Acute canine ehrlichiosis: platelet survival and factor 3 assay

Ten dogs experimentally inoculated with Ehrlichia canis were thrombocytopenic 14 days after inoculation. Circulating platelet concentrations had declined rapidly by postinoculation day 10, but remained at or slightly below the 14-day concentration during the remainder of the 42-day experimental period. The percentage of circulating megathrombocytes also increased to 8.7 times preinoculation concentrations indicating the existence of accelerated thrombocytopoiesis. The release of platelet factor 3 (PF-3) from normal canine platelets was delayed after incubation of the platelets in globulin fractions from infected dogs. Inhibition of PF-3 release was attributed to the effect of an unidentified substance present only in the dialyzed globulin fraction of infected canine serums. The presence of antiplatelet antibody in these dogs was not evident. Survival of 51Cr-labeled platelets was decreased in infected dogs, but was most abbreviated in infected dogs that were thrombocytopenic when injected with labeled platelets. Thus, accelerated nonantibody-mediated destruction of platelets contributes markedly to the development of thrombocytopenia in dogs with acute ehrlichiosis.