Canine distemper virus-induced thrombocytopenia.

Effects of canine distemper virus (CDV) infection on circulating platelet values were studied in gnotobiotic dogs inoculated with R252-CDV. Thrombocytopenia (less than 200,000 platelets/microliter) was present on postinoculation day (PID) 5 and persisted through PID 15. Peak thrombocytopenia occurred on PID 10 (less than 85,000 platelets/microliter). Thrombocytopenia was accompanied by lymphopenia, neutropenia, and monocytopenia. Platelet membrane-bound CDV antigen and IgG were present from PID 7 onward; neither the third component of complement nor IgM was detected on platelets from CDV-inoculated dogs. The mean number of megakaryocytes per unit of bone marrow surface area was unchanged. Megakaryocyte infection was present in dogs euthanatized on PID 4 (0.33%), increased slightly in dogs euthanatized on PID 8 (3%), and increased sharply in dogs euthanatized on PID 9 and 10 to 17.8% and 8.3%, respectively. Phagocytosis of platelets by stellate reticuloendothelial (Kupffer's) cells in the liver was prominent in dogs euthanatized from PID 5 onward. Seemingly, CDV-induced thrombocytopenia was mediated by virus-antibody immune complexes on platelet membranes. Decreased platelet production after PID 8 resulting from direct viral infection of megakaryocytes was a likely contributing factor and occurred against a background of profound virus-induced dysfunctions of all hematopoietic cellular elements.