JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2023

P-F02 (NP)

 

Signalment (JPC #2681634): Thoroughbred foal

 

HISTORY: This foal was born two weeks premature, was able to nurse intermittently, and had a suckle reflex. Thoracic radiographs revealed an increased interstitial pattern. The foal then developed diarrhea, became weak, and died within 24 hours of birth.

 

HISTOPATHOLOGIC DESCRIPTION: Lung: Expanding the pulmonary parenchyma and overlying pleura is a cellular infiltrate that forms variably distinct nodules up to 11mm in diameter and extends into and expands adjacent alveolar septa filling the alveolar lumens. Inflammatory infiltrates are comprised of numerous epithelioid macrophages and multinucleated giant cells (foreign body and Langhans types), fewer lymphocytes and neutrophils, and rare plasma cells admixed with fibrillar eosinophilic material (fibrin), homogenous eosinophilic fluid (edema), and necrotic cellular debris.  There are numerous intrahistiocytic and fewer extracellular 2-4µm round to oval yeast with a clear 1-2µm wide halo surrounding a basophilic center. Bronchioles are variably filled with a mild exudate composed of edema and few inflammatory cells. Alveoli multifocally coalesce and form large clear spaces (emphysema). Multifocally, the pleural mesothelium is cuboidal with increased eosinophilic cytoplasm and round nuclei with open chromatin (reactive).

 

MORPHOLOGIC DIAGNOSIS: Lung: Pneumonia, granulomatous, diffuse, marked, with numerous intrahistiocytic and fewer extracellular yeast, Thoroughbred, equine.

 

ETIOLOGIC DIAGNOSIS: Pulmonary histoplasmosis

 

CAUSE: Histoplasma capsulatum var. capsulatum

 

CONDITION: Histoplasmosis

 

GENERAL DISCUSSION:

• Common, soil borne, facultative, dimorphic fungus; intracellular parasite of the monocyte–macrophage system; causes granulomatous inflammation
• Worldwide distribution; endemic in the Mississippi, Ohio, and St. Lawrence river valleys
• Grows best in nitrogen-rich organic matter such as bird or bat excrement
• Dimorphic fungus: Mycelial phase in environment, yeast phase in host
• Disease is rare in immunocompetent hosts; greater risk if immunosuppressed or receive significant exposure

 

PATHOGENESIS:

• 12 to 16 day incubation period
• Microconidia from the soil are inhaled > transform into the yeast stage in the lung > pulmonary macrophages ingest yeast, which can multiply by budding within the phagolysosome and lyse the host cell
• Yeast are eliminated via cell-mediated immune response; if response is incomplete, organism can incite granuloma formation within the lung or establish a dormant phase within the monocyte-macrophage system (with potential for reactivation upon immunosuppression)
• Hematogenous systemic dissemination to many organs may occur, primarily to organs with many mononuclear phagocytes
• Gastrointestinal histoplasmosis without respiratory involvement has occurred, suggesting that ingestion may also cause disease, but this has not been proven experimentally

 

TYPICAL CLINICAL FINDINGS:

Pulmonary Form:

• Dyspnea, coughing, weight loss, and fever
• Clinical Pathology: Neutrophilia, monocytosis, lymphopenia, and eosinopenia

Disseminated Form:

• Generalized lymphadenopathy, chronic bloody diarrhea, and emaciation
• Clinical Pathology:

• Normocytic, normochromic, nonregenerative anemia of chronic inflammatory disease
• Hypoalbuminemia, hyponatremia, and hyperkalemia from chronic diarrhea 
• Increased ALP, ALT, hyperbilirubinemia, and hypoalbuminemia if there is liver involvement
• Hypercalcemia can occur secondary to any granulomatous disease; macrophages produce 1,25 dihydroxycholecalciferol
• CSF with increased protein and cellularity (pleocytosis) with nondegenerate neutrophils, macrophages/monocytes, lymphocytes, and occasionally intrahistiocytic yeast

 

TYPICAL GROSS FINDINGS:

Pulmonary Form:

Disseminated Form:

• Regional or generalized lymphadenopathy: Affected lymph nodes are firm and uniform in appearance, resembling lymphoma
• Intestines: Nodular thickening of mucosal surface, resembling Johne’s disease of cattle; hemorrhagic enteritis is also possible
• Splenomegaly/hepatomegaly
• Ocular involvement can occur, especially in cats

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Pulmonary Form: Multiple granulomas composed of epithelioid macrophages and multinucleated giant cells with intrahistiocytic yeast; +/- caseation and calcification; or diffuse, extensive proliferation and infiltration of monocytes and epithelioid macrophages
• Disseminated Form: Histiocytic and lymphoplasmacytic to granulomatous pneumonia, lymphadenitis, enterocolitis, hepatitis, splenitis, adrenalitis, myelitis, and meningoencephalitis with intrahistiocytic yeast 

 

ADDITIONAL DIAGNOSTIC TESTS:

• Cytology: Intrahistiocytic round to ovoid yeast measuring 1-4µm diameter with a purple nucleus and lightly basophilic protoplasm surrounded by a thin clear halo; readily recognizable on rectal scrapings, fine needle aspirates of organs (e.g. liver, spleen, skin), lymph nodes, pleural and peritoneal fluids, bone marrow, skin, and CSF
• Histochemical stains:

• PAS (periodic acid-Schiff): Pink to red staining of carbohydrates in the fungal cell wall
• GMS (Gomori’s methenamine silver): Black staining of the fungal cell wall
• Giemsa: Pale light blue ring surrounding the blue protoplasm

 

DIFFERENTIAL DIAGNOSIS:

Intrahistiocytic organisms with similar morphology:

• Histoplasma capsulatum var. duboisii (I-F13, African histoplasmosis): Only reported in baboons and humans; larger (8-15um); narrow-based budding
  • Nonhealing lesions of the sex skin, hands, and feet in baboons is African histoplasmosis (Histoplasma capsulatum var. Duboisii)
  • Can be easily diagnosed on impression smears, but not distinguishable from other Histoplasma variants

• Blastomyces dermatitidis (P-F05): Larger (7-15um); multinucleated; thick “doubly contoured” walls; broad-based budding
• Cryptococcus neoformans (P-F04): Variably sized yeast (2-20um); uninucleate; pleomorphic; single narrow-based budding; usually thick, mucicarmine-positive capsule; rare hyphae
• Leishmania sp. (D-P20, H-P07, I-P15, U-P03; amastigotes within macrophages): 2-4µm; rod-shaped kinetoplasts perpendicular to nuclei
• Toxoplasma gondii (P-P01): 2-6 µm crescentic bradyzoites; stain entirely with H&E; no “halo”; develops pseudocysts; usually found in somatic cells instead of phagocytic cells
• Neospora caninum (I-P17, N-P03): Similar to Toxoplasma gondii
• Pneumocystis carinii ( jiroveci), intra-alveolar cyst form: 4-6µm; primarily extracellular; lacks budding; GMS-positive
• Sporothrix schenckii (I-F07, H-F01): 2-6 µm; pleomorphic; single narrow-based budding, thin cell wall; rare hyphae; uninucleate
• Coccidioides immitis (P-F03): Released endospores may be confused with H. capsulatum

 

COMPARATIVE PATHOLOGY:

• Bats: Serve as reservoirs, uncommonly exhibit clinical disease, and there are no reports of mortality due to natural infections in free ranging bats; yeast are most often located within pulmonary macrophages but can occasionally be found within circulating macrophages in the mesentery, spleen, liver, adrenal gland, and intestine; excrete infective fungi in the feces
• Wild carnivores and omnivores: Rarely reported in bears and procyonids; somewhat frequently reported in coyotes
• Herbivores: Rare; one case in a goat with multicentric lymphoma (thought to be secondary to immunosuppression)
• Rodents:  Sporadic cases in juvenile mice, chinchillas, guinea pigs, and rabbits
• Birds, reptiles and amphibians:  Generally not reported in birds, reptiles and amphibians

 

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