SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
October 2021
D-T11

Signalment (JPC #2414736):  1-year-old Angus/Hereford cross cow

HISTORY:  This cow was one of a group of 17 found in sternal recumbency.  This cow died several hours after having been found.  The pasture in which the cow had been kept bordered a glacial lake that was covered by a thick layer of green algae the day before the heifer was discovered.

HISTOPATHOLOGIC DESCRIPTION:  Liver:  Diffusely there is centrilobular to midzonal (submassive) to rarely periportal (massive) hepatocellular degeneration, necrosis, and loss characterized by dissociation of hepatic cords with replacement by abundant hemorrhage, fibrin, and edema admixed with eosinophilic cellular and karryorhectic cellular debris, occasional necrotic neutrophils, and Kupffer cells.  Remaining centrilobular to midzonal hepatocytes are often individualized and rounded with hypereosinophilic cytoplasm and nuclear pyknosis, karyorrhexis, or karyolysis (single cell death).  Less affected periportal hepatocytes are often swollen with vacuolated cytoplasm and large, vesiculate nuclei (degeneration).

MORPHOLOGIC DIAGNOSIS:  Liver, hepatocytes:  Necrosis and loss, acute, centrilobular to midzonal (submassive), diffuse, severe, with hemorrhage, Angus/Hereford cross, bovine.

ETIOLOGIC DIAGNOSIS:  Microcystin-LR hepatic toxicosis

CAUSE:  Microcystin-LR toxin (produced by Microcystis aeruginosa)

GENERAL DISCUSSION:

• Cyanobacteria, or blue green algae, are classified in the phylum Monera, division Cyanophyta; closely related to bacteria and no longer considered members of the plant family
• Blue green algae bloom in warm, stagnant water that contain a high nutrient content (i.e. fertilizer run-off and animal waste)
• Four genera of cyanobacteria have been associated with toxic water blooms on freshwater systems: Microcystis, Nodularia, Anabena and Aphanizomenon
  • Microcystis:
    • Often concentrate in areas by wind or water currents and form a blue-green “scum”; prevalence is usually greatest in July and August
    • Microcystis induce a lethal hepatoxicosis of ruminants; toxicity has been reported in sheep, horses, domestic poultry, dogs and cats
  • Toxic effects vary depending on the type and amount of toxin ingested; microcystins are about 10 times more toxic than strychnine
  • Microcystis aeroginosa contains a preformed toxin (microcystin LR) - a potent cyclic heptapeptide protein phosphate inhibitor which causes necrosis of sinusoidal lining cells (endothelium) and variable centrilobular to massive necrosis
  • Other species found in New Zealand and the Baltic region can contain a toxin called nodularin
  • Some blooms contain “fast-death factor”; causes sudden extreme weakness, collapse and death with no lesions
  • Cyanobacteria also produce neurotoxins (anatoxin A) and dermatotoxins

PATHOGENESIS:

• Toxins are liberated when microorganisms disintegrate in water after application of copper sulfate for algae control or within the rumen or stomach after ingestion
• Microcystin toxin cannot cross cell membranes and requires bile acid carrier system; a similar carrier is present in renal tubular epithelium
• Ingestion by animal > microcystin transported into hepatocytes by membrane carrier organic anion-transporting polypeptide (OATP) > toxin inhibits cytoplasmic protein phosphatases 1 and 2A > rapid hyperphosphorylation of cytoskeletal proteins > disorganization of hepatocyte and endothelial cytoskeletal actin filaments (disruption of endothelial shape and integrity) > necrosis and apoptosis of sinusoidal endothelial cells > perisinusoidal hemorrhage and necrosis
• Death due to hypovolemic shock and hepatic insufficiency, usually within a few hours post exposure

TYPICAL CLINICAL FINDINGS:

• History of exposure
• Diarrhea, vomiting, weakness, pale mucous membranes and shock
• Animals that survive longer than a few hours develop hyperkalemia, hypoglycemia, nervousness, recumbency, and convulsions
• Anuria (due to renal tubular injury)
• Animals that survive acute intoxication may develop hepatogenous (type III) photosensitization and/or chronic liver disease
• Anatoxin > seizures, muscle fasciculation

TYPICAL GROSS FINDINGS:

• Acute: Red, hemorrhagic liver; hepatomegaly; hemorrhagic gastroenteritis
• Subacute: Fatty liver

