Factors affecting the survival of Australian wild rabbits exposed to rabbit haemorrhagic disease

Abstract

Rabbit haemorrhagic disease virus (RHDV) is foreign to Australia, and first entered populations of Australian wild rabbits (Oryctolagus cuniculus L.) in Australia in late 1995. Rabbits are serious environmental and agricultural pests in Australia, and RHDV, a major new pathogen, was introduced as a biological control agent to reduce their numbers. Our study evaluated some of the factors affecting survival of wild rabbits exposed to rabbit haemorrhagic disease (RHD) at 78 sites across Australia.

Our data on rabbit numbers consist of the number of rabbits per spotlight kilometre present shortly before and shortly after an RHD outbreak at each site. They are a direct measure of survival rather than mortality. By reducing the interval between the pre- and post-RHD counts to the minimum possible, we sought to minimise the influence on the analysis of other causes of change in rabbit numbers. We calculated proportional survival as the ratio (number of rabbits present after RHD)/(number present before RHD), and used regression analysis to relate it to environmental and other variables. Proportional survival was lower at higher densities of rabbits; was lower if RHDV arrived naturally at the site rather than if it was deliberately released; was lower in areas with hot, dry climates than in areas with cold, wet climates; was lower in southern, inland areas than in warm, coastal areas; and, if the outbreak occurred during summer, was lower in areas of winter rainfall than in areas of summer rainfall. Rainfall seasonality was not correlated with survival at other times of the year. Only in the last effect was there a significant interaction with the time of the year that the outbreak occurred.

Our statistical model describes correlations among the data, but does not in itself establish cause and effect. We interpret the properties of our statistical model to draw the following conclusions. First, the effectiveness of RHD is reduced in cold, wet areas and warm, coastal areas, because of the prevalence in these areas of one or more pre-existing caliciviruses in rabbits that impart year-round resistance to RHD. Second, we conclude that the poor summertime performance of RHD in areas that are wet in summer could result from poor survival of RHDV exposed to the combination of high temperature and high relative humidity, although it is also possible that during summer the effectiveness of vectors declines.