JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
October 2021
D-T08

Signalment (JPC #2077046):  1-year-old breed unspecified dog

HISTORY:  This dog developed severe facial edema, lethargy, and methemoglobinemia following administration of an organic compound.

HISTOPATHOLOGIC DESCRIPTION:  Liver:  Diffusely there is necrosis affecting the centrilobular to midzonal region of hepatic lobules, sparing a thin rim of periportal hepatocytes. In the affected regions, there is moderate to severe disruption of hepatic cord architecture with dissociation of hepatocytes, disorganization of sinusoids, and the presence of hemorrhage, necrotic debris, and rare mineralization.  Within affected areas, many hepatocytes are lost or individualized, and remaining hepatocytes have contracted, rounded, or angular cell margins, hypereosinophilic cytoplasm, and pyknotic, karyorrhectic, or absent nuclei (single cell death).  Other less affected hepatocytes, particularly in periportal areas at the margin of areas of necrosis, are swollen with pale eosinophilic, occasionally microvacuolated cytoplasm and vesiculate nuclei (degeneration).   Multifocally the portal connective tissue is mildly expanded by clear space (edema), dilated lymphatics, and mild hemorrhage and fibrin.

MORPHOLOGIC DIAGNOSIS:  Liver, hepatocytes:  Necrosis and loss, centrilobular to midzonal (submassive), acute, diffuse, severe, with multifocal vacuolar degeneration, breed unspecified, canine.

ETIOLOGY:  Acetaminophen toxicity

ETIOLOGIC DIAGNOSIS:  Toxic hepatic necrosis

GENERAL DISCUSSION:

• Acetaminophen is an intrinsic hepatotoxin, meaning the toxicity is dose dependent, predictable, and reproducible in experimental animals, and the underlying mechanism is at least partially understood
• Widely used synthetic non-opiate analgesic and antipyretic that produces dose dependent hepatotoxicity, methemoglobinemia, and Heinz-body formation in dogs and cats
• Dogs are more susceptible to hepatic necrosis; cats are more susceptible to methemoglobinemia and Heinz-body formation
• Cats are more sensitive to acetaminophen toxicosis because they are deficient in glucuronyltransferase and therefore have limited capacity to glucuronidate this drug; in cats, acetaminophen is primarily metabolized via sulfation; when this pathway is saturated, toxic metabolites are produced
• Cytochrome P450 enzyme (CYP450) concentration is highest in the centrilobular area resulting in more severe centrilobular degeneration and necrosis due to higher concentration of toxic metabolites, which result from the phase I bioactivation reactions involving chemicals and other drugs such as acetaminophen

PATHOGENESIS:

• Normal acetaminophen metabolism: Phase I reaction via CYP450 > toxic metabolite N-acetyl-p-benzoquinone-imine (NAPQI) > Phase II reaction (conjugation) via either:
  • Uridine diphosphate (UDP)-glucuronosyl transferase (glucuronidation) > excreted via bile and urine
  • Phenol sulfotransferase (sulfation) > excreted via bile and urine
  • Glutathione (GSH) (glutathione conjugation) > excreted via bile and urine
• Acetaminophen overdose:
• Phase II enzyme systems become saturated > excess toxic NAPQI
  • Depletion of GSH (one of the Phase II enzymes) > excessive oxidative stress and cell damage
    • Depletion of GSH in erythrocyte membranes > oxidation of iron in hemoglobin and formation of methemoglobin (Fe2+ > Fe3+) > interference with oxygen transport, denatured hemoglobin precipitates on erythrocyte membranes (Heinz bodies), oxidation and cross-linking of membrane proteins causes eccentrocyte formation
  • Hepatic necrosis follows in 24 hours; untreated dogs die within 3 days
• In the kidney:
  • CYP450 is in highest concentration in the renal cortex (especially the proximal tubular epithelial cells), although small amounts are found in the medulla; acute over-doses are biotransformed by cytochrome P450 
  • Prostaglandin synthase, another phase I enzyme, is found primarily in the medullary interstitial and collecting duct cells; chronic acetaminophen nephrotoxicity is attributed to the prostaglandin H synthase biotransformation
• The degree and type of inflammation also plays a role in drug induced hepatic injury

