Acquired thrombocytosis

Thrombocytosis may occur in primary myeloproliferative conditions or as a secondary phenomenon in a variety of physiologic and pathologic states. Thrombocytosis is usually due to an increase in platelet production and release (in reactive cases, this is likely due to increased thrombopoietin or other thrombogenic cytokines, such as IL-6), rather than an increased lifespan. Usually, a platelet count higher than the reference range for the species is a reactive thrombocytosis and not of direct pathologic importance. Note that young animals, in particular calves and foals, normally have platelet counts higher than the adult reference range.

Reactive thrombocytosis can be seen in a variety of disorders, including neoplasia, chronic inflammatory diseases, immune-mediated disease (immune-mediated hemolytic anemia, non-regenerative immune-mediated anemia), trauma (fractures, diaphragmatic hernia), drugs (especially antineoplastic agents and corticosteroids), and iron deficiency anemia. Animals in the early stages of recovery from ITP or prior bone marrow suppression may have transient thrombocytosis with platelet counts > 1,000,000/Ál. In one study in dogs and cats, the most common disorders associated with a reactive thrombocytosis were neoplasia (especially lymphoma), gastrointestinal disorders (especially pancreatitis, chronic hepatitis, and inflammatory bowel disease) and endocrine disorders (including diabetes mellitus, hyperadrenocorticism and hypothryoidism). In horses, the most common conditions were gastrointestinal tract disease (colitis), infectious or inflammatory disease (e.g. peritonitis, pleuritis) and miscellaneous causes, including hepatic disease, fractures and neoplasia.

Neoplastic thrombocytosis: In rare cases, the cause of persistent thrombocytosis is essential thrombocythemia, a myeloproliferative disorder affecting megakaryocytes, resulting in extremely high platelet counts (usually > 1,000,000/Ál). A bone marrow aspirate will reveal a megakaryocytic hyperplasia, therefore definitive diagnosis relies upon ruling out other diseases that may produce a thrombocytosis. Essential thrombocythemia has been documented in both dogs and cats. Occasionally, thrombocytosis may also accompany other myeloproliferative diseases, such as polycythemia vera.