JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
August 2021
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 eosinophilic granulomas centered on cores of cellular and karyorrhectic debris, rare negatively staining, poorly discernible hyphae, admixed with abundant degenerate eosinophils, neutrophils, fibrin, hemorrhage, rimmed by hypertrophied fibroblasts, abundant collagen (fibroplasia and fibrosis) admixed with few lymphocytes (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, and remaining smooth muscle myocytes are shrunken, 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 small numbers of hyphae. Hyphae are black-staining and contain 2-8um diameter, nonparallel walls with irregular, non-dichotomous branching and rare septations.

 

MORPHOLOGIC DIAGNOSIS:  Small intestine:  Enteritis, pyogranulomatous, eosinophilic, and fibrosing, focally extensive, transmural, marked, with 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 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
• The only other oomycete recognized to cause disease in mammals is Lagenidium
• 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
  • 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
• 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 insidiosum extract

 

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: narrow (3-6 um) hyphae, numerous septa, acute-angled dichotomous branching; asteroid bodies; often hemorrhagic and infarctive lesions; thrombosis; most common in abomasum
  • Mucoraceous fungi (mucormycosis) (Absidia, 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 um hyphae; eosinophilic sleeve 5-10 um wide; nasal, pharyngeal, SQ, pulmonary lesions in dogs and cats
  • Basidiobolus ranarum (formerly haptosporus): 5-20 um hyphae; eosinophilic sleeve up to 25 um wide; SQ and pulmonary lesions mostly in horses, rarely dogs and cats
• Oomycosis:
  • 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 (range 7-25um)
  • Saprolegnia:
    • 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 - 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. Carmo PM, Protella RA, Silva TR, Oliveira-Filho JC, Riet-Correa F. Cutaneous pythiosis in a goat. J Comp Pathol. 2015; 152(2-3):103-105.
3. Hargis AM, Myers, S. The Integument. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:1083.
4. Maia, LA, Olinda RG, et al. Cutaneous pythiosis in a donkey (Equus asinus) in Brazil. J Vet Diagn Invest. 2016; 28(4):436-439.
5. 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.
6. Souto EPF, Olinda RG, et al. Pythiosis in the nasal cavity of horses. J Comp Pathol. 2016; 155(2-3):126-129.
7. 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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