Hemolytic anemia is a pathologic condition that results from accelerated erythrocyte removal and can be intravascular and extravascular. Intravascular hemolysis occurs when erythrocytes are destroyed within the vascular space. Clinical signs associated with intravascular hemolysis are typically acute in onset and classically include icterus and red- to port-wine-colored urine. Extravascular hemolysis results from accelerated erythrocyte removal by macrophages in the spleen or liver and is characterized by icterus without hemoglobinuria.
Diseases That Primarily Cause Intravascular Hemolysis
Oxidative Erythrocyte Damage
Oxidant damage to erythrocytes can develop following exposure to a variety of oxidizing agents such as phenothiazines, onions, or wilted red maple (Acer rubrum) leaves. The latter cause is by far the most common. The oxidizing agent causes hemoglobin to become denatured with subsequent disulfide bond formation. Oxidized hemoglobin forms precipitates referred to as Heinz bodies that are visible with Romanowsky’s-stained blood smears. These cell changes result in increased fragility of cells with subsequent intravascular hemolysis and enhanced removal by the mononuclear phagocytic system in the spleen and liver. In addition, oxidative damage causes increased permeability of the membrane, thereby altering ion transport mechanisms and osmotic gradients. Due to these changes, erythrocytes may rupture within the vascular lumen, thus resulting in intravascular hemolysis and microangiopathy. In contrast to oxidative damage that leads to erythrocyte destruction, altered oxygen carrying capacity results when hemoglobin is oxidized and forms methemoglobin. Methemoglobinemia occurs when more than 1.77% of the hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state by oxidizing agents.
Because A. rubrum is a common tree in the eastern United States, toxicity associated with red maple leaf ingestion is a fairly common clinical disease in that region. Although the specific toxic agent in the plant has not been identified, a seasonal trend appears to exist for the development of disease because more cases are associated with ingestion of wilted or dried leaves in the fall than for leaves ingested in the spring.
Clinical Signs and Diagnosis
In cases of red maple toxicity, a combination of intravascular and extravascular hemolysis develops over a variable period of 2 to 6 days. In severe cases, clinical signs of hemolytic anemia and tissue hypoxia secondary to methemoglobinemia may develop more rapidly. The prognosis for horses suffering from red maple toxicity is guarded; the approximate survival rate is 60% to 70%. Clinical signs result from the combined effects of tissue hypoxia and hemolysis that results in fever, tachycardia, tachypnea, lethargy, intense icterus, and hemoglobinuria, with characteristic brown coloration of skin, mucous membranes, and — if methemoglobinemia is present — blood. Hematologic abnormalities include anemia, increased mean corpuscular hemoglobin concentration and mean corpuscular hemoglobin, free plasma hemoglobin, anisocytosis, poikilocytosis, eccentrocytes, lysed erythrocytes that produce fragments or membrane ghosts, agglutination, increased red blood cell fragility, variable presence of Heinz bodies, and neutrophilia. Serum chemistry abnormalities include increased total and indirect bilirubin, serum creatinine, and serum urea nitrogen concentrations, reduced red blood cell glutathione and increased aspartate aminotransferase, sorbitol dehydrogenase, creatinine phosphokinase, and gamma-glutamyl transpeptidase activities.
Additional abnormalities may include hypercalcemia and hyperglycemia, with a variable degree of metabolic acidosis. Urinalysis findings could include any combination of the following: hemoglobinuria, methemoglobinuria, proteinuria, bilirubinuria, and urobilinogenuria. Methemoglobin can be quantified spectrophotometrically. Red maple leaf, wild onion, or phenothiazine toxicosis would be strongly suspected if a history of exposure or opportunity for exposure exists. Diagnosis is based on clinical signs of an acute onset primarily of intravascular hemolytic crisis supported by laboratory evidence of oxidative damage — that is, Heinz bodies or methemoglobinemia. Additional differential diagnoses for methemoglobinemia should include familial methemoglobinemia and nitrate toxicity, both of which are exceptionally rare in the horse.
