SUMMARYThis study evaluated the time course distribution of rabbit haemorrhagic disease virus (RHDV) structural protein VP60 in tissues from experimentally infected rabbits from three different age groups. Viral VP60 antigen could not be detected in tissue samples from animals under four weeks, and only a few hepatocytes (0·01 to 0·2 per cent) were stained in the 6-week-old animals. A 6-week-old rabbit euthanised at 72 hpi showed VP60-labelling in hepatocytes and macrophages close to areas of inflammation. Viral VP60 antigen was detected as early as 12 hpi in a few hepatocytes (0·03 per cent) from adult animals. Within this age group, the extent of hepatocyte labelling considerably increased at 18 (3·0 per cent), 24 (25·5 per cent), 36 (50 per cent) and 48 (60 per cent) hpi. Extrahepatic viral VP60 antigen was also detected at 36 and 48 hpi in spleen macrophages and lymphocytes from adult rabbits. These findings support the hypothesis that the hepatocyte is the only cell type in the liver able to support RHDV replication almost immediately after viral infection.

The immune response to foot-and-mouth disease virus (FMDV) elicited by infection or immunization with inactivated virus in adult mice was examined. A model of adoptive transfer of immunocompetent cells was used for this purpose. The results presented here indicate that both short-and long-term secondary immune responses elicited by high doses of inactivated virus are indistinguishable, at the humoral or cellular level, from that observed after infection. The responses to inactivated or infectious virus were both efficiently mediated by B cells. However, immunization with low doses of inactivated virus induced a response which, although effective in aborting infection, was fully dependent on FMDV-specific T cell cooperation. These findings suggest that the different immune responses observed after infection and immunization are mainly the result of the different viral mass presented to the immune system in each case.

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