Platelet function and the von Willebrand factor antigen in the hepatosplenic form of schistosomiasis mansoni

Forty-five individuals with hepatosplenic schistosomiasis mansoni were studied with the aim of measuring levels of von Willebrand factor antigen (vWF:Ag), detecting abnormalities in platelet morphology and aggregation, and identifying changes to surface antigens. Haemograms, platelet aggregation tests, flow cytometry investigations of CD41/CD42b antibody and vWF:Ag assays were performed. Mean platelet counts were low (77 522/mm3) and 82.2% of patients presented thrombocytopenia. An inverse relationship between spleen size and platelet count was seen. Macroplatelets were found in 57.1% of patients, indicating good bone-marrow response, but were insufficient to compensate for the decrease in platelets due to splenomegaly. Decreased or absent platelet aggregation was seen in 50% of patients, probably due to low platelet counts. Markers for GPIIb/IIIa were normal in more than 90% of patients, not supporting the increased capture and destruction of platelets in the spleen that is hypothesized to occur with cirrhosis. Similar to cirrhosis, vWF:Ag levels were high or very high in 70.5% of patients. High levels of vWF:Ag were associated with platelet counts <100 000/mm3, larger spleen diameter and oesophageal varices. In conclusion, hepatosplenic schistosomiasis leads to a lower platelet count due to pooling in the spleen and, consequently, impaired aggregation, but not to increased capture and destruction of platelets in the spleen. High vWF:Ag levels probably promote stabilization of platelet microaggregates and prevent minor manifestations of thrombocytopenia such as petechiae, ecchymosis and gingival bleeding.