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Volume 70 Edition 5  Calicivirus  14 July 1999

The Other Millennium Bug

The plight of the rabbit calicivirus disease has been quiet in the media of late. A year ago it was clear that the disease, although accidentally released on the Australian mainland, was working effectively at reducing the numbers of European rabbit populations in Australia. The rabbit, introduced here by settlers in 1859 for hunting purposes, has caused mass devastation to native habitat, flora and fauna, causing extinction of some species of plants and forcing native mammals out of their natural habitat.

Last year it was reported by the Department of Conservation and Land Management that in arid zones such as the Nullarbor Plain, rabbit calicivirus was spreading more efficiently and effectively than in wetter areas of the continent, like Queensland. Now, it appears, calicivirus is newsworthy once more. Just recently, CALM's Science division held a seminar, to report on the disease's progress in this state and the latest issue of the Australian Geographic journal has an article on the South Australian perspective of the disease. It appears that the latest reports reinforce what was believed to be true last year ­ the virus is continuing to spread and is having a positive effect on native wildlife and habitat revival, with native mammal species reclaiming habitat that was once theirs, and some plant species re-establishing themselves in areas where they were previously gnawed to sticks.

A brief history of the rabbit calicivirus disease (RCD) thus far: In 1989 investigation began into a new disease aimed specifically at killing the European rabbit, the rabbit we love to hate. In 1991 such a disease was imported into Australia, known as the rabbit calicivirus. Tests began to determine the effect of RCD on our native wildlife, and when it seemed certain that all was clear, further tests began on Wardang Island, just off the coast of South Australia near Adelaide ­ the same Island that housed the tests for myxomatosis, the last big rabbit killer in this country.

The disease affects the red blood cells in the rabbit's body, causing blood clots in the main organs so the rabbit eventually falls asleep and doesn't wake up. The clotting causes fluid to fill the lungs inducing heart failure which generally takes between 24 and 48 hours to kill the rabbit, as opposed to the comparatively cruel 13 days taken by myxomatosis.

In October of 1995, 7 months after trials started on Wardang Island, the disease escaped to the mainland ­ the cause of this event is still uncertain, but it is believed to have been carried by bushflies which were blown on to the Island by a hot wind, and then back to the mainland in the afternoon seabreezes. It was believed that the disease could only be transmitted by way of physical contact of rabbit to rabbit ­ rubbing noses, mating, and whatever else rabbits do, not by way of flies and other insects. By May the following year, 8 months down the track, it was found extensively throughout South Australia and Victoria, into New South Wales, and some first reports had come in from Queensland and Western Australia. Three months later in August of 96, its incidence had almost doubled ­ RCD was taking off in a big way and having an enormous effect on rabbit populations with no side effects thus far. Following this, the disease had spread into the Northern Territory, and more extensively throughout New South Wales, South Australia, Victoria and Western Australia, but hadn't really developed well in Tasmania or Queensland. Australia was well on its way to eliminating the rabbit problem.

One thing remains true however: Australia will always have rabbits. Like it or not, it's the same situation with feral animals like the cat, the wild pig, fox and the cane toad to name a few. While the earlier rabbit killer, the myxomatosis virus, was successful to a large degree, it petered out because the rabbits built up immunity to it. The resistant rabbits passed that trait on to their young, and slowly but surely, within 10 years Australia was blessed with another rabbit problem. Unfortunately, a similar immunity is developing with RCD, but only time will tell how effective the disease will be in the long run.

The reason for the difference in the impact of calicivirus in the various Australian states is to do with climate. The more arid environments were showing populations decreasing by up to 90 and 95%, however the wetter climates like Tasmania and the south west of WA were not showing such promising signs because of the difference in the rabbits' breeding cycles. Rabbits in the Nullarbor plain for instance breed only when it is wet and food is abundant, so if the disease comes along at a time when there is not much rain, it has the ability to wipe out whole populations very swiftly. In the wetter environments, the rabbits are constantly breeding, so it is more difficult for the disease to have such a sweeping effect.

Shouldn't the Europeans have a problem with the rabbit too? To an extent they do, but the rabbit is free from many of its natural predators here in Australia and thrives in the same countryside that houses many of our native animal species like bandicoots and dunnarts. The more successful breeding strategies of the rabbit ensure its survival and spread throughout the mainland ­ it goes back to Darwinian theory and the "survival of the fittest" concept ­ calicivirus will hopefully return things at least partially back to the way they were.

