About Myxomatosis
In 1950, the CSIRO released the myxoma virus to combat Australia’s rampant rabbit population, in trials at five locations along the Murray River. The release was a huge success: within days, the myxomatosis disease had spread throughout the Murray River region, ultimately decimating Australia’s wild rabbit population in the following years. These trials had significant benefits for Australia’s agricultural and pastoral industries that had been devastated by rabbit overpopulation for decades.
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Rabbit overpopulation in Australia
The number of rabbits increased rapidly and dramatically following their introduction to Australia in 1859 by a Victorian grazier. Within 2 years, descendants of the original 13 European wild rabbits introduced on the grazier’s estate were threatening agricultural and pastoral activities, causing erosion around waterholes and competing with native animals for food and water.
Identifying a solution
In 1919, the myxoma virus was identified as a possible solution to rabbit overpopulation by Brazilian scientist Dr H de Beaurepaire Aragao.
The NSW Department of Agriculture conducted laboratory experiments with the myxoma virus in 1926, but the results were not encouraging.
Further trials on rabbits were conducted in arid parts of South Australia, but the results were again disappointing. Field trials in Europe also showed myxomatosis was difficult to establish. The virus did not spread from one warren to another, and scientists from the CSIR (now known as the CSIRO) concluded that myxomatosis could not be used to control rabbit populations under most natural conditions in Australia.
By 1949, rabbits were in plague proportions, with numbers estimated at 600 million.
A breakthrough at the Murray River
The unusually heavy rains which fell in the summer of 1950-51 resulted in a build-up of mosquito numbers along the Murray-Darling river system.
A breakthrough in the possible use of the myxoma virus occurred when the newly-established Wildlife Survey Section of the CSIRO identified that insects would likely help to spread the virus by stinging rabbits in their burrows.
A 1951 CSIRO press release said: ‘There is a very obvious and clear relation between the activity of the disease and proximity of weedy lagoons. These are the breeding places of the dusk-biting Culex annulirostris mosquito [which help to spread the myxoma virus].’
This discovery prompted the recommencement of myxomatosis experiments.
Using virus materials supplied by the Glenfield Veterinary Research Station , myxomatosis experiments were conducted along the Murray River downstream from Albury in areas where mosquitoes were likely to be present. The virus was released between May and November 1950 at five locations, including two sites at Balldale.
Ferrets were used to catch live rabbits, which were inoculated with the virus and could be subsequently identified by the bare patch of fur at the inoculation site. Three caged wild rabbits inoculated at Balldale died within nine days.
Observations stopped at Balldale in November 1950 when researchers believed that the disease had died out; however, in December 1950 more dead rabbits were reported in the area. Diseased rabbits soon appeared on the Murray River flats 8 miles south of Balldale. Within days there were reports of more dead and dying rabbits along the Murray River and into its northern tributaries of the Murrumbidgee, Lachlan and Darling River systems.
Concerns about the virus
With the success of the disease came fears that it would cause illness in humans. To alleviate these concerns, CSIRO Chairman Ian Clunies Ross , Frank Macfarlane Burnet from the Walter and Eliza Hall Institute of Medical Research, and Frank Fenner from the Australian National University injected themselves with myxomatosis to prove it was not dangerous to humans.
Declining impact of myxomatosis and new rabbit control methods
As early as 1954, scientists correctly predicted that the impact of myxomatosis would gradually decline as both the myxom a virus and the rabbit population changed genetically.
In the 1980s, a new virus was identified for rabbit control: calicivirus. During field trials conducted by the CSIRO on South Australia’s Wardang Island, the calicivirus escaped via an insect vector and spread rapidly. The new disease decimated wild rabbit populations without any adverse impacts on native ecology..
The lasting impact
The initial release of the myxoma virus led to a dramatic reduction in Australia’s rabbit population. Within two years of releasing the virus, Australia’s wool and meat production increased by approximately $68 million. While the disease’s effectiveness declined over the years, the initial success of myxomatosis and later calicivirus continue to inform Australia’s approach to rabbit control today.
References
CSIRO - https://csiropedia.csiro.au/myxomatosis-to-control-rabbits/ CSIRO web site lists an extensive bibliography of scientific publications.
Bruce Munday, Those Wild Rabbits. How they shaped Australia. Wakefield Press, 2017
Brian Douglas Cooke, Australia’s War against Rabbits. The story of rabbit haemorrhagic disease. CSIRO Publishing, 2014.