Viral biocontrol: grand experiments in disease emergence and evolution

• Virus biocontrol agents provide unique insights into disease emergence and evolution.
• Myxoma virus (MYXV) and rabbit hemorrhagic disease virus (RHDV) have experienced different trajectories of virulence evolution.
• Host resistance appeared to both MYXV and RHDV, reducing the impact of biocontrol.
• There has been no inadvertent jumping of MYXV and RHDV to nontarget host species.

Although viral emergence is commonly associated with cross-species transmission, the processes and determinants of viral evolution in a novel host environment are poorly understood. We address key questions in virus emergence and evolution using data generated from two unique natural experiments: the deliberate release of myxoma virus (MYXV) and rabbit hemorrhagic disease virus (RHDV) as biological control (biocontrol) agents against the European rabbit in Australia, and which have been of enormous benefit to Australia's ecosystem and agricultural industries. Notably, although virulence evolution in MYXV and RHDV followed different trajectories, a strongly parallel evolutionary process was observed in Australia and Europe. These biocontrol agents were also characterized by a lack of transmission to nontarget host species, suggesting that there are major barriers to successful emergence.