JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

September 2024

D-F01 (NP)

 

SIGNALMENT (JPC #2039603): 8-year-old dog

 

HISTORY: Chronic, intractable diarrhea.

 

MICROSCOPIC DESCRIPTION: Colon: There are multifocal to coalescing, transmural inflammatory infiltrates predominantly expanding the submucosa up to 1.5 mm, and to a lesser extent expanding the lamina propria and infiltrate -ing between and through the layers of the tunica muscularis. The inflammatory infiltrates are composed of epithelioid macrophages, lymphocytes, plasma cells, and rare multinucleated giant cells and neutrophils admixed with reactive fibroblasts/fibrosis, fibrin, and edema. The cytoplasm of numerous macrophages contain variable numbers of round to oval, 2-5 µm yeast with 1-2 µm basophilic nuclei surrounded by a 1-2 µm peripheral clear zone and enclosed by a thin cell wall; there are rare extracellular yeast as well. Multifocally crypt epithelial cells are necrotic (shrunken, hypereosinophilic, with pyknosis) and are often sloughed into the crypt lumina. Occasionally crypts contain few 1x3 µm bacilli. Submucosal lymphatics are mildly ectatic and occasionally contain pale, homogenous, eosinophilic fluid (edema). 

 

MORPHOLOGIC DIAGNOSIS: Colon: Colitis, granulomatous, chronic-active, diffuse, moderate, with intrahistiocytic yeast, breed unspecified, canine.

 

ETIOLOGIC DIAGNOSIS: Colonic histoplasmosis

 

CAUSE: Histoplasma capsulatum var. capsulatum

 

CONDITION: Histoplasmosis

 

GENERAL: 

• Common, soil-borne, dimorphic, intracellular (within cells of the monocyte-macrophage system) fungus that causes granulomatous pneumonia or histiocytic/lymphoplasmacytic enterocolitis (or rarely gastritis) with dissemination or systemic disease to many organ systems (including bones/joints, eyes, middle ear, lymphoid and/or splenic, hepatic, endocrine (e.g., adrenal gland), skin and subcutaneous disease)
• Clinical disease occurs most frequently in dogs and cats but has been reported in other species 
• Worldwide distribution: Endemic in Mississippi, Ohio, and Missouri River valleys of the United States and Ottawa and St. Lawrence River valleys in Canada - thrives in moist, nitrogen-rich organic matter such as bird and bat excrement

 

PATHOGENESIS: 

• Dimorphic fungus with free-living, mycelial stages and infectious microconidia; infection through inhalation of airborne conidia or rarely by ingestion or skin contact with infective spores (including possibly infected sputum)
• Inhalation or ingestion of microconidia -> Macrophages ingest microconidia trapped in the mucus layer -> Microconidia germinate in phagosomes into the yeast form -> Yeast synthesizes proteins that inhibit acidification of the phagolysosome thereby escaping death -> Disseminate via lymphatic and hematogenous routes -> Small intestine -> Yeasts are released upon the death of the macrophage (6-16 day lifespan) -> Inflammatory cells accumulate in lamina propria > Thickened walls and ulcerated mucosae -> Protein-losing enteropathy
  1. Disease is usually self-limiting (e.g., mild bronchopneumonia); however, immunosuppression can lead to dissemination to lymph nodes, liver, bone marrow, spleen, GI tract, and adrenal glands
• Cell-mediated response is necessary to clear infection, but can also incite granulomatous inflammation
• Virulence factors:
  1. Yeast form produces proteins that inhibit acidification of phagolysosome and lysosomal protease activity
  2. Yeasts synthesize melanin or melanin-like compounds
  3. Modifications in surface polysaccharides (genomic variation) allows fungi to evade and/or neutralize detection by pattern recognition receptors (PRR, which usually recognize the α and β-glucan surface polysaccharides) on macrophages, neutrophils, and dendritic cells 

 

TYPICAL CLINICAL FINDINGS: 

• Intractable chronic diarrhea with anorexia and weight loss
• Lethargy and poor hair coat
• Predominantly a disease of young dogs that are usually presented with weight loss, 

generalized lymphadenopathy, hemorrhagic diarrhea, and tenesmus 

• Clinicopathologic abnormalities
  1. Neutrophilia, monocytosis, nonregenerative anemia (chronic inflammation); thrombocytopenia
  2. Hypercalcemia (associated with granulomatous disease)

 

TYPICAL GROSS FINDINGS:                        

Pulmonary form:

Intestinal and disseminated form: 

