Rabbit Advocacy Animal Matters
Plants that make rabbits sterile
Research being done at the Murdoch University-based State Agricultural Biotechnology Centre could help deal with Australia's increasing rabbit population. Murdoch PhD student Peiling Tan is genetically engineering plants that could be used to reduce rabbit fertility. The research is funded by the Cooperative Research Centre (CRC) for Legumes in Mediterranean Agriculture and the Vertebrate Biocontrol CRC.
Ms Tan is modifying plants to produce an animal protein that can be used as a vaccine to immunise rabbits. Controlling fertility using vaccines is a concept known as immunocontraception. Delivery of the protein into the rabbit creates an antibody response. This can result in reduced fertility if the antibodies can be tricked into attacking the rabbit's own eggs as though they were foreign cells.
"Our hope is to make them sterile, but that may not be possible," said Ms Tan. "A reduced fertility is the next best thing."
Traditional approaches to the rabbit problem, such as poisoning, have proven ineffective and rabbits have developed some resistance to the disease Myxomatosis. Sterilisation is a more humane way to control the rabbit population.
The process of modifying the plants is simple. Cut plant material is soaked in a solution of soil bacteria called Agrobacterium. The bacteria have been modified to contain the animal DNA. The bacteria infect the plant material, transfer the animal DNA into plant cells where it integrates into the plant DNA.
The plant tissue is regenerated in culture using hormones to grow shoots and then transferred to a tub containing hormones that promote root growth. Once roots grow the plant is transferred to soil.
Research has not proceeded to the stage of delivering the animal protein into the rabbit's system, but Ms Tan said this could be done by extracting the protein from the plant, then purifying and injecting it into the rabbit, or the plant could be crushed and fed to the rabbit.
While other research is being done into infecting rabbits by way of viruses, Ms Tan said using plants was potentially a cheaper and easier way of delivering the vaccine. "The reason why I'm using plants is that this particular protein hasn't been produced in plants before," she said. "So we want to see whether it can be produced in plants. The other reason is if it can be produced in plants it will reduce the cost, because it's cheaper to grow plants than to use other systems such as viruses."
Ms Tan is genetically modifying two types of plants — tobacco and clover. "Tobacco is a very easily modified plant," said Ms Tan. "But clover is the main aim, because rabbits will eat clover. It's part of their diet." Ms Tan is using an animal protein from the zona pellucida. The zona pellucida is a membrane that surrounds the egg in a female animal.
She is using the zona pellucida protein from pigs, which is very similar to the protein in rabbits. The pig protein has been shown to induce antibodies in a range of animals. Rabbit zona pellucida protein may be studied at a later date. However, the rabbit protein may not work because the rabbit's immune system might not produce antibodies in reaction to the presence of 'self' rabbit proteins. Ms Tan said she expected to continue her research for another two years.
Virus could sterilise Australia's rabbits
After more than a decade of trying, Australian researchers have created a highly infectious virus that could wipe out the country's rabbit pests by making them sterile. The team, at the Pest Animal Control Cooperative Research Centre (PAC CRC) in Canberra, has already applied for permission to carry out field trials with a similar virus that makes European mice infertile. Advocates of this form of biological control say it is more humane than existing strategies such as poisoning, shooting or spreading lethal diseases.
From cats to camels, feral mammals cost Australia hundreds of millions of dollars each year in lost agricultural production and environmental damage, and have driven some native mammals and birds to extinction.
"Australia has lost more mammal species than the rest of the world combined in the past 400 years," says project director Tony Peacock. But the prospect of genetically engineered viruses being released into the wild is still likely to spark a fierce debate.
The viruses make females infertile because they have an added gene for a protein from the zona pellucida, the thick layer surrounding the egg. Females infected with one of the transgenic viruses produce antibodies against their own eggs, damaging them and blocking fertilisation - a process called immunocontraception. For rabbits, geneticists chose to engineer the myxoma virus, which devastated populations when it was released in Australia half century ago. But many rabbits have become resistant, and less lethal strains have edged out the original virus.
Four years ago, researchers selected the rabbit ZPB gene as the most promising of three zona pellucida genes to insert into the myxoma virus. But this sterilised less than 25 per cent of rabbits. Now they have switched to the ZPC gene, with much better results.
In two trials in May, New Scientist has learned, the latest transgenic myxoma virus sterilised eight out of 11 female domestic rabbits, while a ninth carried only a single embryo - a success rate of over 70 per cent. It is a highly infectious but non-lethal strain that should give most rabbits no more than a fever for a few days.
Peacock says if the virus could sterilise 70 to 80 per cent of wild females, rabbits would decline to densities similar to those in Europe and become a relatively minor pest. But even if it were only 50 per cent successful, it would still end the episodic plagues that have ravaged the country.
