African swine fever virus ASFV/NH/P68 is a naturally occurring, non-haemadsorbing and non-fatal isolate. Longitudinal clinical and immunological studies on 31 pigs inoculated oronasally or intramuscularly with this isolate defined two discrete groups of animals : those developing ASF chronic type lesions and those remaining asymptomatic. Animals developing lesions had viraemia and fever late after infection, NK activity levels close to that of control animals and high levels of anti-ASFV specific antibodies together with a marked hypergammaglobulinaemia involving IgG1, IgG2, IgM and IgA immunoglobulin isotypes. Pigs remaining asymptomatic after infection, on the other hand, did not have viraemia or fever after day 14 post-infection and had elevated NK cell activity, but normal plasma Ig concentrations and relatively low specific anti-virus antibody concentrations throughout the duration of the experiments. Importantly, the latter group of pigs virus were resistant to subsequent challenge with the highly virulent ASFV/L60 isolate and survived with no major changes in any of the parameters examined and referred to above. Finally, lymphoproliferative responses to the mitogens concanavalin A, phytohaemagglutinin and pokeweed mitogen were not depressed in either of the two clinically defined groups of pigs. Thus further studies with this infection model may provide new insights on mechanisms of protective immunity to ASFV.

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Cross-reaction in hemadsorption inhibition tests with African swine fever viruses and swine immune sera permitted the separation of two antigenic subgroups, A and B. However, there are African swine fever viruses which cannot be included in these subgroups, thus constituting at least one third subgroup. Nonhemadsorbing African swine fever viruses are described which were isolated from pigs dying with chronicM lesions of pneumonia as shown by autopsy; several hypotheses to explain the appearance of these nonhemadsorbing strains are advanced. Electron microscopy studies suggest that the subgroup-specific antigen is associated with the outer structural component of the virion, which derives from the cytoplasmic membrane of the infected cell.