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ERYTHROCYTOSIS is an increase in red blood cell (RBC) count. Relative erythrocytosis, as caused by dehydration-related haemoconcentration or splenic contraction, is more common (Watson 2000) than absolute erythrocytosis, in which there is an actual increase in the number of RBCs. Absolute erythrocytosis can be primary or secondary (Lording 2008) and is uncommon in horses (Muñoz and others 2009, Sellon and Wise 2010).
Primary erythrocytosis is characterised by a proliferation of cells of the erythroid lineage without an increase in erythropoietin (Collatos 2003). Secondary erythrocytosis involves an increase in erythropoietin and can be appropriate (in response to tissue hypoxaemia), as in congenital cardiovascular disease, severe chronic pulmonary or cardiovascular conditions, high altitude or changes in haemoglobin (Watson 2000, Thrall 2004), or inappropriate. Inappropriate erythrocytosis involves the release of erythropoietin or erythropoietin-like substances in response to renal hypoxia or neoplasia (Lennox and others 2000, Thrall 2004, Koch and others 2006).
If the haematocrit rises above 60 per cent, the advantages of greater oxygen transport are overcome by the harmful effects of increased blood viscosity (Watson 2000, Thrall 2004). In horses, the most prominent clinical signs are lethargy, weight loss, dark-coloured blood, deep red to purple mucous membranes, normal to slightly elevated capillary refill time, tachycardia and tachypnoea, mild cyanosis, haemorrhagic episodes and thrombosis (McFarlane and others 1998, Watson 2000, Collatos 2003, Thrall 2004, Sellon and Wise 2010). Congenital cardiac anomalies are the main cause of absolute erythrocytosis in large animals. Erythrocytosis …