Treatment for oxidative injury involves reducing the fragility of erythrocytes, maximizing tissue oxygenation, maintaining renal perfusion, and providing supportive care. The horse needs to be removed from the environmental source of the toxin and treated with activated charcoal (8-24 mg/kg up to 2.2 kg PO) via nasogastric tube to reduce further absorption of red maple toxin. Dexamethasone (0.05-0.1 mg/kg IV ql2-24h) may be helpful to stabilize cellular membranes and reduce extravascular removal of erythrocytes by phagocytes. Ascorbic acid (10-20 g PO q24h) is often used to maintain cellular α-tocopherol in the reduced form and as a scavenger of free radicals. No data support the use of methylene blue as a reducing agent in horses; in fact, it may exacerbate oxida-tive damage. Oxygen insufflation may be required for horses that suffer from severe hypoxia. Whole blood transfusion from a compatible donor should be considered in those horses that demonstrate severe hemolytic disease. These signs include a severe reduction in venous oxygen tension (PVo2) and increased anion gap or packed cell volume (packed cell volume) 10% to 12% with evidence of cardiovascular and respiratory distress. In cases of severe hemolysis, pigmentary nephropathy — induced by the combination of excess filtration of hemoglobulin and hypoxemia — may be a complication. Renal function should therefore be monitored during the course of treatment. Intravenous fluids are administered as needed, but hemodilution in anemic patients should be avoided. If acute renal failure develops, diuretic agents such as dopamine, furosemide, or mannitol may be indicated. In high-risk patients, drug therapy should be designed to avoid additional potentially nephrotoxic agents such as aminoglycosides and non-steroidal antiinflammatory drug (NSAID) agents.
Complications are a major component of morbidity following severe hemolysis. Poor tissue oxygenation may lead to cerebral anoxia and altered mentation, renal failure, and myocarditis. Blood transfusion may result in colic secondary to reperfusion of hypoperfused bowel. Laminitis is always a concern in horses with severe illness and may be compounded by the use of corticosteroids. Disseminated intravascular coagulopathy may develop secondary to severe hemolysis.
In conclusion, horses should not be housed in areas with access to red maple trees or other potential oxi-dants. Good quality forage should be available at all times to reduce the likelihood of horses ingesting leaves as they fall or blow into pastures. Other maple varieties of trees should be considered potentially dangerous; reports have suggested clinical signs consistent with red maple toxicity when affected animals were exposed to red maple hybrids.
Additional Causes of Intravascular Hemolysis
Microangiopathic hemolysis may be associated with vessel thrombosis. Hemolysis is secondary to intravascular fibrin accumulation and may be characterized by the presence of schizocytes. This is a potential complication that is characteristic of chronic disseminated intravascular coagulation (DIC) in horses. Fulminant liver failure also carries the potential complication of hemolysis. Clinical evidence is consistent with intravascular hemolysis that includes icterus and hemoglobinuria. The severity of hemolysis contributes to mortality in most cases. Lesions at necropsy are consistent with DIC. It has been proposed that alterations in exchangeable red blood cell lipoprotein are affected by increased bile acids, thus contributing to altered metabolism of red cells resulting in hemolysis.
Toxin exposure may result in acute intravascular hemolysis. Snakebites that result in envenomation with potential hemolysis include: rattlesnakes (Crotalus spp.), pigmy rattlesnakes (Sistrurus spp.), copperhead (Agk-istrodon spp.), cottonmouth (Agkistrodon piscivorus), or water moccasins. Snake toxins comprise more than 90% proteins that include proteolytic and phospholipase enzymes. These toxins are well known to induce episodes of severe hemolysis and altered coagulation mechanisms. Bacterial exotoxins produced from Clostridium spp. and some staphylococcal pathogens carry the potential of inducing severe intravascular hemolysis, which is especially apparent in septic neonatal foals. Although rare in horses, infection by Leptospira pomona and Leptospira icterohaemorrhagiae serotypes have been reported to cause acute intravascular hemolysis in several large animal species. Intravenous iatrogenic administration of hypotonic fluids or undiluted dimethyl sulfoxide (DMSO) or excessive administration of water enemas to neonates may result in hemolysis. Although rare, heavy metal intoxication may also cause hemolysis.