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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • Results from the ingestion of wilted or dried Acer rubrum (red maple) leaves
  • Most frequently reported in the eastern half of North America
  • The specific toxin has not been identified. Pyrogallol and gallic acid, metabolites formed from intestinal bacterial metabolism of tannic acid found in wilted or dry leaves, have been suggested
  • Clinical findings are consistent with oxidative injury to red blood cells, resulting in the formation of methemoglobin, Heinz bodies, and hemolytic anemia
  • Affected organ systems include:
    • Cardiovascular—tachycardia secondary to anemia
    • Hemic—methemoglobinemia, hemolytic anemia, and Heinz bodies
    • Renal—pigmenturia, hematuria, and proteinuria; renal failure secondary to hemoglobin deposition in the kidney
    • Reproductive—abortion secondary to fetal hypoxia
    • Respiratory—polypnea secondary to anemia

Signalment!!navigator!!

No breed, gender, or age predilections.

Signs!!navigator!!

  • Acute death can result from rapid formation of methemoglobin. Alternatively, hemolytic crisis can develop over several days as the hemolysis and methemoglobinemia progressively worsen
  • Lethargy, weakness, anorexia, and perhaps colic or fever. Yellow or brown mucous membranes, red or brown urine, tachycardia, polypnea, and dehydration

Causes and Risk Factors!!navigator!!

In summer and fall months after an event that results in leaf wilting such as tree pruning, fallen branches after a storm, or autumn leaves falling.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • All causes of hemolytic anemia
  • Hemolytic anemia accompanied by Heinz bodies and/or methemoglobinemia indicates oxidant toxicosis. The most common causes in horses are onions, red maple, and phenothiazine anthelmintics, which can only be differentiated by a history of ingestion

CBC/Biochemistry/Urinalysis!!navigator!!

  • Interpretation may be difficult due to hemolysis
  • Decreased packed cell volume, hemoglobin, and erythrocyte count confirm anemia, whereas increased mean corpuscular hemoglobin concentration and mean corpuscular hemoglobin support intravascular hemolysis with hemoglobinemia
  • Heinz bodies are not present in all cases. They may be seen on routinely stained blood smears but are more apparent in new methylene blue-stained smears
  • Serum bilirubin, especially unconjugated bilirubin, is increased because of hemolytic anemia and inappetence
  • Urinalysis results include proteinuria and hemoglobinuria, with few or no intact erythrocytes
  • Increased albumin and total protein result from dehydration
  • Blood urea nitrogen and creatinine increase if a pigment nephropathy develops and causes acute renal failure
  • Elevated liver enzymes and creatine phosphokinase may occur, probably secondary to cell damage caused by anemia-induced hypoxia
  • Eccentrocytes and ghost cells have been reported

Other Laboratory Tests!!navigator!!

The percentage of methemoglobin in the blood often is elevated.

Pathologic Findings!!navigator!!

  • Gross findings include generalized icterus, enlarged spleen, and discolored kidneys. Petechiae and ecchymoses may be present
  • Histopathologic findings include erythrophagocytosis by macrophages, renal pigment casts and sloughed epithelial cells, splenic and hepatic hemosiderin, and centrilobular hepatic lipidosis. Pulmonary thrombosis possible

Treatment ⬆ ⬇

TREATMENT

  • Inpatient or outpatient according to severity
  • IV fluids to maintain renal perfusion
  • Blood transfusion with severe anemia
  • Continuous nasal oxygen administration may be helpful

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Ascorbic acid has been used for its antioxidant effects (30–50 mg/kg every 12 h added to IV fluids).

Contraindications/Possible Interactions!!navigator!!

  • Do not treat methemoglobinemia with methylene blue because of its poor efficacy and it may increase Heinz body formation
  • NSAIDs to control pain but can compromise renal function

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Monitor methemoglobinemia and anemia to adjust therapy.

Prevention/Avoidance!!navigator!!

Remove fallen branches immediately after storms or pruning.

Expected Course and Prognosis!!navigator!!

  • Prognosis depends on the quantity of leaves ingested
  • Death is attributed to severe methemoglobinemia, anemia, or to renal failure

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Laminitis can occur during or after the course of the disease.

Pregnancy/Fertility/Breeding!!navigator!!

Anemia and methemoglobinemia can result in fetal hypoxia, followed by abortion.

Abbreviations!!navigator!!

NSAID = nonsteroidal anti-inflammatory drug

Suggested Reading ⬆ ⬇

Agrawal K, Ebel JG, Altier C, Bischoff K. Identification of prototoxins and a microbial basis for red maple (Acer Rubrum) toxicosis in equines. J Vet Diagn Invest 2013;25(1);112–119.

Alward A, Corriher CA, Barton MH, et al. Red maple (Acer rubrum) leaf toxicosis in horses: a retrospective study of 32 cases. J Vet Intern Med 2006;20;1197–1201.

Burrows GE, Tyrl RJ. Sapindaceae juss. In: Burrows GE, ed. Toxic Plants of North America, 2e. Ames, IA: Wiley, 2013:1110–1124.

Author(s) ⬆ ⬇

Author: Scott L. Radke

Consulting Editors: Wilson K. Rumbeiha and Steve Ensley

Acknowledgment: The author and editors acknowledge the prior contribution of Konnie H. Plumlee.

Additional Further Reading ⬆

Click here for Additional Further Reading