JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

August 2025

I-F02

Signalment (JPC# 21474-7,-8): 1-year-old, breed not specified, dog

HISTORY: This dog from Texas had severe, ulcerative dermatitis involving the legs.

HISTOPATHOLOGIC DESCRIPTION: Haired skin and subcutis: Multifocally effacing the dermis, separating and surrounding collagen fibers and adnexal structures, infiltrating the subcutis, and extending to all borders are coalescing, disorganized nodules composed of a core of cellular debris, fibrin, and drop-out (lytic necrosis) centered on faint, negative images of 6-10 µm diameter hyphae with nonparallel walls and poorly discernible, irregularly angled, non-dichotomous branching. Hyphae are surrounded by numerous degenerate and fewer viable neutrophils, epithelioid macrophages, fewer multinucleated giant cells (foreign body and Langhans types), eosinophils, plasma cells, and lymphocytes. Inflammatory nodules are admixed with abundant hemorrhage, fibrin, hemosiderin-laden macrophages that often exhibit erythrophagocytosis, and colonies of 1-2 µm basophilic cocci. Nodules of inflammation are separated by numerous reactive fibroblasts and loose, fibrous connective tissue with perpendicularly arranged small caliber blood vessels (granulation tissue) that progresses to mature fibrous connective tissue. The wall of a large blood vessel is focally discontinuous with loss of endothelial cells and replacement by eosinophilic cellular and karyorrhectic debris and few previously described inflammatory cells (vascular necrosis), and the lumen is partially occluded by a large fibrin thrombus. Small blood vessels are often lined by reactive endothelial cells and are variably occluded by moderate amounts of fine to coarse fibrin with enmeshed erythrocytes and few inflammatory cells (fibrin thrombi).

Gridley’s stain: Hyphae are 6-10 µm (occasionally up to 15 µm) wide, and rarely septate, with irregularly angled, non-dichotomous branching and thick, unevenly stained, non-parallel walls.

MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Dermatitis and panniculitis, pyogranulomatous and eosinophilic, multifocal to coalescing, severe, with hyphae, vascular necrosis and fibrin thrombi, breed not specified, canine.

ETIOLOGIC DIAGNOSIS: Cutaneous pythiosis

CAUSE: Pythium insidiosum

CONDITION: Oomycosis (formerly phycomycosis)

GENERAL DISCUSSION:

• Pythium insidiosum is an aquatic dimorphic water mold, member of the Oomycetes
  • Oomycetes differ from true fungi: Bi-flagellate aquatic zoospores; cell walls contain cellulose and beta-glucan (lack chitin); nuclear division, mitochondria, and Golgi structures; plasma membranes lack sterols (ergosterol)
• Pythiosis is a chronic cutaneous, enteric, or multisystemic granulomatous disease primarily in horses (cutaneous), dogs (enteric more often than cutaneous), humans, cattle, sheep; sporadic reports in camels, donkeys, cats, and other species
• Requires contact (most likely of broken skin) with stagnant water that contains zoospores, or ingestion
• Found globally in tropical to sub-tropical regions; more common in Gulf states in the U.S. though reported in others
• Equine disease was formerly referred to as bursatee, Florida horse leeches, granular dermatitis, hyphomycosis, destruens equi, phycomycosis, phycomycotic granuloma, and swamp cancer
• Not transmissible or zoonotic
• See D-F04 for enteric pythiosis

PATHOGENESIS:

