Epidemiology of rabbit haemorrhagic disease (RHD) in the South Island, New Zealand, 1997–2001

Abstract

Rabbit haemorrhagic disease (RHD) was illegally released in New Zealand in August 1997 and subsequently spread by farmers and naturally over all areas inhabited by rabbits. The disease has persisted, causing either annual or biennial epidemics that appear to start each spring by infecting susceptible adult rabbits and running through to the autumn infecting young rabbits of the year. Rabbit densities have been suppressed by more than 90% in many areas where numbers were initially high and epidemics returned annually, and by about 50% in areas with low initial densities and biennial epidemics, leaving between 35% and 20%, respectively, of the survivors immunised. In some areas the disease has not reduced rabbit densities and has left more than 80% (at worst) of the survivors across all age cohorts with antibodies to RHD. A possible cause of this apparent longitudinal transmission is that seropositive adult rabbits (which may retain virus) may be infectious and pass virus on to their offspring at just the right dose and age to impart immunity. A pen trial, in which we orally dosed 9-week-old rabbits born to seropositive mothers, showed that they survived and did not seroconvert, but were fully susceptible when re-dosed at about 17 weeks. We have evidence that the possible pre-existing calicivirus does not impart immunity to RHD in at least one part of New Zealand.