Erythrocytosis in Horses 

• Definition: An increase in the red cell concentration, hemoglobin concentration, and/or calculated hematocrit or PCV.

• Clinical Signs and Physical Examination Findings:

  • Lethargy

  • Weight loss

  • Dark blood

  • Deep red to purple mucous membranes

  • Hypertension

  • Prolonged capillary refill time

  • Tachycardia

  • Tachypnea

  • Cyanosis

  • Episodic hemorrhage or thrombosis

  • Loss of performance (illustrated by a case example)

  • Colic (illustrated by a case example during exercise)

• Types of Erythrocytosis:

  • Relative Erythrocytosis:

    • Not attributed to a true increase in the body's overall red cell mass.

    • Associated with a relative increase in red cells due to loss of plasma volume or redistribution of red cells.

    • More common than absolute erythrocytosis.

    • Causes:

      • Dehydration: Reduced proportion of plasma due to water loss results in a relative increase in red cells.

        • Treatment involves correcting dehydration and underlying conditions causing water loss or deprivation.

        • Erythrocytosis is expected to resolve upon rehydration.

      • Splenic Contraction: Release of a large number of red cells sequestered in the equine spleen.

        • Can be released when the animal is excited, fearful, in pain, or exercising.

        • Can substantially increase hematocrit (approximately 40% above baseline on average).

        • PCV may remain within or increase above the reference interval depending on the original concentration.

        • Not expected to be associated with concurrent panhyperproteinemia or clinical signs of dehydration.

        • Laboratory changes associated with catecholamine release may occur, such as mild to moderate hyperglycemia, mild mature neutrophilia and lymphocytosis, and/or mild to moderate thrombocytosis.

        • Effects are transient; PCV and other CBC changes should return to baseline within 40–60 minutes.

  • Absolute Erythrocytosis:

    • Due to a true increase in the body's red cell mass from increased erythropoiesis.

    • Not due to reduced plasma volume or redistribution of red cells.

    • Uncommon in horses.

    • Primary Absolute Erythrocytosis:

      • Due to increased, unregulated, neoplastic, autonomous production of red cells.

      • Considered a diagnosis of exclusion for animals with persistent erythrocytosis without evidence of dehydration, splenic contraction, or identifiable secondary causes.

      • In humans, dogs, and cats, serum erythropoietin concentration is typically within the reference interval or decreased.

      • A mutation in the Janus kinase 2 (JAK2) gene has been described in humans and dogs.

      • Very rare diagnosis in horses, only described twice in literature as isolated case reports.

    • Secondary Absolute Erythrocytosis:

      • Due to increased synthesis or activity of erythropoietin, the main hormone stimulating erythropoiesis.

      • Can be further categorized as “appropriate” or “inappropriate”.

      • In humans, diagnosis and treatment are directed to identifying and treating underlying causes.

      • Appropriate Secondary Absolute Erythrocytosis:

        • Occurs when there is increased release of erythropoietin in an attempt to improve the blood’s oxygen-carrying capacity.

        • The increased production of erythropoietin is considered an appropriate response to poor tissue oxygenation (hypoxemia).

        • May occur with cardiac or respiratory disease, high altitudes, or hemoglobin disorders.

        • Hypoxemia with PaO2 < 80-mmHg and O2 saturation < 92% is sufficient to induce an erythrocytosis.

        • In horses, most often connected with congenital cardiac defects associated with right to left shunting or lung disease (including chronic pneumonia or pleuropneumonia).

        • Most equine lung conditions are not severe enough to cause hypoxemia sufficient to result in an erythrocytosis.

      • Inappropriate Secondary Absolute Erythrocytosis:

        • Typically a paraneoplastic finding attributed to autonomous synthesis of erythropoietin or prostaglandins enhancing erythropoietin action by a neoplasm.

        • Can also be a response to renal hypoxia.

        • In horses, reported associated neoplasms include hepatoblastoma, lymphoma, hepatocellular carcinoma, and a metastatic carcinoma.

        • Administration of erythropoietin or erythropoietin-like compounds can also result in a secondary inappropriate erythrocytosis.

          • Usage of these agents is prohibited by authorities like the Horseracing Integrity and Safety Authority and Association of Racing Commissioners International, but they are still illegally and unethically used.

• Implications:

  • While an erythrocytosis should theoretically enhance oxygen-carrying capacity, in practice, it can increase blood viscosity such that it negates any improvements to oxygen-carrying capacity.