The immunocontraception idea has already been proved in another major pest, the European house mouse (New Scientist print edition, 26 April 1997). An engineered herpes virus, murine cytomegalovirus, has consistently produced 100 per cent sterilisation of female mice in lab trials.
The PAC CRC will apply to the Office of the Gene Technology Regulator later this year for permission to conduct the first contained field trial of the transgenic mouse virus at Walpeup, in north-western Victoria. Peacock says extensive consultation would precede any release.
The virus appears to sterilise female mice for life, but it is too early to know if the same will be true with rabbits, which are longer lived. So the team plans to add other genes to the myxoma virus to try to boost the contraceptive effect.
If the government decides to release such viruses, Peacock thinks populations should decline rapidly as breeding slows and natural mortality and predation take their toll. Other researchers think the effects will only be temporary, as natural selection will favour animals with a mutated zona pellucida protein that evades the immune response.
But Peacock points out that sperm can only fertilise eggs if the proteins on their head bind to the ZP proteins. So for resistance to appear, both egg and sperm proteins would have to mutate simultaneously yet still be able to bind to each other, which is extremely unlikely.
However, the viruses could be accidentally - or deliberately - transferred to another continent. Another worry is that the viruses could spread to other species, but the modified viruses are no more likely to jump the species barrier than wild strains. And the myxoma virus has not done this in the 50 years it is been in the country.
For Australia's worst feral predator, the fox, the PAC CRC team has not been able to identify a virus that does not also infect domestic and wild dogs, including dingoes. So the plan is to modify a canine herpes virus so it can only replicate when an antibiotic such as tetracyline is present. The virus and antibiotic would be added to baits that are irresistible to foxes, but are shunned by dogs and dingoes. Because the virus doesn't infect any native mammals, they would be safe even if they ate the baits.
The PAC CRC is also working with a New Zealand research team to develop an immunocontraceptive virus for New Zealand's worst feral predator, the stoat, which also inflicts a heavy toll in Hawaii.
Sterilisation plan for grey squirrels
Scientists are planning a mass sterilisation programme to halt the growth of Britain's grey squirrel population which is now feared to have reached five million. Teams in Britain and America are working against the clock to develop a method of rendering the pests infertile using treated bait.
Some estimates put the number of greys in Britain at five million and it is feared the local red population could die out within two decades unless dramatic steps are taken to curb their bigger, stronger rivals. As well as forcing out red squirrels, greys destroy trees by stripping bark and have taken their toll on songbird populations by taking eggs. They also steal food from garden bird tables and infest loft spaces.
The problem of the burgeoning grey population was highlighted by the Daily Telegraph in an article by associate editor Simon Heffer which prompted a flood of letters and emails from readers. He called for a cull to stop them damaging the countryside.
Government scientists are working on a two-year programme commissioned by the Department for Environment, Food and Rural Affairs to find an effective oral contraceptive for grey squirrels. They are also trying to pinpoint the best way of giving it to the rodents without affecting other animals. If successful, the treatment could be adopted in around five to 10 years.
Brenda Mayle, who is leading the research, said sterilisation injections had already proved successful in horses and deer in the US. "If it eats part of the bait and leaves the rest it is a risk to non-target species. We are looking for a food package that the squirrel will eat in its entirety and not cache, which it does with acorns," said Ms Mayle, who is research programme manager for Forest Research – part of the Forestry Commission – in Surrey.
The contraceptive would work by attacking the immune system of the squirrel, suppressing its fertility. Scientists are desperate to find ways of tackling the grey squirrel threat before it causes more damage to the red population.
Greys have been found to carry a disease called squirrelpox virus (SQPV) which does not harm them but kills reds. It is spreading through Britain and has recently been found as far north as Lowland Scotland.
Ms Mayle said: "It's very important that we do find something to reduce the rate of their spread, particularly because we are seeing the squirrelpox virus spreading north in Scotland now." But she added: "It's not an alternative to culling. It will become another tool in our ability to manage wildlife populations but it's not an alternative to lethal methods."
A spokesman for Defra ruled out a national cull, saying it had been considered but would be too expensive with no guarantee of success. "The Government is committed to preventing the further spread of grey squirrels, however, eradication is considered impracticable at national level. Some local programmes of control are under way, particularly in areas where grey squirrels are threatening remaining populations of reds or to protect forestry," she said.
Grey squirrels were introduced to Britain from America in the Victorian era but swiftly pushed out red squirrels, which are now only found in the Isle of Wight, Brownsea Island, western Wales, northern England and parts of Scotland.
Comment: During the 1930s, it was discovered that hormones prevented ovulation in rabbits. In later research (1990’s) at a Halifax University, Nova Scotia, scientists developed a feed that sterilized deer (and possibly rabbits) for 10 years with a single feeding.
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