Working on myxomatosis

What caused you to change to the topic of myxomatosis?

Myxomatosis was brought into Australia in 1950. At a meeting here in Canberra, a man I had never met came up to me and said, 'I've just recently been appointed to the Chair of Microbiology at the National University. I am going to study myxomatosis. We believe it might be mosquito-borne. Would you be interested in taking on the mosquito side of it while I do the biology?' This was Frank Fenner, a very remarkable person, and that was the beginning of a friendship that has lasted ever since. Frank is a superb collaborator, and for the next five years we worked on the mechanisms of transmission of myxomatosis.

There was a long background to myxo being brought into Australia. Dame Jean McNamara, a polio expert, had kept pressing the government to do something about introducing this disease to control rabbits. Rabbits had always been a serious problem in Australia, but during the war years, when there was no manpower available, absolutely staggering numbers built up – an enormous plague of rabbits. It is really hard now to appreciate how many there were in Australia, but hundreds of millions of pairs of carcases per annum (they were sold in pairs, for some reason) were exported, mainly to the UK, without any effect on the population. And for every rabbit that was sent overseas, probably four or five more were fed to the dogs or eaten by the locals. Every large, and sometimes small, pastoral station had rabbit trappers, generally permanently on the property. It was a huge business. It is never possible to estimate properly how many were killed in the year that myxo was brought in, but if you went into the countryside there was a smell everywhere of dead flesh. It was unbelievable.

Rabbit trapping was one of the most exhausting jobs I could conceive of. One of my uncles had a property up at Quirindi, where I used to stay occasionally as a boy, and I went out with the rabbit trappers. They would go at 4 o'clock in the afternoon, each carrying 50 steel traps over his shoulder. It nearly killed you to carry these things, but there was a 15 year-old kid who could set one of these traps in his hand as he walked along, whereas I couldn't even set it by standing on it.

At 8 o'clock you would go out and collect the rabbits, and reset the traps. You would carry the rabbits over your shoulder – which is just about as difficult as carrying traps – and then park them. At 10 o'clock, or later, you would go out for the second catch. This time you were doing it in complete darkness, and if you trod on a trap it would cut your foot off. Yet these people could walk in the dark straight to the traps they had set, take the rabbits out and carry them and the traps as well. And then you had to take those rabbits in to the cooler during the day. Can you imagine a tougher job?

Myxo was brought in by the wildlife research section of CSIRO, as it became, under Francis Ratcliffe, and was liberated in about May at the little town of Gunbar. How it was transmitted was not yet known. August, through the winter, despondent, cold and wet – they came back to Canberra and issued a press statement: 'It hasn't worked. Nothing happened.' The myxo killed the rabbits in the warrens where it had been introduced, but it didn't spread.

Three weeks after they got back to Canberra it was reported at Corowa, miles from where they had released it. Then that summer it spread 1000 miles in one direction, 700 miles in another. Nobody knew how it was being spread. It was going so fast that although the Division was planning to go to Macquarie Marshes, in northern New South Wales, to get ahead of the disease and watch it come through, before they had left Canberra it was already 200 miles past Macquarie Marshes.

We still don't know quite how it spread so fast, but it was clearly mosquitoes which were moving. This was a bad year for mosquitoes – a lot of rain. Other species of mosquitoes were also transmitting Murray Valley encephalitis, and so overnight CSIRO went from being seen as the saviour of our country (everybody in Australia knew about myxomatosis; it was a huge thing) to a killer. People were dying. A few deaths from Murray Valley encephalitis. And when Lord Casey, the then Minister, said, 'Myxomatosis and encephalitis are different viruses,' the Parliament said to him, 'Prove it.' Macfarlane Burnet, Frank Fenner and Ian Clunies-Ross – who was then the Chairman, and took this matter extremely seriously and was very worried by it – were all inoculated by Burnet or Frank with myxoma, to demonstrate that it wasn't a killer. This was not known about for quite a long while, though.

Of course, we really didn't know an awful lot about myxoma, and I still recall being bitten by a mosquito which I knew was carrying the virus and feeling a little queasy the next morning, wondering whether there might have been a stage of development.

What were you able to find out about the spread of myxomatosis?

Whereas the Americans believed that the myxo virus – like yellow fever and some other viruses – multiplied in the vector, in the mosquito, we believed the transmission was purely mechanical. We turned out to be right, but we had a lot of work to do to demonstrate that myxo was being transmitted by a different mechanism. I had been sensitised to the idea of mechanical transmission because that is the way a lot of the aphids transmit their viruses. The aphid people talk about persistent viruses and non-persistent, the non-persistent ones being affected as though it is a mechanical operation. And that is the way myxo was being spread. We took a long time to demonstrate it unequivocally, but we did.

The work was intense but fascinating. All the ecology was being done by the Division of Wildlife Research, we were doing the lab stuff, and it meshed well together. Any mosquito species which feeds on a rabbit is capable of being a vector. Different strains of the virus which were showing up behaved differently, and we were able to demonstrate how it was that some of the less virulent strains were spreading by comparison with the very virulent ones, which were killing 99 per cent of the rabbits while the others were much less affected.

Myxo is still an important feature in the Australian landscape but it goes in a cyclical way. It will kill a group of rabbits, die out there, then spread and appear somewhere else. But it is hard now to realise just how many rabbits there used to be. You couldn't drive the hundred kilometres south to Cooma, as I did regularly, without seeing a dead rabbit – as soon as you had passed one, there was another one in view, squashed on the road. Now you can do that trip 20 times without seeing one. The impact of that unbelievable change goes even further: it has done more to arrest soil erosion, more for regeneration of various species of plants, for the wool clip, which alone increased tremendously, than any other single thing that has ever happened.

Focus questions

• Why do you think scientists suspected that mosquitoes were important in the transmission of myxomatosis?

• What do you think Day meant when he said that myxomatosis was spread by mosquitos using ‘mechanical transmission’?