JPC SYSTEMIC PATHOLOGY

URINARY SYSTEM

January 2024

U-T12

 

Signalment (JPC#1901183): 12-year-old, male horse, breed unspecified.

 

HISTORY: This horse was one of three pastured horses observed eating large quantities of wilted leaves from a fallen red maple tree (Acer rubrum). Clinically, the horses were depressed, lethargic, anorectic, cyanotic, and icteric.  

 

HISTOPATHOLOGIC DESCRIPTION: Kidney: Diffusely within the cortex and extending into the medulla, tubules exhibit one or more of the following changes: ectatic, lined by attenuated epithelium, contain brightly eosinophilic to red, hyaline to granular material with occasional crystalline structures (hemoglobin or hyaline cast) or eosinophilic homogenous material (proteinosis) occasionally admixed with sloughed epithelial cells and necrotic cellular debris (granular casts); tubular epithelial cells are either swollen with cytoplasmic vacuolization and large, vesiculate nuclei (degenerate) or are shrunken and hypereosinophilic with pyknosis (necrotic); and/or the tubular basement membrane is fragmented (tubulorrhexis). Multifocally, glomerular uriniferous spaces are distended with proteinaceous fluid, and there is multifocal periglomerular fibrosis. Multifocally, there are few neutrophils within tubules and the surrounding interstitium. There is multifocal thickening of the basement membranes of tubules and Bowman’s capsule.

 

MORPHOLOGIC DIAGNOSIS: Kidney: Tubular degeneration and necrosis, subacute, diffuse, marked, with tubulorrhexis and hemoglobin, hyaline, and granular casts, breed not specified, equine.

 

ETIOLOGIC DIAGNOSIS: Hemoglobinuric nephrosis

 

CAUSE: Red maple leaf (Acer rubrum) toxicosis

 

GENERAL DISCUSSION: 

• Red maple (Acer rubrum) is a deciduous tree native to eastern North America
• Ingestion of wilted leaves results in methemoglobinemia and Heinz body hemolytic anemia (acute intravascular hemolysis), leading to hemoglobinuric and hypoxic acute tubular injury (not directly nephrotoxic)

 

PATHOGENESIS:

• Hemoglobinuric acute tubular injury – mix of direct cytotoxicity by hemoglobin and ischemia (e.g. reduction in renal blood flow and anemia) 
• Heinz body anemia - Gallic acid from wilted or dried leaves and bark of A. rubrum à oxidative injury to erythrocytes (hemoglobin oxidizes à denatures/conformation change à membrane bound precipitants [Heinz bodies]) à affected erythrocytes have reduced deformability and are removed by the spleen (extravascular hemolysis) OR may lyse due to increased fragility (intravascular hemolysis à hemoglobinemia) 
• Oxidizing the heme iron in hemoglobin to ferric (Fe3+) iron > generates methemoglobin (incapable of transporting oxygen)

 

TYPICAL CLINICAL FINDINGS:

• Methemoglobinemia > tissue anoxia and sudden death
• Heinz body formation à intra- and extravascular hemolysis à weakness, icterus, etc 
• Clinical pathology: 
  1. Anemia with polychromasia and reticulocytosis 
  2. Heinz bodies, eccentrocytes
  3. Hyperbilirubinemia and bilirubinuria 
  4. Hemoglobinemia and hemoglobinuria 
  5. Methemoglobinemia and methemoglobinuria 

 

TYPICAL GROSS FINDINGS: 

• Yellow (icteric) to brownish (hemoglobin/methemoglobinemia) discoloration of tissues and blood, including kidneys
• Cyanotic mucous membranes
• Splenomegaly

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Kidney: Varying degrees of acute tubular injury +/- tubulorrhexis, hemoglobin casts (orange-red granular refractile material)

• Liver: Cholestasis with bile plugs
• Spleen: Erythrophagocytosis 

 

ADDITIONAL DIAGNOSTIC TESTS:

• Heinz bodies – Slightly pale, rounded, protruding structure that creates a membrane defect or occurs free in the blood; best visualized with new methylene blue (stains pale blue)
• Differentiating red to brown pigmenturia:
  1. Hematuria – Color clears with centrifugation and erythrocytes are present in sediment
  2. Hemoglobinuria – Color does not clear with centrifugation, no erythrocytes in sediment; positive heme (occult blood) reaction; color clears with 80% ammonium sulfate (unreliable); plasma pink to red
  3. Myoglobinuria – Color does not clear with centrifugation, no erythrocytes in sediment; positive heme (occult blood) reaction; color clears with 100% ammonium sulfate (unreliable); plasma clear 
  4. Methemoglobinuria – positive heme (occult blood) reaction
  5. Horses may have artifactual (non-pathologic) pigmenturia when urine is stored or exposed to snow due to pyrocatechin/pyrocatechol normally present in their urine

 

 

DIFFERENTIAL DIAGNOSIS:

• Nephrotoxic acute tubular injury is difficult to differentiate from ischemic acute tubular injury, but nephrotoxicity often doesn’t have tubulorrhexis 
• Causes of hemolytic anemia in the horse: 

1. Red Maple (Acer rubrum): Intravascular hemolysis
2. Pistacia spp.: Intravascular hemolysis
3. Babesia caballi; B. equi: Intravascular hemolysis
4. Leptospira bratislava: Intravascular hemolysis
5. Wild onion (Allium sp.): Intravascular hemolysis
6. Immune-mediated hemolytic anemia: Intravascular hemolysis
7. Phenothiazine: Intravascular hemolysis
8. Neonatal isoerythrolysis: Intravascular hemolysis
9. Anaplasma phagocytophilum: Extravascular hemolysis
10. Equine Infectious Anemia (equine Lentivirus): Extravascular hemolysis

 

COMPARATIVE PATHOLOGY:

Other oxidative agents (Heinz body anemia): 

• Allium spp. (Garlic, onions, chives) – all species
• Brassica spp (rape, kale, turnips) – cattle, other ruminants
• Chronic copper toxicosis – sheep, ruminants 
• Zinc toxicity – dogs
• Methylene blue – cats (used as a urinary acidifier)
• Selenium deficiency (Se required for glutathione peroxidase, a catalyst for GSH)
• Propofol – dogs, cats 
• Acetaminophen – dogs, cats 
• Propylene glycol – cats 

 

References:

1. Cianciolo RE, Mohr FC. Urinary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 422, 424, 429.
2. Duncan, M. Perissodactyls. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier. 2018:438.
3. Durham AC, Boes KM. The urinary system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:711, 735, 831.
4. Stockham SL, Scott MA. Fundamentals of Veterinary Clinical Pathology. 2nd ed. Ames, IA: Blackwell Publishing; 2008: 140-141, 186-187, 441, 465-466. 
5. Sula MM, Lane LV. The urinary system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:735
6. Uzal FA, Plattner BL, Mostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 271.
7. 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; 2016: 125-126. 
8. Zabolotzky SM, Walker DB. Peripheral Blood Smears. In: Valenciano AC, Cowell RL, ed. Cowell and Tyler’s Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier; 2020: 449-450. 

 

 

 

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