Overview and Clinical Importance
Polycythemia (erythrocytosis) refers to an increase in the red blood cell (RBC) count, packed cell volume (PCV), hematocrit (HCT), and hemoglobin concentration above reference intervals. While the terms polycythemia and erythrocytosis are often used interchangeably, polycythemia technically implies an increase in all blood cell lines, though this is exceptionally rare in companion animals. Clinical signs typically do not manifest until PCV exceeds 60%, with some of the highest recorded values exceeding 85%. Understanding the classification, pathophysiology, and management of polycythemia is essential for the NAVLE, as this condition represents a critical diagnostic and therapeutic challenge in small animal practice.
| Type |
Mechanism |
Causes |
| Relative Polycythemia |
Increased PCV without increased RBC mass due to decreased plasma volume |
Dehydration: Vomiting, diarrhea, inadequate water intake
Splenic contraction: Excitement, fear, epinephrine release |
| Absolute Primary (Polycythemia Vera) |
Autonomous clonal expansion of erythroid precursors independent of EPO |
Myeloproliferative disorder; JAK2 mutations documented in some dogs |
| Absolute Secondary Appropriate |
Physiologic response to systemic hypoxia stimulating EPO release |
Right-to-left cardiac shunts (reversed PDA, Tetralogy of Fallot)
Severe chronic pulmonary disease
Methemoglobin reductase deficiency |
| Absolute Secondary Inappropriate |
EPO production without systemic hypoxia |
EPO-secreting renal tumors (carcinoma, lymphoma, nephroblastoma)
Non-neoplastic renal disorders (amyloidosis, cysts)
Extra-renal EPO-producing tumors (hepatoma, cecal leiomyosarcoma) |
Definitions and Terminology
Erythrocytosis specifically refers to an increase in RBCs only, while polycythemia may imply increases in all blood cell lines (RBCs, WBCs, and platelets). However, concurrent leukocytosis and thrombocytosis along with erythrocytosis is exceptionally rare in dogs. Normal canine PCV ranges from 37-56%, with sighthounds (Greyhounds) normally having mild erythrocytosis compared to standard reference intervals. Clinical signs do not typically develop until PCV exceeds 60%.
| System |
Clinical Signs |
| Mucous Membranes |
Brick-red or ruddy color (hyperemia); cyanotic in cases with right-to-left shunting; congested scleral vessels |
| Neurological |
Seizures (most common presenting complaint), behavioral changes, lethargy, depression, ataxia, weakness, blindness, head tilt, circling, tetraparesis |
| Hemorrhagic |
Epistaxis, hyphema (bleeding into anterior chamber), hematemesis, melena, hematuria |
| Ocular |
Dilated, tortuous retinal vessels; retinal hemorrhage; blindness; uveitis |
| Systemic |
Polyuria, polydipsia, exercise intolerance, syncope, intermittent collapse |
| Thromboembolic |
Dyspnea (pulmonary thromboembolism), acute neurological deterioration (cerebral infarction) |
Classification of Polycythemia
High-YieldRelative polycythemia from dehydration will have concurrent hyperproteinemia and prerenal azotemia, while absolute polycythemia will have normal protein levels. This is a key distinguishing feature on the NAVLE!
| Test |
Findings and Interpretation |
| CBC with PCV/TS |
PCV greater than 56% (dogs); if TP elevated, suspect dehydration; if TP normal, suspect absolute polycythemia |
| Chemistry Panel |
Prerenal azotemia with dehydration; hypoglycemia may occur with severe polycythemia (increased glucose utilization by expanded RBC mass) |
| Arterial Blood Gas |
PaO2 less than 80 mmHg or SaO2 less than 92% indicates secondary appropriate erythrocytosis; normal values support primary or secondary inappropriate causes |
| Serum EPO Level |
Low/low-normal: Primary polycythemia; Elevated: Secondary (appropriate or inappropriate); Normal: Does not rule out secondary causes (significant overlap exists) |
| Thoracic Radiographs |
Evaluate for pulmonary disease, cardiac enlargement, or metastatic disease |
| Echocardiography |
Essential to rule out right-to-left shunting cardiac defects (reversed PDA, Tetralogy of Fallot, VSD); bubble study confirms shunting |
| Abdominal Ultrasound |
Evaluate kidneys for masses, cysts, or structural abnormalities; check liver for tumors |
| Bone Marrow Aspirate |
Shows erythroid hyperplasia in all absolute causes; NOT useful to distinguish primary from secondary (both show same findings) |
Pathophysiology
Erythropoietin Feedback Loop
Under normal conditions, decreased oxygen delivery to the kidney stimulates erythropoietin (EPO) secretion from peritubular interstitial cells. EPO then acts on erythroid progenitor cells in the bone marrow to increase RBC production. When oxygen delivery normalizes, EPO secretion decreases, completing the negative feedback loop.