JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

September 2024

D-F04

 

Signalment (JPC #2841523): Dog, age and breed not specified.

 

HISTORY: This dog had a small intestinal mass. 

 

SLIDE A:

HISTOPATHOLOGIC DESCRIPTION: Small intestine: Affecting 60% of the section, markedly expanding and transmurally effacing the intestinal wall up to 1.5 cm in thickness, and compressing the intestinal lumen, are multifocal to coalescing, poorly formed nodules of inflammation with central pallor centered on cores of cellular and karyorrhectic debris, rare, negatively staining, and poorly discernible fungal hyphae. Rimming this core are abundant degenerate eosinophils and neutrophils admixed with fibrin, hemorrhage, and edema, further rimmed by multiple epithelioid macrophages that are occasionally large and multi-nucleated, hypertrophied fibroblasts, abundant collagen (fibroplasia and fibrosis), and few lymphocytes and plasma cells (eosinophilic granuloma). Multifocally, degranulating eosinophils surround individualized, brightly eosinophilic, fragmented collagen (major basic protein, collagen degeneration). Previously described inflammatory cells separate and surround smooth muscle myocytes, which are multifocally pale, swollen, and contain variably sized sarcoplasmic vacuolations (degeneration) or are shrunken and hypereosinophilic with pyknotic nuclei (necrosis). In less affected areas, the submucosa is expanded by previously described inflammatory cells, rare granulomas, and increased clear space and ectatic lymphatics (edema). Multifocally, there is blunting, fusion, and loss of villi. 

 

SLIDE B: GMS: Primarily within centers of granulomas are low numbers of hyphae. Hyphae are black-staining with 2-8µm diameter, nonparallel walls with irregular, non-dichotomous, acute-angle branching and rare septations.

 

MORPHOLOGIC DIAGNOSIS: Small intestine: Enteritis, pyogranulomatous and eosinophilic, transmural, multifocal to coalescing, severe, with fibrosis, necrosis, and rare hyphae, canine.

 

ETIOLOGIC DIAGNOSIS: Enteric pythiosis 

 

CAUSE: Pythium insidiosum

 

CONDITION: Pythiosis, oomycosis

 

GENERAL DISCUSSION:

• A chronic, progressive, gastrointestinal, cutaneous, or multisystemic granulomatous disease of horses, dogs, cattle, cats, sheep, several captive exotic species, and humans in tropical, subtropical, and (occasionally) temperate climates
• Fungus-like, aquatic mold of class Oomycetes

• Opportunistic pathogen that lives in warm, stagnant water; only P. insidiosum is pathogenic in mammals 

• Oomycetes differ from true fungi in the composition of their cell walls (contain cellulose and beta-glucan, but not chitin) and in the production of motile, biflagellate zoospores; more closely related to algae than fungi 
• The only other oomycete recognized to cause disease in mammals is Lagenidium spp.
• Two forms of pythiosis (usually one or the other):
  • Gastrointestinal: Most common form in the dog
  • Cutaneous: Most common form in the horse
• Difficult to diagnose/treat 

 

PATHOGENESIS:

• Oospore colonizes aquatic plant leaves -> sexual reproduction to produce sporangia -> asexual reproduction within sporangia -> infective stage (biflagellate motile aquatic zoospore), released in warm/wet weather -> exposure by standing in or drinking stagnant water or exposure to contaminated soil or wet grasses with newly emerged zoospores (zoospores are chemotactically attracted to animal hair, damaged skin, and intestinal mucosa) -> zoospore encysts on skin/GI mucosa and forms a germ tube (hypha)-> hypha invades tissue, including blood vessels, produces proteases -> granulomatous reaction
• Zoospores do not form in tissue, no zoonotic potential

 

TYPICAL CLINICAL FINDINGS:

• In dogs, young adult, large breed, and German Shepherds/Labrador Retrievers may be predisposed
• Clinical pathology: Anemia, eosinophilia, hypoalbuminemia, hyperglobulinemia, rare hypercalcemia
• GI form: weight loss, vomiting, diarrhea, hematochezia, +/- palpable abdominal mass; systemic signs apparent after intestinal obstruction or perforation
• Cutaneous form: rapidly-growing nodular lesions, +/- pruritic (extremely pruritic in horses)

 

TYPICAL GROSS FINDINGS:

• GI form:
  • Segmental thickening of GIT (anywhere), irregular mucosal ulceration
  • Transmural granulomatous inflammation +/- perforation with granulomatous peritonitis, +/- omental adhesions, +/- obstruction/infarction
  • Transmural, mass-like, +/- obstructive thickening is most common in stomach, small intestine, or ileocolic junction
  • Enlarged mesenteric lymph nodes, thickened lymphovascular channels
• Cutaneous form:
  • Proliferative lesion with ulceration and draining tracts
  • Horses only form cores of gray/white, creamy necrotic coagula (called leeches or kunkers) 
  • Most common sites: Extremities (may involve entire circumference of limb), ventral thorax or abdomen, base of tail, and perineum; has been known to cause pyogranulomatous vaginitis 
• Concurrent cutaneous and GI lesions in the same animal are rare

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Eosinophilic granulomatous-pyogranulomatous inflammation +/- granulomas, MNG cells, epithelioid macrophages, lytic/caseous necrotic center, fibrosis 
• Inflammation mostly in submucosa, may be transmural> adhesion/peritonitis; enlarged lymph nodes may become embedded in granulomatous mass at root
• Cutaneous: Granulomatous inflammation with foci of lytic necrosis & extensive fibrosis, frequently involving dermis & subcutis, with acanthotic/ulcerated epidermis
• Hyphae: in centers of necrosis and occasionally seen in walls of small arterioles; 2-8um diameter, have non-parallel walls, irregular branching (non-dichotamous), rare septations; hyphae often surrounded by Splendore Hoeppli reaction
• Organism stains poorly with H&E (hyphal ghosts); GMS preferred over PAS