One of CALM's Principal Research Scientists in this field, Tony Start, was at the recent seminar on the progress of RCD, and I asked him about the success of the calicivirus, and what we can expect for the all important next five years.

"I wish I had a crystal ball. If the disease dies out on the Nullabor Plain, we may see a resurgence in rabbit numbers. The other issue is one we've seen with myxomatosis, where with time, rabbits can evolve a better resistance to the virus and the virus may evolve into a less virulent strain so it becomes less effective in controlling rabbit numbers. How long that might take and whether in fact it will happen, I'm not sure. My guess is that in the next five years we will see lower rabbit numbers persisting in the arid areas."

If the rabbits disappear then native mammal species should return. Tony Start explains that, "Native animal species will take quite a long time to recover from the very low population levels that they have got to. When rabbit numbers are reduced, the first thing that recovers is the vegetation. When the vegetation recovers then the herbivores can recover, and when the herbivores recover then the carnivores can recover. You have a time lag all the way through, it's very erratic and can take quite a long time for a new equilibrium."

And then there is the issue of predators. If the rabbits all disappear, what will the foxes turn to? That is one question which would have been better off answered while trials were still being carried out on Wardang Island. As rabbit numbers are depleted, it would be fair to draw the conclusion that predators such as the fox and feral cat, not to mention native predators like the wedge-tailed eagle and quolls, would turn towards whatever native prey are left, possibly further endangering those species and additionally disrupting the food chain.

Tony Start says, "Rabbit numbers have dropped off before when epidemics of myxomatosis have gone through, creating exactly the same situation, but really if there's alternatives for foxes to switch to they are eating, or have eaten, them already. We've intensified fox baiting in areas where other native species are susceptible."

So why are scientists raving about RCD? At the same point in time after the release of myxomatosis there was excitement in the air - more than there is currently with RCD. The lack of headlines can be put down to the fear of a back lash when it is revealed that the disease is less successful than what they hoped for. CALM and other scientists involved in its research and release don't want to end up with egg on their faces with bold and sweeping statements that this will be the be all and end all of Australia's rabbit problem ­ again. It's fair to say that myxo is still effective at controlling rabbit populations, but it has been nowhere near as successful as was hoped and hyped in the initial stages of its release. At the recent CALM seminar, it was revealed that if young rabbits (kittens) contract the disease before 8 weeks of age, they develop immunity to it and carry that immunity for the remainder of their lives. This resistance is not passed on to the next generation however, as was myxomatosis . Contrary to this report however, is the Australian Geographic article that states: "Antibodies in the mothers' milk protects most young rabbits from RCD until they are about 8-10 weeks old. Many succumb once this immunity is lost." As this shows, much is still to be learned about the new killer. Perhaps a second strain of the virus is the answer? One that will kill all the rabbits that are currently resistant?

"We think that calicivirus is a much more stable organism than myxomatosis, and therefore it probably will evolve at a much slower rate. We hope that it will remain potent than for much longer than myxomatosis."....but no second strain yet....?

So let's look at the positive aspects to the disease: By eliminating rabbits, we are decreasing the chance of erosion due to soil damage and vegetation destruction. Native plants and animals are hopefully able to re-establish their populations in their old habitat and by decreasing the amounts of rabbits as prey for cats and foxes, we will eliminate such predators as well. That's the plan at least, and it seems to be going well ­ studies in South Australia show signs of fewer numbers of foxes and cats, but for those still hunting, their diets have not significantly changed.

Then there's the negative side of the debate. Feral predators like the fox and cat, and native predators like the wedgetail eagle and quolls, could eventually switch from using the rabbit as prey, to using native mammals like the bandicoots and dunnarts as a source of food. Will we see an invasion of weeds instead of the native plants returning? And what about the build up of resistance to calicivirus? When will we see that take a turn for the worse? And there's even the chance that, being a virus, it will mutate to continue dominating its hosts' immune systems. In doing so it may possibly jump species and affect natives, but as a quote in the Australian Geographic article puts it: "The risk that the virus may cross over to another species has to be weighed up against the real and the inevitable loss of more native species if we failed to use it."

While calicivirus is good for the time being, its not too late to start looking for a new, even more effective, virus or disease now, keep numbers at a constantly low rate in all areas, and get the Australian countryside back to the way it was before 1859. Possible but unlikely, especially with a myriad of other environmental problems slowing slipping beyond our control...

Stuart Mutzig



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Pelican  Volume 70 Edition 5  14 July 1999
 
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