Myxomatosis to control rabbits

Rabbits were introduced to Australia in 1859 by a wealthy Victorian grazier keen on the sport of hunting. Hunters, however, could not keep up with the extraordinary rate at which the animals multiplied and soon millions of rabbits were competing with Australia’s livestock for feed and were damaging the environment.

The initial release of the myxoma virus led to a dramatic reduction of Australia’s rabbit population. Within two years of the virus’s release in 1950 Australia’s wool and meat production recovered from the rabbit onslaught to the tune of $68 million.

The episode was also memorable for a public scare over the simultaneous outbreak of human encephalitis in northern Victoria. To calm public anxiety that myxomatosis might have been the cause of this deadly human brain disease, CSIRO Chairman Ian Clunies Ross and two other notable scientists, Macfarlane Burnet and Frank Fenner, injected themselves with myxoma virus. They were unaffected proving conclusively that the suggestion was without foundation.

As predicted by CSIRO’s Francis Ratcliffe and ANU’s Frank Fenner, the impact of myxomatosis gradually declined over time as both the myxoma virus and the rabbit population changed genetically. News of a new rabbit control agent, calicivirus, emerged in the 1980s and led to CSIRO conducting field trials on Wardang Island off the South Australian coast. During these trials the virus escaped via an insect vector and spread rapidly. The most likely vectors involved in the escape were bushflies and/or mosquitoes.

In the end the rabbit calicivirus did the job it was predicted and intended to do, without any adverse impacts and with major benefits to primary production and native ecology.

The Australian landscape devastated by rabbit plagues

Thomas Austin introduced rabbits to Australia in 1859, for sporting hunters. But with no natural predators and litters of five or more baby bunnies seven times a year, soon there was a rabbit plague. Farmers ripped their warrens, laid poison and shot them but still they multiplied. By the late 1940s after the second world war rabbits had reached plague proportions and were ravaging vast areas of Australia.

In many places not a blade of grass remained and even bushes were stripped bare of leaves and bark. Farmers and graziers had been away at the war, unable to carry out normal trapping, shooting and poisoning which kept rabbits to manageable numbers. Rabbits swarmed around waterholes like a seething carpet of brown fur. Where rabbit-proof fences existed it was as if they separated two halves of a surrealist painting of devastation: on one side lush growth, for these were the good years, and on the other naked earth often scarred by erosion.

Early use of myxoma in inland South Australia disappointing

In 1919 the Brazilian virologist Aragao had suggested that myxomatosis, a virus disease of rabbits, might be used in Australia to control the rabbit population. But while the rabbit was regarded as a serious pest by many, it was a source of income to others, and the suggestion was opposed. The New South Wales Department of Agriculture did conduct laboratory experiments with the myxoma virus in 1926, but the results were not encouraging.

The use of myxomatosis was again advocated in 1934, this time by Dame Jean MacNamara, a distinguished authority on poliomyelitis. CSIR arranged for laboratory tests with the myxoma virus to be carried out in England by Sir Charles Martin. These were followed in 1936 by laboratory experiments in Australia by Dr Lionel Bull of the CSIR Division of Animal Health. Having confirmed that the virus would not adversely affect domestic stock or native animals, Bull commenced a lengthy series of field trials in the arid regions of South Australia. The results were disappointing; although it proved possible to exterminate rabbits in warrens in which the virus had been introduced, the infection failed to spread from one warren to another.

In 1944 Bull and his colleague Bill Mules concluded that: myxomatosis could not be used to control rabbit populations under most natural conditions in Australia with any promise of success

They did, however, say that it might be possible to use the disease with some promise of temporary control of a rabbit population, but only under special conditions, including the presence of insect vectors in abundance and the absence of predatory animals

Renewed calls for the use of myxomatosis

By 1949 the situation was desperate. The traditional methods of control were quite inadequate. New and radical measures were called for. Dame Jean MacNamara once more took up her advocacy of using myxomatosis campaigning vigorously in the Melbourne-published Stock and Land and the Herald.

