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 8 out of 11 female domestic rabbits, while a ninth carried only a single embryo- a success rate of over 70 per cent. It's 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, 26 April 1997, p 4). 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's 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 doesn't 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.
Graeme O'Neill, Mildura, Australia
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New Scientist issue: 10 AUGUST 2002
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