Erythrocytosis

Erythrocytosis (polycythemia) refers to an increased RBC count, Hgb concentration, and HCT. Erythrocytosis can be either relative or absolute in nature (Box 24.3). In general, relative erythrocytosis is far more common than absolute and these causes should be considered or ruled out first.

■ BOX 24.3

Causes of Erythrocytosis in Large Animals Relative Erythrocytosis

Dehydration

Endotoxic shock

Splenic contraction

Absolute Erythrocytosis

Primary

Primary erythrocytosis

Secondary Appropriate

Chronic pulmonary disease Right-to-left cardiovascular shunting Residence at high altitude

Secondary Inappropriate Erythropoietin-secreting neoplasms

Renal pathology causing local hypoxia

increased plasma protein concentration and clinical evidence of dehydration (e.g., skin tent, tacky mucous membranes, sunken eyes) would be supportive. Epinephrine-mediated splenic contraction as the result of excitement, pain, fright, or vigorous exercise can also cause a relative erythrocytosis as a result of redistribution of relatively high HCT splenic blood into the peripheral circulation. This is most often seen in the horse. Plasma protein concentration is not expected to increase with splenic contraction as compared with dehydration.

Absolute erythrocytosis occurs when there is a true increase in total erythrocyte mass as a result of increased erythropoi­esis and can be primary or secondary in nature. Absolute erythrocytosis is considered secondary when it is in response to increased erythropoietin (EPO) levels or activity. This can be an appropriate physiologic response to chronic hypoxemia (e.g., chronic pulmonary disease, right-to-left cardiovascular shunting, or animals living at high altitude); however, tumor production of EPO or EPO-like substances or renal pathol­ogy causing local tissue hypoxia can result in a secondary erythrocytosis in the absence of systemic hypoxemia. Primary absolute erythrocytosis is considered a chronic myeloprolif­erative disease (primary erythrocytosis or polycythemia vera) where erythropoiesis occurs independent of EPO. There are rare reports of primary erythrocytosis in the horse.¹⁶ Absolute erythrocytosis of unknown cause has also been described in a herd of Jersey calves and in a Japanese black heifer.¹⁷

Clinical evaluation of animals with an erythrocytosis should first begin by considering causes of relative erythrocytosis because these are much more common. If dehydration, shock, and splenic contraction are ruled out and if the increased HCT persists over time without response to fluid therapy, then the erythrocytosis may be considered absolute. Arterial blood gas, thoracic radiographs, and echocardiography can help to identify decreased cardiorespiratory function and systemic hypoxia as the cause. Neoplasia (e.g., lymphoma, hepatoblastoma, hepatocel­lular carcinoma) and renal disease should also be ruled out before considering the erythrocytosis primary in nature.^18-21 Bone marrow aspiration and biopsy are generally not helpful in differentiating causes of absolute erythrocytosis (i.e., primary vs. secondary) because erythroid hyperplasia would be expected in both cases. Serum EPO concentrations would be expected to be normal to low in primary absolute erythrocytosis and increased in secondary absolute erythrocytosis. This assay, however, is not widely available, and there is likely some overlap in EPO levels among patients with primary and secondary absolute erythrocytosis, making interpretation difficult in some cases.