• Infectious motile, biflagellate, aquatic zoospores demonstrate chemotactic preference for animal hair, damaged skin, intestinal mucosa, and plant leaves (Rodrigues Hoffmann, Vet Path 2023)
• Zoospores encyst on tissue and attach by secreted glycoprotein
• Develops germ tube toward and into affected tissue (hypha); penetration and invasion of dermis and blood vessels aided by secreted proteases
• Causes eosinophilic granulomatous inflammation which forms ulcerative nodules with draining tracts
• TYPICAL CLINICAL FINDINGS: Cutaneous signs:
  • Lesions found anywhere but most common on distal extremities, ventral abdomen and chest in horses; and extremities, face and tailhead of dogs. Lesion is often intensely pruritic in horses and self-mutilation can be severe
  • Peripheral eosinophilia +/- lymphadenopathy
• Gastrointestinal signs (see D-F04):
  • Anorexia, weight loss, diarrhea, vomiting, dysphagia, palpable abdominal mass, mesenteric lymphadenopathy
  • Stomach and duodenum most common sites; invasion to pancreas, mesenteric lymph nodes or bile ducts possible
  • Peripheral absolute eosinophilia

TYPICAL GROSS FINDINGS:

• Cutaneous form (limbs; distal to carpus and hock is most common and ventral thorax and abdomen)
  • Rapidly progresses from poorly circumscribed dermal nodule to multiple ulcerative nodules with fistulous tracts draining hemorrhagic purulent exudate
  • Characteristic gray-white to pale yellow coral-like concretions found in sinus tracts and may extrude from skin surface, unique to horses, known as “kunkers”
• Enteric form
  • Segmental thickening of GIT (anywhere) with irregular mucosal ulceration
  • Transmural granulomatous inflammation +/- perforation with granulomatous peritonitis, +/- omental adhesions, +/- obstruction/infarction
  • Enlarged mesenteric lymph nodes, thickened lymphovascular vessels

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Granulomatous to pyogranulomatous lesions consisting of necrotic foci with eosinophils, neutrophils, epithelioid macrophages, plasma cells and multinucleated giant cell infiltrates and fibrosis with negatively stained hyphae at periphery of necrotic eosinophilic debris
• Two inflammatory patterns (dogs):
  • Necro-eosinophilic: broad zones of eosinophilic necrosis, cell debris, and variable numbers of eosinophils
  • Granulomatous: epithelioid macrophages and Langhans giant cells surrounded by connective tissue
• Difficult to see hyphae on H&E, may see clear spaces outlined by eosinophilic material (often resembling Splendore-Hoeppli phenomenon)
• Hyphae stain readily with Gomori methenamine silver (GMS)

ULTRASTRUCTURAL FINDINGS:

• Irregularly branching, non-dichotomous, rarely septate hyphae with 2-7 µm diameter nonparallel thick walls

ADDITIONAL DIAGNOSTIC TESTS:

• Difficult to impossible to distinguish from the oomycete Lagenidium and fungi of the Order Entomophthoromorales (Basidiobolus and Conidiobolus) on histologic sections
• Staining: H&E, GMS, Gridley’s
• Cytology: Pale staining, 2-7 µm diameter, linear, branching hyphae with nonparallel walls and infrequent septations within and amongst aggregates of inflammatory cells
  • Hyphae easily missed with Romanowski stains; use GMS if suspected (PAS will not stain hyphae)
• PCR, IHC, culture (lacks specificity)
• Serological assays (ELISA, immunoblot) – high sensitivity and specificity
• Hemagglutination test – sheep RBCs coated with P. insidiosum extract

DIFFERENTIAL DIAGNOSIS:

• Infectious granulomas:
  • Bacterial: Deep pyoderma, opportunistic mycobacterial infections, actinomycosis, nocardiosis
  • Zygomycosis: Basidiobolus, Conidiobolus, Mucor spp.
  • Oomycosis: Lagenidium spp. (dogs)
  • Dimorphic fungal: Cryptococcosis, blastomycosis, histoplasmosis
  • Parasitic: Demodicosis, habronemiasis
  • Algal: Protothecosis
• Neoplasia: Squamous cell carcinoma, equine sarcoid
• Excessive granulation tissue
• Acral lick dermatitis
• Foreign body granulomas
• Idiopathic nodular dermatoses

COMPARATIVE PATHOLOGY:

• Horses (pythiosis also called Florida horse leech, bursattee, and swamp cancer)
• Predominantly cutaneous form - lesions on areas likely to touch water- lower limbs distal to carpus & hock (often circumferential), ventral abdomen, and chest that rarely disseminate to internal organs (may invade bone)
• Subcutaneous nodules or masses up to 45 cm in diameter; ulcerated, scarred surface with multiple draining tracts; kunkers/leeches; often pruritic
• Less commonly, infection of the small intestine causes eosinophilic enteritis and granulomas and may result in colic
• Cattle - uncommonly in beef cattle <12 months old; predominantly cutaneous/subcutaneous form; lesions on lower limbs contain yellow punctate foci (do not form leeches/kunkers)
• Cats - rare; cutaneous/subcutaneous lesions affecting inguinal, tailhead, or periorbital regions; ulceration uncommon; rare gastrointestinal pythiosis
• Sheep - predominantly cutaneous/subcutaneous form over limbs, abdomen, and prescapular regions with rare lung and lymph node dissemination; ovine rhinofacial pthyiosis, or “bull nose,” reported in Brazil.
• Amphibians: Saprolegniasis affects captive and wild animals throughout their lifecycles (eggs to larvae to adults); Saprolegnia, Aphanomyces, and Achyla spp. reported as causative genera
  • UVB radiation (egg masses), poor water quality, and stress, as well as disruption of skin barrier may predispose to infection
  • Fish: Freshwater fish are commonly infected by water molds including Pythium spp. as well as Saprolegnia spp., and Leptomitus spp. Skin wounds and stress predispose fish to infection; eggs can also be infectedGross: cotton-like proliferative growth on gills and/or skin
  • Histo: lesions mainly confined to dermis; minimal inflammation; hyphae often visible on H&E but silver stain can be helpful
  • Saprolegnia parasitica is a particularly important primary pathogen of fish, including farmed catfish; colloquially known as “winter kill”; serious pathogen of cultured eggs
  • Aphanomyces invadens: cause of epizootic ulcerative syndrome (EUS) aka red spot disease (RSD), mycotic granulomatosis (MG), and ulcerative mycosis (UM); worldwide cause of disease in wild and farmed freshwater fish in 94 fish species
  • Branchiomyces spp.: cause “gill rot” aka branchiomycosis in 17 families of cultured and wild freshwater fish
• Infrequently reported in many other species

REFERENCES:

1. de Souto EPF, Kommers GD, Souza AP, Miranda Neto EG, Assis DM, Riet-Correa F, Galiza GJN, Dantas AFM. A Retrospective Study of Pythiosis in Domestic Animals in Northeastern Brazil. J Comp Pathol. 2022 Jul;195:34-50.
2. do Carmo PMS, Uzal FA, Riet-Correa F. Diseases caused by Pythium insidiosum in sheep and goats: a review. J Vet Diagn Invest. 2021 Jan;33(1):20-24.
3. 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 Mosby; 2014:82-83.
4. Frasca SJ, Wolf JC, Kinsel MJ, Camus AC, Lombardini ED. Osteichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018: 984-986.
5. Haddad JL, Marks Stowe DA, Neel JA. The gastrointestinal tract. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and hematology of the dog and cat. 5th ed. St. Louis, MO: Elsevier; 2020: 300.
6. Hostetter SJ. Oral cavity, gastrointestinal tract, and associated structures. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023: 305.
7. 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 Saunders; 2016: 657-659.
8. Noga EJ. Fish Disease: Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley Blackwell; 2010: 156-161.
9. Pessier AP. Amphibia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018: 939-940.
10. Rodrigues Hoffmann A, Ramos MG, Walker RT, Stranahan LW. Hyphae, pseudohyphae, yeasts, spherules, spores, and more: A review on the morphology and pathology of fungal and oomycete infections in the skin of domestic animals. Vet Pathol. 2023;60(6):812-828.
11. Solano-Gallego L, Masserdotti C. Reproductive system. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023: 465-466.
12. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier Ltd; 2016: 177-180.
13. Welle MM, Linder, KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022: 1177-1178. 

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