JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
August 2021
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 1.5mm, and to a lesser extent expanding the lamina propria and infiltrating 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 um yeast with 1-2 um basophilic nuclei surrounded by  a 1-2 um 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 1 x 3 um 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, etiology consistent with Histoplasma capsulatum, 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 inflammation of the intestinal tract with dissemination to many organ systems
• 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: 

• The yeast invades tissue, causes necrosis and replicates in macrophages
• 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
• Cell-mediated response is necessary to clear infection, but can also incite granulomatous inflammation
• Disease is usually self-limiting (mild bronchopneumonia); however, immunosuppression can lead to dissemination to lymph nodes, liver, bone marrow, spleen, GI tract, and adrenal glands
• Virulence factors:
  • Yeast form produces proteins that inhibit acidification of phagolysosome and lysosomal protease activity
  • Yeasts synthesize melanin or melanin-like compounds
  • 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

• Anemia
• Hypercalcemia: Associated with granulomatous disease (macrophages produce vitamin D analog)

TYPICAL GROSS FINDINGS:                       

Pulmonary form:

• 1-2cm, grey to white, firm to calcified, round nodules scattered in the parenchyma; occasionally enlarged hilar lymph nodes

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 marrow, and eyes may also show involvement
• In cats, disseminated disease is most common and may involve lungs, lymph nodes, liver, spleen, kidney, adrenal glands, bone marrow, and the gastrointestinal tract; primary intestinal histoplasmosis is rare in cats
• One case of disseminated disease has been reported in horses

TYPICAL MICROSCOPIC FINDINGS: 

• Yeast are 2-5um in diameter, oval to round, with a 1-2um basophilic center surrounded by a 2um 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 GALT and mesenteric lymph nodes
• Mycelial form can coexist with yeast form

ADDITIONAL DIAGNOSTIC TESTS:

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

DIFFERENTIAL DIAGNOSIS: 

Gross differential:

• Intestinal lymphoma

 

Intrahistiocytic organisms with similar morphology:

• Histoplasma capsulatum duboisii (African histoplasmosis): Only reported in baboons and humans; larger (8-15um); narrow-based budding
• Blastomyces dermatitidis: Larger (7-15 um); multinucleated; thick “double contoured” walls; broad-based budding
• Cryptococcus neoformans: Variably sized yeast (2-20 um); uninucleate; pleomorphic; single narrow-based budding; mucicarmine-positive capsule; rare hyphae
• Candida (Torulopsis) glabrata: Slightly larger (2-5 um); variably sized; oval to elongate; broad-based budding; amphophilic; stain entirely with H&E; no “halo” or pseudocapsule
• Leishmania (amastigotes within macrophages): 2-4um; kinetoplasts perpendicular to nuclei
• Toxoplasma gondii: 2-6 um crescentic bradyzoites; stain entirely with H&E; no “halo”; develops pseudocysts; usually found in somatic cells instead of phagocytic cells
• Neospora caninum: Similar to Toxoplasma gondii
• Pneumocystis carinii ( jiroveci), intra-alveolar cyst form: 4-6um; primarily extracellular; lacks budding; GMS-positive
• Sporothrix schenckii: 2-6 um; pleomorphic;single narrow-based budding, thin cell wall; rare hyphae; uninucleate
• Coccidioides immitis and posadasii: Released endospores may be confused with H. capsulatum

COMPARATIVE PATHOLOGY: 

• Similar lesions reported in cats, cattle, horses, swine, rodents, guinea pigs, pinnipeds, lagomorphs, non-human primates, humans, and a wide variety of non-domestic mammals - raccoons
• Dogs and cats are most susceptible to infection; birds are not susceptible because of their higher body temperature
• Second most commonly reported systemic fungal disease in cats
• 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
• One reported case of histoplasmosis found in a Northern Sea Otter in Alaska.
• Bats tolerate systemic infections with minimal inflammation and may serve as reservoirs.

REFERENCES: 

1. Bromel C, Greene CE. Histoplasmosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. St. Louis, MO: Saunders Elsevier; 2012: 614-621.
2. Boes KM, Durham AC. Bone marrow, blood cells, and the lymphoid, lymphatic system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:800-801.
3. Farina LL, Lankton JA. Chiroptera. Pathology of Wild Life and Zoo Animals. Cambrige, MA, Elsevier, 2018: 623.
4. Lopez A, Martinson SA. Respiratory system, mediastinum, and pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:547-548.
5. Robinson WF, Robinson NA. Cardiovascular system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier Ltd; 2016:202-203.
6. Schlemmer SN, Fratzke AP, Gibbons P, et. al. Histoplasmosis and multicentric lymphoma in a Nubian goat. J Vet Diagn Invest. 2019: 31(5):770-773.
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; 2016:97, 202-3.
8. Valli VEO, Kiupel M, Bienzle D.  Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier Ltd; 2016:186-187.
9. Wilcock BP, Njaa BL. Special senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Vol 1. St. Louis, MO: Elsevier Ltd; 2016:449-450.

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