The presence of cross-reactive antibodies to rabbit haemorrhagic disease virus in Australian wild rabbits prior to the escape of virus from quarantine.

Sera collected from Australian wild rabbits prior to the escape of rabbit haemorrhagic disease virus (RHDV) from Wardang Island were examined for RHDV antibodies using purified recombinant capsid protein VP60 expressed from baculovirus. A VP60-based indirect ELISA showed that 196 of 392 wild rabbit sera reacted (OD(450) >0.15) with VP60. Twenty sera (OD(450) ranging from 0.15-2.47), randomly chosen from the 196 positive sera, recognized the 64 kDa VP60 in Western blot analysis, indicating that the reactivity detected in ELISA is indeed specific to the VP60 antigen. In a separate study, sera of 23 rabbits from an RHD-free area after the escape of RHDV were tested by ELISA and 21 of the 23 rabbits were found to be positive. When these rabbits were challenged with a lethal dose of RHDV, 11 out of the 23 rabbits survived. The presence of RHDV-protective antibodies in some of these rabbits suggested that they had been exposed to a pre-existing non-virulent rabbit calicivirus closely related to RHDV. These results highlight the need to study the prevalence of, and to characterize, this viral agent in order to effectively control rabbit populations in Australia and New Zealand.