CODEX Entry 6022: The biocontrol virus myxomatosis


In Australia, the introduction for hunting of a few European rabbits in the mid-1800s resulted in a ballooning population of hundreds of millions that were threatening crops. In 1950, researchers discovered that a smallpox variant found in South American rabbits was lethal for it’s European relatives. Australian authorities released the virus into the wild, cutting the rabbit population by 99%. A few years later, the virus was released in France and eventually spread to the United Kingdom. Billions of rabbits died. But within a decade, rabbit numbers were on the rise again, as resistance increased. The rabbits became resistant by making genetic changes to their DNA, usually a shift in the frequency of particular versions, or alleles, of a gene. Strikingly, these changes were shared by the rabbits in Australia and Europe, evidence of parallel evolution. One allele shift affected the rabbits’ interferon, a protein released by immune cells that sound the alarm when there is a viral attack, and help trigger an immune response. Compared with pre-1950’s rabbits, modern rabbits make an interferon that is better at responding to the biocontrol virus. In the 1970s the virus then developed a greater ability to suppress the rabbit’s immune responses. Unlike the parallel evolution in rabbits, myxoma smallpox viruses on the different continents took different genetic paths to regaining potency. But it has been the new rabbit hemorrhaging virus (RHV) that brought numbers down again in the 1990’s. As rabbit numbers rise again, scientists are looking to weaponize RHV2 to decimate the rabbits once again. Evolutionary microbiologists suggest the myxoma viral counterattack “is a cautionary tale” for researchers aiming to take charge of the evolutionary arms race by introducing biocontrol agents or by making humans or livestock more resistant to disease, suggesting caution with the evolution and counterevolution as ‘the rabbit hasn’t won’.