Mechanism of thrombocytopenia in African swine fever

Pigs were inoculated with an African swine fever (ASF) isolate of moderate virulence, and the changes in the number of circulating blood platelets during infection were correlated with the appearance of antiviral antibody and fluctuations in total plasma hemolytic complement concentrations. Thrombocytopenia was detected by postinoculation days (PID) 7 and 8, and antiviral antibody was detected by PID 7, using an indirect immunofluorescence technique. The total hemolytic complement concentration was moderately and transiently decreased from PID 5 to 9, but was consistently low from PID 18 to 26. Pigs inoculated with an ASF virus isolate of greater virulence had a decrease in platelet counts on PID 6 and 7, and the total plasma hemolytic complement levels decreased in all pigs by PID 6 to 7. Antibody to ASF virus was not detected in pigs inoculated with the more virulent isolate. Pigs sensitized to ASF viral antigen with an inactivated-virus vaccine or by previous infection with ASF were challenge exposed. Sensitized pigs became clinically ill and thrombocytopenic by 24 to 72 hours earlier than did inoculated, nonsensitized pigs. Vaccinated pigs inoculated with homologous virus had lower blood virus concentrations than did nonvaccinated pigs. African swine fever virus-sensitized pigs inoculated with heterologous virus had a higher fatality rate than did nonsensitized pigs, and the pigs died peracutely, with only a few gross lesions in evidence. In vitro experiments demonstrated that ASF virus antigen induced platelet aggregation in platelet-rich plasma from recovered, nonviremic pigs. Viral antigen, antibody, or complement was not demonstrable on the surface of platelets from pigs inoculated with ASF virus isolate, by direct immunofluorescence testing.