European rabbits are an introduced pest species that cause serious damage to the environment and agricultural industries in Australia. Biological control using the two rabbit pathogens myxoma virus (MYXV) and Rabbit Haemorrhagic Disease Virus (RHDV) has not only greatly reduced rabbit numbers and impacts, it has also provided a unique opportunity to study the initial spread and establishment of emerging pathogens and their co-evolution with their host.
Following the release of MYXV in 1950, wild rabbits rapidly developed genetic resistance and less virulent forms of the virus became dominant in the field, to facilitate virus transmission by biting insect vectors. These interactions between MYXV and wild rabbits in Australia have become the text book example for the trade-off hypothesis of virulence evolution.
By contrast, since the release of RHDV in 1995 a different pattern of co-evolution has been occurring. It is only in recent years that some Australian rabbit populations have begun to develop genetic resistance to RHDV, and consequently rabbit numbers are again on the rise. Furthermore, rapid attenuation of RHDV has not been observed in the 16 years since its release. Indeed recent research suggests that there may be selective pressure for RHDV to maintain high levels of virulence in the face of developing genetic resistance in the host. These findings suggest that, similar to MYXV, RHDV virulence may also be selected towards maximum virus transmission, with the key difference that rabbit carcasses, and not the diseased animals, are the likely source of mechanical transmission by insects.
This is an important contribution towards understanding what drives host-pathogen co-evolution in rabbits, a prerequisite for ongoing effective rabbit control in Australia. It also highlights the unique opportunity that Australia’s rabbits and their viruses provide as a model system for the study of the evolution of emerging diseases in general, and in particular of mechanisms promoting high levels of virulence.