanthosis, hyperkeratosis
• Pelodera sp. has the following characteristics: ~0.6 mm long, smooth cuticle, paired lateral alae (usually absent in tissue section), platymyarian-meromyarian musculature, rhabditiform esophagus (corpus, isthmus, bulb), uninucleate intestinal cells with a low microvilli border, reflexed ovaries with comparatively large ova
  • The small size of the nematode may make many of these features difficult to discern in histologic sections

ADDITIONAL DIAGNOSTIC TESTS:

• Deep skin scraping; biopsy

DIFFERENTIAL DIAGNOSIS:

• Grossly:
  • Bacterial dermatitis
  • Contact dermatitis
  • Demodicosis (I-P07): Mites are 40x300 um with 2 rows of 6-8 legs; within follicular lumina and rarely in sebaceous glands
  • Sarcoptic acariasis (I-P06): Mites are 200-400 um in length; within stratum corneum
  • Superficial bacterial folliculitis
  • Dermatophytosis (I-F11)
• Microscopically:
  • Hookworm dermatitis: lesions on areas of body in frequent contact with the ground, especially distal limbs, feet, interdigital areas; larval migration tracts may be seen histologically
  • Dirofilariasis (C-P03)
  • Strongyloidiasis
  • Demodicosis (I-P07)

COMPARATIVE PATHOLOGY:

• Horse: Rare; marked alopecia; skin may be thickened and wrinkled with pustules on ventral abdomen and mammary gland; pruritus is variable
• Sheep: Rare; loss of wool, hyperkeratosis, lichenification
• Cattle: Dermal pruritus can be absent; lesions become lichenified

REFERENCES:

1. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: Armed Forces Institute of Pathology 2006:14-15.
2. Fisher DJ. Cutaneous and subcutaneous lesions. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and hematology of the dog and cat. 5th ed. St. Louis, MO: Elsevier; 2020:84.
3. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, Vol 1. 6th ed. Philadelphia, PA: Elsevier; 2016:689.
4. 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; 2022:60.
5. Welle MM and Linder KE. The Integument. In: Pathologic Basis of Veterinary Disease. 7^th ed. Ed. Zachary JF. St. Louis, MO: Elsevier; 2022:1238.

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