A plague on the pest rabbit calicivirus disease and biological control
Box 2 | The history of myxoma virus in Australia
The ancestors of the rabbits in Australia lived in North Africa, whereas the myxoma virus evolved in South America where it has kept the rabbit population under control. The history of the interaction of rabbits and the myxoma virus has become the most completely documented example of the interaction of a host animal species and a disease organism.
Wild rabbits were introduced into Australia in 1859, and by the 1880s they had become a major pest. In 1919 Dr Aragao, a South American researcher, suggested the introduction into Australia of the myxoma virus. However, in spite of the devastation rabbits were causing to farming land, Australian authorities were reluctant to damage the thriving rabbit meat and fur industries.
Laboratory experiments with myxoma virus were finally carried out in Australia in 1924, but were not promising. It was not realised that the spread of the virus depended on insect vectors, such as mosquitoes.
In 1933 Dr Jean Macnamara, a medical specialist from Melbourne, visited New York to study poliomyelitis. She met Dr Richard Shope, who was investigating myxomatosis in domestic rabbits on Californian fur farms. She realised the potential of myxomatosis for controlling the Australian rabbit population and had samples of the virus sent to Australia. However, authorities would not let the samples into the country.
Jean Macnamara then persuaded Stanley Bruce, Australian High Commissioner in London, to help. Sir Charles Martin, who had been chief of the Division of Animal Nutrition at CSIR (Council for Scientific and Industrial Research, the precursor of CSIRO), had moved to England and was working on myxomatosis at Cambrige University on behalf of the CSIR. In 1936 he reported that the virus was specific to rabbits and thus safe to import into Australia, although he questioned how well it would spread throughout the rabbit population.
Experiments in Australia in 1936 confirmed Martin's results. It was difficult to obtain sites for field tests because of the vigorous protests of rabbiters and rabbit merchants. Eventually sites were chosen in South Australia, but the trials were not a success. The area was drought-stricken and there were few insect vectors to spread the disease.
Work on the myxoma virus ceased in 1943 and Jean Macnamara publicly criticised the work as 'a pathetically limited inquiry'. CSIRO resumed work on myxomatosis in 1950, with trials at five sites in the Murray Valley, all of which appeared to be failures. However, at the end of 1950 and early in 1951 myxomatosis swept through the Murray-Darling river systems. Heavy rains at the time meant that mosquitoes bred and then carried the virus from infected to uninfected rabbits, sometimes several hundred kilometres away. Within 3 years, the disease had been carried to all parts of Australia and rabbit numbers were drastically reduced.
However, as early as 1953 scientists studying the virus and rabbits noticed that the virulence of the virus had changed from being 99.9 per cent effective to 95 per cent, a small but significant drop. The less-virulent virus took 3 to 4 weeks to kill a rabbit instead of 6 to 10 days, so that sick rabbits could be bitten by mosquitoes and fleas for 3 to 5 times as long as a rabbit suffering from the highly virulent strain. The milder strain was therefore more successful in infecting rabbits, and it spread rapidly. Through this selection the virus evolved to a less-virulent form.
At the same time, evolutionary selection processes were working in the rabbit population. If one rabbit in a thousand had a natural resistance to the myxoma virus, it alone would survive to leave offspring and their chance of surviving would be greatly increased by lack of competition from other rabbits. They would also inherit their parents' resistance. Thus in a few generations the proportion of resistant rabbits would increase.
The two factors, attenuation of the virus and inherited immunity to the virus, have led to the situation where, today, the myxoma virus may kill only 50 per cent of the rabbit population during an epidemic.
Posted February 1997.