Feline Leukemia Virus-Induced Thrombocytopenia and Macrothrombocytosis in Cats:

The Kawakami-Theilen strain of feline leukemia virus (FeLV-KT). an exogenous anemia-inducing retrovirus, induced significant macrothrombocytosis and thrombocytopenia during acute infections of cats. The geometric mean platelet volumes of both freshly isolated fixed platelets and ethylenediamine tetraacetic acid- sphered platelets from infected cats were increased, but the ratio of fresh-fixed to ethylenediamine tetraacetic acid-sphered platelet volumes. an estimate of ethylenediamine tetraacetic acid-induced isovolumetric shape change, was normal. Total plasma membrane increased as the platelet volumes increased in FeLV-KT-infected cats, but estimated surface connected canalicular system surface area did not. Platelet concentrations were decreased 2 to 6 weeks after inoculation. but marked macrothrombocytosis resulted in a trend toward increased platelet mass on weeks 5 and 6 after inoculation. Thus. FeLV-KT-induced macrothrombocytosis may serve as a model of impaired platelet volume regulation. The platelet volume and platelet membrane surface area abnormalities found suggest that this model would allow studies of the megakaryocyte/platelet axis, particularly in the area of membrane formation and territorial demarcation. The mechanisms of platelet volume regulation are complex and poorly under~tood.~ Increased throm- bopoiesis frequently results in macrothrombocytosis'' and most acquired macrothrombocytic disorders of man involve increased platelet turnover and reduced life~pan.~ IJ 74 However, in some acute leukemias. ac- quired intrinsic megakaryocytic abnormalities rather than platelet turnover are critical determinants of platelet volume7 and in human hereditary throm- bocytopenias accompanied by macrothrombocytosis the platelet lifespans may be normal, decreased or in- crea~ed.~ Animal models of macrothrombocytosis are needed to further investigate the regulatory systems of the megakaryocyte-platelet axis. Naturally occumng FeLV infections in cats induce a wide spectrum of hematological and immunological dysfunctions,I8 some of which mimic human diseases, e.g., preleukemia and acute leukemia, in which macro- thrombocytosis and platelet dysfunction are frequently noted.' ' '' Macrothrombocytosis in FeLV-infected cats has not been generally recognized but bizarre macro- thrombocytes have been describediX and a transient thrombocytopenia occurs in viremic cats after infec- tion by contact exposure to FeLV.'' In viremic cats the megakaryocytes and platelets are productively infected" and acquire FeLV ant1gens.I" I3 This paper describes a prospective study of platelets in cats ex- perimentally infected with the Kawakami-Theilen strain of feline leukemia virus (FeLV-KT). Platelet size. concentration, mass, volume distribution, ethylene- diamine tetraacetic acid-induced shape change, and membrane surface areas were determined in control and FeLV-KT-infected cats.