快猫短视频

WHEN Australian sheep farmer David Lord says he feels 鈥渋ncredibly lucky and
privileged鈥�, he is not talking about witnessing a religious revelation or the
latest medical miracle. He is talking about the deaths of hundreds of thousands
of rabbits on his family鈥檚 farm, Thackaringa Station in New South Wales, and the
benefits he sees coming for Australia鈥檚 farmers and the environment.

Within weeks of rabbit calicivirus (RCV) escaping from Wardang Island off the
South Australian coast in October 1995, it had killed over 80 per cent of the
rabbits on Lord鈥檚 property. The disease it causes was being tested on the
island, under strict quarantine, to assess its effectiveness as a means of
ridding Australia of its rabbit plague (This Week, 9 December 1995, p 4).

And Lord is not alone in his hatred of rabbits and his enthusiasm for rabbit
calicivirus. 鈥淚n Australia, rabbits are regarded as lower than the lowest
vermin,鈥� says Keith Murray, deputy director of the Australian Animal Health
Laboratory (AAHL) in Geelong, which evaluated the risk of the virus spreading to
other species. 鈥淚f you set out to invent a new virus that would kill a species
in the most efficient and quickest way possible, you couldn鈥檛 dream up anything
better than rabbit calicivirus.鈥�

But many scientists, particularly in Europe and North America, are equally
passionate opponents of the control strategy. When Australian government
scientists started deliberately releasing RCV on the mainland late last year,
and New Zealand鈥檚 Ministry of Agriculture began public consultations on whether
to import the virus the debate escalated.

鈥淲e just don鈥檛 know enough about this virus. They should hold back until we
know more. Once you鈥檝e let it out you can鈥檛 bolt the door,鈥� says clinical
virologist David Cubitt of Great Ormond Street Hospital in London. Cubitt, an
expert on human caliciviruses, points out that caliciviruses are relative
newcomers to the microbiology laboratory, and their biology is still very poorly
understood.

Cubitt and others fear that in desperation to rid their country of what is,
by any estimate, a plague of Biblical proportions, Australian virologists and
wildlife biologists have thrown caution to the winds, and released a virus that
may prove deadly to Australia鈥檚 native wildlife. In Europe, where there have
been natural outbreaks of RCV since at least 1987, the knock-on effects have not
always been benign.

The European rabbit (Oryctolagus cuniculus) originated in Spain. In
1859, rabbits were brought to Australia, where, with few competitors or
predators, they colonised the country at a rate of between 20 and 100 kilometres
a year, the fastest known for any mammal other than humans.

And as the rabbits spread, devouring grazing land, they provoked the wrath of
the farming community and, much later, environmentalists. By the 1950s it was
open warfare. Myxomatosis, caused by the myxoma virus, loosened the rabbits鈥�
grip on the land鈥攂ut not for long. Today, rabbits cost the country about
A$600 million (nearly 拢286 million) per year in lost agricultural
production, according to a 1995 study for the International Wool Secretariat, an
industry group based in Melbourne.

鈥淭he large-scale plagues of rabbits we have here are beyond the ken of
Europeans,鈥� says Murray. Lord claims that before RCV arrived his 66 000 hectare
property was riddled by 25 000 rabbit warrens each housing up to 50 rabbits.
Together, they could eat 125 tonnes of grass a day, he says. 鈥淭hat鈥檚 bloody
丑耻驳别.鈥�

And rabbits play havoc with Australia鈥檚 indigenous wildlife. Their burrows
contribute to soil erosion, and they eat growing shoots, preventing many plants
and trees from regenerating. Birds, insects and animals that depend on this
vegetation for food and shelter die. For example, the bilby or rabbit-eared
bandicoot, which is classified by the World Conservation Union as a 鈥渧ulnerable鈥�
mammal species, now survives only in rabbit-free pockets of land.

When RCV escaped to the mainland from Wardang Island, to many it seemed a
godsend. The Australian authorities seized the moment and, after analysing the
safety implications, followed up with deliberate releases of the virus. By last
month, infected rabbits had been released at 327 sites across Australia. More
releases are planned for March.

Palatable virus

RCV causes blood to clot in the rabbits鈥� lungs, heart and kidneys. They die
from heart failure or asphyxiation usually within a few days of infection.

But even the symptoms and the name of the virus provoke sharp disagreement.
In Australia and New Zealand the virus is called RCV, while elsewhere it is
usually called rabbit haemorrhagic disease virus. The CSIRO, Australia鈥檚
national research organisation that runs the AAHL, maintains that the common
name is misleading because the rabbits do not necessarily haemorrhage. The
critics counter that the use of the less common name is a cynical attempt to
make an unpleasant means of biocontrol palatable to the public.

The virus, which appeared out of the blue in Europe in the mid-1980s,
reaching Britain in 1992, kills over 90 per cent of infected rabbits in the
early stages of an outbreak.