 

ADDITIONAL DIAGNOSTIC TESTS:

• PCR, culture, immunohistochemistry (lacks specificity)
• Serologic assays (ELISA, immunoblot) - highly sensitive and specific
• Hemagglutination test - sheep RBCs coated with P. insidiosum extract 
• Cytologic findings: Often have mixed inflammation (pyogranulomatous) with an eosinophilic component
  1. Organisms (if present on cytology): Poorly staining, broad/wide, poorly septate, occasionally branching hyphae (2-7µm diameter) with parallel cell walls; hyphae will not stain w/ Romanowsky, but will stain with GMS (more so than with PAS).
  2. Common locations for obtaining diagnosis: Stomach, small intestine, ileocolic junction 
  3. NOTE: Cytology alone cannot distinguish between Pythium, Lagenidium, and zygomycete organisms (fungi) – requires other confirmatory tests (listed above)

 

DIFFERENTIAL DIAGNOSIS: 

• Granulomatous enteritis:
  • Fungal enteritis – zygomycosis, aspergillosis
  • Algal enteritis – protothecosis
  • Mycobacterial enteritis 
• Granulomatous dermatitis: 

• Neoplasia (sarcoid, SCC)
• Excessive granulation tissue (proud flesh)
• Habronemiasis
• Fungal dermatitis – aspergillosis, zygomycosis
• Oomycotic dermatitis – lagenidiosis 

 

• Alimentary tract mycoses:
  • Pythiosis, lagenidiosis, and entomophthoromycosis cannot be differentiated via normal histologic sections
  • Aspergillus sp.: Narrow (3-6 µm) hyphae, numerous septa, acute-angled dichotomous branching; asteroid bodies; often hemorrhagic and infarctive lesions; thrombosis; most common in abomasum
  • Mucoraceous fungi (mucormycosis) (Absidia sp., Mucor sp., Rhizopus sp.): broad (6-25 um) hyphae, hemorrhagic and infarctive lesions; mycotic omasitis > rumenitis > reticulitis >abomasitis; CNS/disseminated disease in dogs, cats
  • Entomophthoracetes (entomophthoromycosis; saprophytic fungi):
    • Conidiobolus coronatus: 5-13 µm hyphae; eosinophilic sleeve 5-10 µm wide; nasal, pharyngeal, SQ, pulmonary lesions in dogs and cats
    • Basidiobolus ranarum (formerly B. haptosporus): 5-20 µm hyphae; eosinophilic sleeve up to 25 µm wide; SQ and pulmonary lesions mostly in horses, rarely dogs and cats
  • Lagenidium sp. (only reported in dogs):
    • Lesions may be more generalized (great vessels, sublumbar and inguinal lymph nodes, lung, and cranial mediastinum)
    • Usually visible on H&E stained sections
    • Hyphae have larger diameter than Pythium sp. (range 7-25µm)
  • Saprolegnia sp.:
    • Fish and amphibians - causes cutaneous oomycosis (proliferative cotton-like growth on skin and/or gills)

 

COMPARATIVE PATHOLOGY:

Pythiosis in other species:

• Horses 
  • Predominantly cutaneous form (AKA leeches, kunkers, bursattee, swamp cancer)- Lesions on areas likely to touch water (i.e. lower limbs distal to carpus and hock [often circumferential], ventral abdomen, and chest); lesions rarely disseminate to internal organs (may invade bone)
  • Subcutaneous nodules or masses up to 45cm in diameter; ulcerated, scarred surface with multiple draining tracts; kunkers/leeches; usually pruritic
  • Less commonly, infection of the small intestine causes eosinophilic enteritis and granulomas and may result in colic
• Cattle – Uncommonly reported in beef cattle <12 months old; predominantly cutaneous/subcutaneous form; lesions on lower limbs 
• Cats - Rare; cutaneous/subcutaneous lesions affecting inguinal, tailhead, or periorbital regions; draining lesions on extremities; rare gastrointestinal pythiosis
• Sheep and Goats - Granulomatous rhinofacial, cutaneous, and digestive tract; hyphae seen in center of necrosis; often surrounded by Splendore Hoeppli reaction

 

REFERENCES:

1. Carmo PMS, Uzal FA, Riet-Correa F. Diseases caused by Pythium insidiosum in sheep and goats: a review. J Vet Diagn Invest. 2021;33(1):20-24 
2. Fisher DJ. Subcutaneous and Subcutaneous Lesions. In: Valenciano AC, Cowell RL. Diagnostic Cytology and Hematology of the Dog and Cat. 5^th edition. St. Louis, MO: Elsevier Ltd; 2020:82-83. 
3. Haddad JL, Stowe DAM, Neel, JA. The Gastrointestinal Tract. In: Valenciano AC, Cowell RL. Diagnostic Cytology and Hematology of the Dog and Cat. 5^th edition. St. Louis, MO: Elsevier Ltd; 2020:294,300. 
4. Hargis AM, Myers, S. The Integument. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Elsevier, 2022:1177-1178.
5. Hostetter SJ. Chapter 7: Oral Cavity, Gastrointestinal Tract, and Associated Structures. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:304-305.
6. Mauldin, EA, Peters-Kennedy, J. Integumentary System. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier Ltd; 2016:657-659.
7. 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):812-28.
8. Solano-Gallego L, Masserdotti C. Chapter 13: Reproductive 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; 2022:465-466.
9. 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.

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