Dame Jean had been particularly provoked by an article which quoted Francis Ratcliffe, head of the newly formed CSIRO Wildlife Survey Section, as saying, Myxomatosis has failed because very close contact with rabbits is needed for its spread

There seemed little scientific justification for further research. But the situation was serious and late in 1949 Ratcliffe decided that further trials should be made to test the practical advantages of the known limited capacity of myxomatosis to spread. Ratcliffe’s proposal had the enthusiastic support of the newly appointed Chairman of CSIRO, Ian Clunies Ross.

New trials in the Murray Valley again disappointing

This time, however, the area chosen for the trials was not the dry inland of South Australia but the Murray Valley. An irrigated dairy farm at Gunbower in the Murray Valley, 240 kilometres downstream from Albury, was selected for the experiments. The first inoculations were a failure because of faulty virus material. Then, in May 1950, the infection was successfully released in several warrens. Soon after the start of the experiment 77 diseased rabbits were counted in an estimated population of 4000. But sightings became fewer and by the end of July not a single sick rabbit had been seen for weeks.

The research workers then introduced the disease in late August on four grazing properties within a 65 kilometre radius of Albury: at Wymah, Rutherglen, Coreen and Balldale 24 kilometres from Corowa. A check at the beginning of December showed that the disease had apparently died out except at Rutherglen, and even there it was fading fast.

A few days later the owner of the Balldale property phoned CSIRO to say that his men had seen numbers of sick rabbits while out fumigating. Unfortunately by the time an investigating team arrived a few days later, the men had completed their job and destroyed the evidence.

Success at last

Then a trickle of reported sightings began coming in from other points up and down the Murray. By the end of January 1951 the trickle had become a flood. Every day elated farmers called to report diseased rabbits. Reports came from along the Murrumbidgee, the Lachlan and even up the Darling.

A CSIRO press release of 29 January 1951 reported outbreaks at points up to 290 kilometres apart (Corowa and Hillston) and said:

The disease tends to be confined to the river flats and frontage country. In the Corowa ‘ Rutherglen area, where the most detailed observations have been carried out, 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.

By mid February, nine weeks after the Balldale outbreak the disease ranged over an area 1 760 kilometres by 1 600 kilometres. The Melbourne Argus carried a headline: ‘Farmers’ New Ally Kills 90% of Rabbits’. The Herald ran a story saying:

Along the river flats the stench of death lies heavy these hot days ‘ myxomatosis is striking at the millions of rabbits that swarm along the lagoons and river reaches.

The reason for the epidemic spread of myxomatosis was to be found in the unusually heavy rains which fell during 1950. These had resulted in a massive build-up of mosquito numbers along the Murray-Darling river system, thus providing the ‘special conditions’ that Bull and Mules had earlier stipulated as being necessary for the successful use of the virus.

Hype followed by panic

The hype surrounding the project did, however, lead to an outbreak of panic fuelled by rumours and half-truths. Shortly after myxomatosis was introduced, a human encephalitis epidemic began spreading through the Riverina region of NSW. In no time the public was pointing accusingly at CSIRO scientists and blaming the deadly new rabbit disease for the potentially fatal human brain disease.

In a remarkable demonstration designed to quell public anxiety, Professor Frank Fenner (Australian National University) and two other top Australian scientists, Dr Macfarlane Burnet (the Director of the Walter and Eliza Hall Institute for Medical Research in Melbourne) and CSIRO’s Chairman Dr Ian Clunies Ross, injected themselves with doses of myxoma virus. It did them no harm and the public’s fears were allayed.