• Nodular to diffuse thickening of the intestines with occasional ulceration
• Lesions are most prominent in the small intestine
• Hepatosplenomegaly; mesenteric lymphadenomegaly
• Systemic lymph nodes, bone (including bone marrow and joints), and eyes may also show involvement

 

TYPICAL MICROSCOPIC FINDINGS: 

• Yeasts are 2-5 um in diameter, oval to round, with a 1-2 um basophilic center surrounded by a 2 um clear halo (artifact due to cell shrinkage, not a true capsule)
• Replication is by narrow-based budding
• Granulomatous interstitial pneumonia with intrahistiocytic yeast, multinucleated giant cells, +/- caseous necrosis
• Non-necrotizing histiocytic/lymphoplasmacytic inflammation with intrahistiocytic yeast in the liver, spleen, lymph nodes, bone marrow, intestinal tract, lungs, adrenal glands, pancreas, eye, skin, heart, kidney, and rarely CNS
• Multifocal and coalescing transmural granulomatous inflammation may occur in the stomach, or small and/or large intestine
• Macrophages filled with organisms are prominent in the lamina propria, GALT, and/or mesenteric lymph nodes

 

ADDITIONAL DIAGNOSTIC TESTS: 

• Cytology (rectal scrapings, organ/lymph node fine needle aspirates, pleural and peritoneal fluids): Pyogranulomatous to granulomatous inflammation with intrahistiocytic organisms
• Fungal stains:
  1. PAS (periodic acid-Schiff): Stains pink to red
  2. Silver-based stains (GMS [Gomori’s methenamine silver], Gridley’s): Stains cell wall black
  3. Giemsa: A pale light blue ring surrounding the blue protoplasm
• Fungal culture: Not recommended because of zoonotic potential of microconidia

 

DIFFERENTIAL DIAGNOSIS: 

Gross differential:

 

Intrahistiocytic organisms with similar morphology:

• Leishmania sp. (amastigotes within macrophages): 2-4µm; kinetoplasts perpendicular to nuclei 
• Toxoplasma gondii: 2-6 µm crescentic bradyzoites; stain entirely with H&E; no “halo”; develops pseudocysts; usually found in somatic cells instead of phagocytic cells
• Blastomyces dermatitidis: Larger (7-15 µm); multinucleated; thick “double contoured” walls; broad-based budding 
• Cryptococcus neoformans: Variably sized yeast (2-20 µm); uninucleate; pleomorphic; single narrow-based budding; mucicarmine-positive capsule; rare hyphae
• Candida (Torulopsis) glabrata: Slightly larger (2-5 µm); variably sized; oval to elongate; broad-based budding; amphophilic; stain entirely with H&E; no “halo” or pseudocapsule
• Neospora caninum: Similar to Toxoplasma gondii
• Pneumocystis carinii (jiroveci), intra-alveolar cyst form: 4-6 µm; primarily extracellular; lacks budding; GMS-positive
• Sporothrix schenckii: 2-6 µm; pleomorphic; single narrow-based budding, thin cell wall; rare hyphae; uninucleate
• Coccidioides immitis and C. posadasii: Released endospores may be confused with H. capsulatum

 

COMPARATIVE PATHOLOGY: 

• Histoplasmosis is reported in a wide variety of species, most common in dogs and humans and occasionally other species
• In dogs, (primarily young dogs), usually confined to the lungs, but dissemination can occur, or rarely solely gastrointestinal disease
• Second most commonly reported systemic fungal disease in cats (is predominately disseminated)

• Similar lesions have been reported in cattle, horses, swine, rodents, guinea pigs, pinnipeds, lagomorphs, non-human primates, humans, and a wide variety of non-domestic mammals
  1. Bats tolerate systemic infections with minimal inflammation and may serve as reservoirs.
  2. Rarely reported in avians (polyostotic osteomyelitis and keratitis reported)
  3. Reported in conjunction with lymphoma in a Nubian goat (proposed that primary lymphoma led to immunocompromise, resulting in secondary, opportunistic infections) (Schlemmer, J Vet Diagn Invest. 2019)

Other species of Histoplasma

• Histoplasma capsulatum var. farciminosum (epizootic lymphangitis of horses): resembles ulcerative lymphangitis and cutaneous glanders; chronic suppurative lymphadenitis of solipeds; in humans there is often caseous necrosis of the adrenal cortex with adrenal enlargement and resultant Addison's disease

• Histoplasma capsulatum var. duboisii (African histoplasmosis) (I-F13): Only reported in baboons and humans and causes cutaneous (and possibly deeper) granulomatous lesions 8-15 µm in diameter (2x larger than H. capsulatum)

 

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