Giant platelet disorder in the Cavalier King Charles Spaniel.

OBJECTIVE: The aim of this study was to describe the clinical, functional, and morphologic characteristics of platelets in Cavalier King Charles Spaniel dogs (Cavaliers). MATERIALS AND METHODS: Blood from 69 clinically normal Cavaliers was collected and anticoagulated with ethylenediamine-tetraacetic acid (EDTA) and citrate. Automated and manual platelet counts were obtained. Percent platelet aggregation in response to ADP (2, 4, 8, 16, and 32 microM) was determined. Electron microscopy was performed to examine platelet internal morphology and dense granule distribution. A cardiologist recorded the quality of murmurs. RESULTS: Thrombocytopenia (<100,000/microL) was present in 51.43% (36/69) of Cavaliers. Macrothrombocytes (>3 microm) were present in 33.33% (22/69). Mean manual platelet count was 118,770/microL. Manual (EDTA blood) and automated (EDTA and citrated blood) methods of platelet counting were correlated. Prevalence of cardiac murmurs was 38% (26/69). There was no association between affected dogs and murmur, signalment, or coat color. Mean percent platelet aggregation was significantly higher in controls than in Cavaliers (79% vs 38%, p=0.001). Response to ADP was unaffected by thrombocytopenia, macrothrombocytes, murmur, or any combination thereof. Platelet electron microscopy showed normal and giant sized platelets with normal internal morphology. CONCLUSIONS: A benign inherited giant platelet disorder affects approximately 50% of Cavalier King Charles Spaniels. It is characterized by thrombocytopenia, macrothrombocytes, or decreased platelet aggregation in response to ADP. Platelet ultrastructure is normal. Citrated or EDTA blood provides accurate platelet counts. Further studies are indicated to determine platelet glycoprotein structure and any association with mitral endocardiosis. Cavaliers may be useful models of inherited giant platelet disorders.