JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

October 2024

D-T06 (NP)

 

Signalment: (JPC #2019285): 9-month-old castrated Merino sheep

 

HISTORY: This sheep was dosed with a toxic preparation over a 25-day period. During the last 3 days the animal became increasingly weak and was anorectic.

 

HISTOPATHOLOGIC DESCRIPTION: Liver: Diffusely hepatic cord and sinusoidal architecture is lost. Hepatocytes are haphazardly arranged and separated by moderate to abundant fine, loose, immature fibrous connective tissue (fibrosis). Hepatocytes are characterized by one of the following: often swollen with variably sized, discrete, intracytoplasmic vacuoles (lipid-type degeneration); individualized and shrunken with hypereosinophilic cytoplasm and a pyknotic nucleus (single cell death); or rarely 2-3 times normal size with abundant, granular, eosinophilic cytoplasm, a large vesiculate nucleus, and occasional multinucleation. Hepatocyte mitotic figures are common, with an average of 1 per individual HPF (0.237mm²), and atypical mitoses are common. Portal areas contain increased numbers of bile ducts (biliary hyperplasia) and are infiltrated by low numbers of lymphocytes. Occasionally bile ducts and bile canaliculi contain luminal, brown to bright yellow, globular material (cholestasis). Multifocally, macrophages are distended with intracytoplasmic bright yellow to brown pigment (bile or hemosiderin). Multifocally lymphatics are ectatic (edema).

 

MORPHOLOGIC DIAGNOSIS: Liver: Hepatocellular degeneration and necrosis, panlobular, chronic, diffuse, severe, with fibrosis, cholestasis, lipid-type degeneration, atypical mitoses, multinucleated hepatocytes, and biliary hyperplasia, merino, ovine.

 

ETIOLOGY: Phomopsin mycotoxin

 

ETIOLOGIC DIAGNOSIS: Phomopsin hepatopathy

 

CONDITION: Mycotoxicosis

 

GENERAL: 

• Phomopsin is a cyclic hexapeptide mycotoxin produced by the saprophytic fungus Diaporthe toxica, which grows on Lupinus spp. plants. 
  • Several wild lupin plant species are themselves toxic (via alkaloids), resulting in syndromes characterized by teratogenic effects and limb malformations, which are separate from the condition caused by mycotoxicosis
• Intoxication is most common in animals grazing lupine stubble, which has the highest toxin concentrations; warmth and rain enhance fungal growth and Phomopsin toxin production
• Two toxic metabolites exist: Phomopsin A and phomopsin B 
• This entity is separate from the toxicosis induced by ingestion of Lupine spp. plants which have teratogenic and neurotoxic effects 

 

PATHOGENESIS: 

• Phomopsin is a potent microtubule inhibitor that blocks the polymerization of tubulin, the major component of microtubules
• Defective microtubule formation arrests mitosis in metaphase
• Evidence exists that purified phomopsin A directly stimulates the onset of mitosis in hepatocytes, leading to increased numbers of mitotic figures
• Secondary photosensitization (also known as Type 3 or hepatogenous photosensitization), particularly in sheep
  • Cholestasis > impaired capacity of the liver to excrete phytoporphyrins (previously referred to as phylloerythrin, a photodynamic breakdown product of chlorophyll normally excreted in bile) > increased blood levels of phytoporphyrins > within the dermis, phytoporphyrins absorb specific wavelengths of light > molecular activation > metastable triplet state > reacts directly with biologic substrate or with molecular oxygen > reactive oxygen intermediates > damages to cell nucleus, cell membrane, and cellular organelles > dermatitis

 

TYPICAL CLINICAL FINDINGS:

• History of exposure to lupines followed by a typically subacute to chronic syndrome
• Anorexia, jaundice, photosensitization
• Stumbling gait, recumbency (from hepatic encephalopathy)
• Elevated GGT, AST, serum/liver copper and iron

 

TYPICAL GROSS FINDINGS: 

• Icterus and ascites 
• Swollen, firm, friable, pale yellow liver in acute cases
• Small fibrotic liver in chronic cases

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Hepatocyte degeneration and single cell death
  • Enlarged hepatocytes with pale granular cytoplasm and vesicular nuclei, or chromatin clumping
• Biliary hyperplasia, ductular reaction, diffuse hepatic fibrosis, +/- nodular regeneration
• Numerous mitoses which are often abnormal
  • Mitotic arrest at late metaphase 
• Lipid type vacuolar change (dependent on the fat reserves of the animal)
• Pigment (lipofuscin, ferric iron, copper) accumulation within macrophages
• Hepatocytes may contain vesicular intranuclear pseudoinclusions

 

DIFFERENTIAL DIAGNOSIS:

• Gross evidence of hepatic necrosis: 
  • Hepatotoxins such as aflatoxin (D-T03), pyrrolizidine alkaloids (D-T04), copper (D-T05), Lantana (D-T12), Xanthium (cocklebur, D-T01), microcystin (blue-green algal toxin, D-T11), Amanita mushrooms

 

COMPARATIVE PATHOLOGY: 

• Cattle: Phomopsin toxicity causes anorexia and ketosis in pregnant or lactating cows; chronically, fibrotic hepatitis with nodular regeneration (cirrhosis) occurs
• Horses: Chronic hepatic fibrosis as in cattle; may have concurrent hemolytic anemia of unknown pathogenesis
• Rats: Phomopsin toxicity produces nodular cirrhosis and biliary hyperplasia; cholangiomas and cholangiocarcinomas/hepatocellular carcinomas are reported

 

REFERENCES: 

1. Cullen JM, Stalker MJ. Liver and Biliary System. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier Ltd; 2016:288, 334-335. 
2. Van Wettere AJ, Brown DL. Hepatobiliary System and Exocrine. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:522. 

 

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