TYPICAL CLINICAL FINDINGS:

• History of exposure
• Clinical signs primarily from methemoglobinemia:
  • Cyanosis, dyspnea, muddy brown mucus membranes
  • Facial edema, depression, dyspnea, hypothermia, vomiting
  • Eccentrocytes, Heinz-body formation > hemolytic anemia
  • Methemoglobinuria (dark brown urine)
• ALT and AST increase up to 1500 fold by 72 hours
• Hyperbilirubinemia, hypoalbuminemia, hypocholesterolemia

TYPICAL GROSS FINDINGS:

• Renal and hepatic hemorrhage and congestion
• Dark chocolate-colored blood
• Pulmonary edema
• Icterus secondary to acute hemolysis in cats or chronic cholestasis in dogs

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Diffuse centrilobular necrosis with hepatocyte vacuolar degeneration
• Bile duct proliferation and bile stasis

DIFFERENTIAL DIAGNOSIS:

• Chronic passive congestion
• Idiosyncratic drug reactions (submassive to massive necrosis): examples include trimethoprim-sulfonamide, zonisamde
• Xylitol (centrilobular to massive necrosis)
• Carprofen (diffuse and massive necrosis)
• Anticonvulsants: primidone, phenytoin, phenobarbital, cause chronic hepatic disease and cirrhosis (bridging portal fibrosis, biliary hyperplasia) in dogs
• Mebendazole (D-T07, centrilobular to massive necrosis)

COMPARATIVE PATHOLOGY:

• Cats are highly susceptible because of deficiency of UDP-glucuronyltransferase > failure to form glucuronide conjugates from bioactivated molecules > limited phase II metabolism, increased depletion of glutathione
  • Cats are also predisposed to methemoglobin formation (oxidative injury) because of high number of sulfhydryl groups on their hemoglobin molecules, compared to other species; NAPQI results in methemoglobinemia in cats exposed to acetaminophen
• Ferrets have a similar predisposition as cats

REFERENCES:

1. Allen AL. The diagnosis of acetaminophen toxicosis in a cat. Can Vet J. 2003;44(6):509-510.
2. Bischoff K. Toxicity of over-the-counter drugs. In: Gupta RC, ed. Veterinary toxicology basic and clinical principles. 1^st San Diego, CA: Elsevier; 2007:363-368. 
3. Court MH. Acetaminophen UDP-glucuronosyltransferase in ferrets: species and gender differences, and sequence analysis of ferret UGT1A6. J Vet Pharmacol Ther. 2001;24:415.
4. 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: 325-328.
5. Guilford WG, Center SA, Strombeck DR, Williams DA, Meyer DJ. Strombeck’s Small Animal Gastroenterology. 3rd ed. Philadelphia, PA: WB Saunders; 1996:688-691.
6. Kumar V, Abbas AK, Aster JC. Environmental and nutritional diseases. In: Kumar V, Abbas AK, Aster JC, eds. Robbins and Cotran Pathologic Basis of Disease. 10th ed. Philadelphia, PA: Saunders Elsevier; 2020:422-423.
7. Osweiler GD. The National Veterinary Medical Series for Independent study: Toxicology. Philadelphia, PA: Williams and Wilkins; 1996:303-304.
8. Parkinson A. Biotransformation of xenobiotics. In: Klaassen CD, ed. Casarett and Doull’s Toxicology: The Basic Science of Poisons. 5^th New York, NY: McGraw-Hill; 1996:114.
9. Stewart JE, Haslam AK, Puig J. Pathology in Practice. Acetaminophen (paracetamol) toxicosis. J Am Vet Med Assoc. 2016;248(9):1009-11.
10. Taylor NS, Dhupa N. Acetaminophen toxicity in cats and dogs. Compend Contin Educ Pract Vet. 2000;22:160-169.
11. Wancket LM, Meng X, Rogers LK. Mitogen-activated protein kinase phosphatase (Mkp)-1 protects mice against acetaminophen-induced hepatic injury.  Toxicol Pathol.  2012;40(8):1095-1105.

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