Frank Fenner had returned to Australia in 1949 to take up the Chair in Microbiology at the John Curtin School of Medical Research in Canberra and switched his research program to the myxoma virus when CSIRO’s myxomatosis trials were having an impact. As Fenner recalled in an interview with Max Blythe:

Yes, this ‘myxomatosis research’ fell into my lap. Recently, in the Burnet archives at the University of Melbourne, I found Burnet’s diary entries about it. There was no virologist working on myxomatosis, only zoologists. In his diary entries for 31 January and 1 February 1951, Burnet had written that Lionel Bull ‘ Chief of the CSIRO Division of Animal Health, who had done the early work on myxoma virus in 1936-43 before war pressures and so on caused it to be dropped ‘ thought there ought to be some virological work done. On 31 January 1951 Burnet wrote that he was thinking of talking to the ecologist Ratcliffe, the head of the CSIRO field section, who was working on myxomatosis. Then on 1 February he wrote that I had approached him to say I wanted to work on myxoma virus. In brackets he had a note that he thought Dame Jean McNamara, a controversial paediatrician who had been needling the CSIRO to get onto myxomatosis again, had something to do with my request. But he was wrong there. I hadn’t been prodded. I just wanted to get into virology and I saw this as an opportunity. And then I was delighted, as I dug into it a bit more, to find that this was a poxvirus ‘ in line with my previous work with Ectromelia. So that kept me in pox virology.

Fenner, who had already established an international reputation for his work on Ectromelia or mousepox (smallpox of mice) went on to make major contributions to myxoma virus research ranging from epidemiology to molecular genetics. He published detailed findings on the pathogenesis, morphology, classification, relationship with other poxviruses, immunity both passive immunity and active immunity. As Frank Fenner recalled in the interview with Max Blythe:

But the most significant work was the study of the changes in virulence, which occurred very early and went on progressively, and the concurrent changes in the resistance of rabbits: the fact that when the mortality rate fell from 99 per cent, which it was originally, to 90 per cent, there were enough survivors for selection for genetic resistance to occur. For a number of years we followed both the changes in virulence and the changes in rabbit resistance. They have subsequently been taken up intermittently by other people, with some very interesting results.

The economic benefit

In 1952-53, Australia’s wool and meat production jumped by $68 million as pastures recovered from the ravages of rabbits. In the two to three decades after that myxomatosis brought incalculable benefit to Australia. However, eventually the rabbits became genetically resistant to the myxoma virus as predicted by Ratcliffe and Fenner, and by 1995 had multiplied to an estimated 300 million. To combat these changes two additional insect vectors, the European rabbit flea and the arid-adapted Spanish flea, were introduced but with minimal impact, and the effectiveness of myxomatosis continued to fall.

Calicivirus and ongoing research

So again CSIRO scientists were asked to come up with a solution. Rabbit Calicivirus Disease (RCD) is a viral disease which affects only European rabbits. The virus probably originated from a less virulent form present in rabbit populations for many years. It was first reported in China in 1984 and soon after in other countries in Asia and Europe and in Mexico.

These reports alerted scientists to a potentially new biological control for wild rabbits in Australia and New Zealand. The virus was taken into quarantine at the CSIRO Australian Animal Health Laboratory in Geelong for comprehensive testing over three years from June 1991.

In March 1995, a quarantine station was set up on tiny Wardang Island off the coast of South Australia to test rabbit calicivirus, which had kept down rabbit populations in Europe. It was due for release in 1998, but, after only 6 months, it escaped from the island, most likely carried by insects. CSIRO has commented:

There would have been a much bigger discussion phase about whether or not to release the virus and that was just left behind. But beyond that, the effects have been as good as we ever would have expected. In the arid zones we have had a remarkable reduction. The numbers of rabbits are down to ten or fifteen percent of their original numbers.

And as the rabbits disappeared, the barren landscape flourished once again. The virus has since spread throughout most of Australia, mainly by natural spread. To date its impact has generally been greatest in the arid and semi-arid zone. Its effectiveness in wetter areas has been lower.

Scientists are also aware that because myxomatosis was only effective for 15 to 20 years, rabbits could also become resistant to calicivirus. Thus the rabbit war is one that is likely to continue indefinitely.

Feature video: Rabbit calicivirus (1999)

Sources

• McKay A, 1976, ‘Myxomatosis’, In: Surprise and Enterprise, Fifty Years of Science for Australia, White F, Kimpton D (eds), CSIRO Publishing, pp.38-39.
• Blythe M, 1992 and 1993, Interviews with Australian Scientists: Professor Frank Fenner ‘ Microbiologist and virologist (Australian Academy of Science)