JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
September 2018
D-F04

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

HISTORY:  This dog had a small intestinal mass.

HISTOPATHOLOGIC DESCRIPTION:  D-F04a:  Small intestine:  Affecting 60% of the section, transmurally replacing normal architecture, and markedly expanding the intestinal wall up to 1.2 cm in thickness, are multiple coalescing granulomas centered on cores of abundant necrotic debris admixed with numerous degenerate eosinophils and neutrophils, and small amounts of fibrin and hemorrhage.  Necrotic cores contain low numbers of negatively staining and poorly-discernible hyphae, and are surrounded by a band of numerous macrophages, including epithelioid macrophages, and fewer multinucleated giant cells (Langhans and foreign body type), admixed with eosinophils and neutrophils.  These are further surrounded by numerous haphazardly arranged hypertrophied fibroblasts and anundant collagen (fibrous connective tissue) admixed with previously described inflammatory cells.  There are multifocal areas of degranulating eosinophils surrounding individualized collagen bundles (collagenolysis). Collagen bundles and smooth muscle fibers throughout the tunica muscularis are replaced by plump fibroblasts and widely separated by the degranulated eosinophils, previously described inflammatory infiltrate, edema, and few small areas of hemorrhage. There are multifocal areas of smooth muscle fiber loss or necrosis, and the tunica muscularis is replaced by abundant fibrosis. In less affected areas there is expansion of the submucosa and lamina propria by infiltrates of epitheloid macrophages, neutrophils, eosinophils, increased clear space and ectatic lymphatics (inflammation and edema), fusion and blunting of the villi, and rare granulomas.  There is focally extensive compression of the intestinal lumen and effacement of mucosal architecture by the aforementioned inflammation and fibrosis.

D-F04b:  GMS:  Scattered throughout the tissue, but primarily within the center of granulomas, are small to moderate numbers of GMS-positive, 4-8 um wide hyphae with nonparallel walls, non-dichotomous acute and right angle branching, and rare septations.

MORPHOLOGIC DIAGNOSIS:  Small intestine:  Enteritis, granulomatous, eosinophilic, and fibrosing, focally extensive, transmural, marked, with rare GMS-positive 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 – lives in warm, stagnant water; only insidiosum is pathogenic in mammals
• 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 (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 with newly emerged zoospores (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
• Exposure to contaminated soil or wet grasses
• 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
• Anemia/eosinophilia/hypoalbuminemia/hyperglobulinemia/rare hyperCa²⁺
• 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
• Organisms found in areas of necrosis have broad, irregularly branching (non-dichotamous), rarely septate hyphae (range 2-8um diameter) with thick, nonparallel walls (occasionally seen in walls of small arterioles)
• Organism stains poorly with H&E (hyphal ghosts); GMS preferred (+/-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:
  • Other fungal enteritis – protothecosis, zygomycosis, aspergillosis
  • Mycobacterial enteritis
• Granulomatous dermatitis:
• neoplasia (sarcoid, SCC)
• excessive granulation tissue (proud flesh)
• habronemiasis
• fungal dermatitis – aspergillosis, lagenidiosis, zygomycosis
• Alimentary tract mycoses:
  • Pythiosis, lagenidiosis & 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:

• Horses (AKA leeches, kunkers, bursattee, 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 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 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 - predominantly cutaneous/subcutaneous form over limbs, abdomen, and prescapular regions with rare lung and lymph node dissemination; recent report of an outbreak of granulomatous rhinitis due to insidiosum

REFERENCES:

1. do 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.
2. 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.
3. Maia, LA, Olinda RG, et al. Cutaneou pythiosis in a donkey (Equus asinus) in Brazil. J Vet Diagn Invest. 2016; 28(4):436-439.
4. 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.
5. Souto EPF, Olinda RG, et al. Pythiosis in the nasal cavity of horses. J Comp Pathol. 2016; 155(2-3):126-129.
6. 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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