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Liver:
  • Variable pattern of necrosis; typically centrilobular to massive necrosis
  • Characteristic centrilobular hepatocyte dissociation, rounding, and shrinkage
  • Bile pigments accumulate in the cytoplasm
  • Slight biliary proliferation and fibrosis
  • Subacute cases: Severely fatty liver with individualized necrotic hepatocytes 
• Kidney: Toxic tubulonephrosis with retention of basement membranes

ULTRASTRUCTURAL FINDINGS:

• Condensation and margination of hepatocellular chromatin; loss of cell-cell contact; loss of microvilli in space of Disse
• Pyknosis, karyorrhexis, and apoptotic bodies

ADDITIONAL DIAGNOSTIC TESTS:

• Algal organism or toxin identification in samples of water, gastric content/vomitus, or liver
• Toxin detection using high performance liquid chromatography-mass spectrometry

DIFFERENTIAL DIAGNOSIS:

• Necrosis of sinusoidal lining cells: Ngaione toxicity; ingestion of furano-sesquiterpenoid oils from Mycoporum leaves
• Centrilobular hepatic necrosis:
  • Pyrrolizidine alkaloid-containing plants (D-T04, e.g. Senecio, Heliotropium spp. Crotalaria spp.): Centrilobular to massive necrosis with megalocytosis, biliary hyperplasia and fibrosis
  • Aflatoxins (D-T03): Centrilobular hepatocytes replaced by inflammatory cells, fibroblasts and primitive vascular channels to centrilobular necrosis; hepatocellular steatosis
  • Cycad (Cycas or Zamiaceae) toxicosis: Centrilobular necrosis with megalocytosis, nuclear hyperchromasia, cholestasis, fatty change, variable fibrosis; acute renal tubular injury
    • Cestrum toxicosis: Marked centrilobular and midzonal coagulative necrosis and hemorrhage
    • Trema (poison peach) toxicosis: Centrilobular necrosis, coagulative necrosis and hemorrhage
  • Gossypol toxicity (D-T02, Gossypium): Centrilobular to submassive necrosis, often with cardiomegaly, myocardial necrosis and segmental rhabdomyositis
  • Cresol poisoning: Multifocal centrilobular necrosis less severe than gossypol
• Lantana camara (D-T12): Megalocytosis, canalicular cholestasis, focal hepatocellular necrosis; necrosis of renal tubular epithelium; photosensitization
• Mycotoxins
  • Phomopsin (D-T06), produced by Phomopsis leptostromiformis: Diffuse hepatic fibrosis and biliary hyperplasia; photosensitization
  • Sporidesmin (D-T13), produced by Pithomyces chartarum: Severe periportal fibrosis with obliteration of bile ducts; photosensitization

COMPARATIVE PATHOLOGY:

• Dogs- Aflatoxin, mebendazole, xylitol, carprofen, Cycas plants and amanita:  Centrilobular to massive hepatic necrosis
• Dogs and cats- Acetaminophen toxicity:  Necrosis of sinusoidal lining cells and centrilobular to submassive pattern of necrosis
• Pigs- Iron dextran and Vit E / Selenium deficiency (hepatosis dietetica):  Massive hepatic necrosis
• Blue-green algae have become popular as a health food supplement; continuous intake of toxins may cause tumor promotion

REFERENCES:

1. Brown DL, Van Wettere AJ, Cullen JM. Hepatobiliary system and exocrine pancreas. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:449.
2. Cantor GH, Beckonertz O, Bollard ME, et. al. Integrated histopathological and urinary  metabonomic investigation of the pathogenesis of Microcystin-LR toxicosis.Vet Pathol. 2013;50(1):59-71.
3. 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; 2016: 285, 330.
4. Puschner B, Hoff B, Tor E. Diagnosis of anatoxin-A poisoning in dogs from North America. J Vet Diagn Invest 2008;20:89-92.
5. Radostits OM, Gay CC, Blood DC, Hinchcliff KW. Veterinary Medicine. 9th ed. New York NY: Saunders;2000:1701-1703.
6. Simola O, Wiberg M, Jokela J, Wahlsten M, Sivonen K, Syrjä P. Pathologic findings and toxin identification in cyanobacterial (Nodularia spumigena) intoxication in a dog. Vet Pathol 2012;49: 755-59.

Click the slide to view.

---