Is increased juvenile infection the key to recovery of wild rabbit populations from the impact of rabbit haemorrhagic disease?

Abstract

The frequency and timing of rabbit haemorrhagic disease (RHD) epizootics and their impact on different age groups of rabbits were studied for 15 years in a recovering rabbit population in South Australia. We recorded the number and body size of rabbits dying during RHD epizootics, collected tissue for genetic analysis of rabbit haemorrhagic disease virus variants and compared the number of carcasses found to the number of susceptible rabbits present at the beginning of each epizootic. All RHD epizootics occurred between late winter and spring, but, progressively, epizootics started earlier and became more frequent and prolonged, fewer susceptible adult rabbits were present during epizootics, and the age of rabbits dying of RHD declined. Increased infection and virus shedding in juvenile rabbits offers the most plausible explanation for those epidemiological changes; the disease is now increasingly transmitted through populations of kittens, starting before young-of-the-year reach adult size and persisting late in the breeding season, so that most rabbits are challenged in their year of birth. These changes have increased juvenile mortality due to RHD but reduced total mortality across all age groups, because age-specific mortality rates are lower in young rabbits than in older rabbits. We hypothesise that this may be the proximate cause of recovery in rabbit populations across Australia and possibly elsewhere.