The demise of rabbits has 鈥渄evastated the ecology in Europe鈥�, says Diana
Bell, head of the rabbit research group at the University of East Anglia in
Norwich. In Spain, where wild rabbits are the staple prey of at least 29
predators, RCV has halved the rabbit population. Red fox litter sizes have
declined in areas where the rabbit is an important food source, says Rafael
Villafuerte of Do帽ana Biological Research Station, in Seville. And
there鈥檚 great concern, he says, that the threatened Iberian lynx and Spanish
Imperial eagle, animals that are far more dependent on rabbits, are also
suffering. In the worst hit parts of Britain, polecats and wildcats, which eat
rabbit, as well as creatures like the stone curlew, and butterflies that
flourish on short-cropped grass, may also be adversely affected, says Roger
Trout a rabbit expert and private consultant based in Farnham, Surrey. Bell says
she recognises that Australia must tackle its considerable rabbit problem, but
she questions the wisdom of using a still mysterious virus to do the job. 鈥淏ut
it鈥檚 happened, so we can only wish them luck.鈥�

The harshest comments have come from Alvin Smith of the College of Veterinary
Medicine at Oregon State University in Corvallis, who studies a calicivirus
called the San Miguel sea lion virus, and David Matson of the Eastern Virginia
Medical School in Norfolk, who studies four caliciviruses that are known to
infect humans. In a letter to the Prime Minister of New Zealand, Jim Bolger,
Smith wrote that Australia had embarked on an 鈥渦ncontrollable and unpredictable鈥�
biological experiment. His advice was that New Zealand 鈥渟hould not come under
the same sword of Damocles鈥�.

Smith, Matson, Cubitt and others say that to justify releasing the virus in
the first place, the Australian government should have first obtained clear
proof that it infects just one species, the rabbit. AAHL researchers claim to
have done just that. Between 1991 and 1996, they exposed 31 species of native
and domestic animal to the virus. Samples from two New Zealand species that had
been exposed to the virus were sent to the AAHL for analysis. The researchers
measured the amount of antibodies and virus in the blood and organs of these
animals. They also looked for any signs of sickness. According to CSIRO鈥檚 public
statements, those tests showed that the virus did not replicate or cause disease
in any test animal. 鈥淥ur testing of rabbit calicivirus is the most comprehensive
study that we know of into the host range of an animal virus,鈥� says Murray.

Smith and Matson are far from satisfied. They argue that the doses of virus
used were too low and they were administered incorrectly. The AAHL scientists
are equally as adamant that the tests were adequate. 鈥淭he dose was chosen as
sufficiently large to present a realistic challenge,鈥� says Harvey Westbury, who
oversaw the studies. The idea, he points out, was to test whether the injected
virus would replicate to create enough virus to trigger an immune response. Each
animal was injected in its muscles with a dose of virus 1000 times the amount
that would kill 50 per cent of rabbits.

Hopping species

鈥淭hat sounds a lot,鈥� says Smith, 鈥渂ut to a researcher that鈥檚 a nothing dose.鈥�
He says that the AAHL researchers should have tried to replicate the amount of
virus the animal would be exposed to under natural conditions. For example, a
fox that ate one infected rabbit liver would get 30 000 times the test dose,
says Smith. And he and Matson argue that the virus should not have been
injected. After all, in the wild, an animal is more likely to inhale the virus
or eat it in contaminated rabbit flesh.

Smith and Matson also question the way the AAHL screened the animals for RCV
antibodies, and interpreted their findings. In a letter sent to Australian
government ministers last August, Smith claims that the AAHL scientists set an
arbitrarily high cutoff point for what they considered significant antibody
levels. According to Smith and Cubitt, the CSIRO studies did discover antibodies
to the virus in falcons, seagulls, kiwis, mice, bats, and echidnas.

AAHL researchers acknowledge only that the kiwis鈥攁nd possibly the
mice鈥攕howed some antibody response, but downplay its significance. The
antibody-detecting assays were only one part of a wide-ranging testing regime,
says Murray, and taken together the results clearly showed that 鈥渢he [virus] did
not grow in any of the animals鈥�.

Even if the trials had been perfect, though, the critics may not have been
swayed. Smith says that there is one more concern that the trials could never
address. Even if RCV cannot infect other species at present, could it mutate to
a form that could jump species?

No, says the CSIRO. It points out that wild rabbits in Europe have been
infected with RCV since the 1980s, apparently without infecting other species.
And in a paper presented last September to the Australian Prime Minister鈥檚
Science and Engineering Council, a government advisory body, CSIRO scientists
acknowledged that the virus would mutate, but said that mutations were far more
likely to alter the virulence of the virus, than its target species

Smith is scornful of that idea. Unlike the myxoma virus, he says, RCV is an
RNA virus, and so mutates one million times faster. 鈥淥n average each [RCV]
genome has one to seven mistakes. It鈥檚 a fickle virus. It skitters all over the
place.鈥� What鈥檚 more, he says, viruses in three of the five calicivirus groups
are known to hop species. The human hepatitis E virus also infects swine and
monkeys, for example, and the sea lion virus infects at least 17 different
species. And because parts of the genomes of RCV and European brown hare
syndrome virus are similar, some researchers suspect that a common ancestor of
RCV has already hopped between two species, the rabbit and the hare.

To make matters worse, the large-scale spread of RCV among Australia鈥檚
rabbits could provide ideal conditions for pushing the virus over the edge. 鈥淚f
you are going to put a new virus with a very rapid mutation rate into a very
large, virgin population of animals, you must up the chances of generating
something that can hop species,鈥� reasons Cubitt. He muses about the possibility
of the virus infecting a migratory bird.

Ecologists are also battling over the possible effects of deliberate RCV
releases. Assuming other animals do not fall prey to a mutated version of the
virus, there is evidence that the demise of the rabbit could be good for the
Australian landscape and its native inhabitants.

In 1991, a study led by Brian Cooke, an ecologist at the CSIRO鈥檚 Division of
Wildlife and Ecology in Canberra, eliminated rabbits from 80 hectares near the
mouth of the Murray River by laying poisoned bait and bulldozing burrows. Soon
after, the she-oak, a native tree with a grey, weeping foliage, and perennial
native grasses returned. By 1994, wombat and western grey kangaroo numbers had
increased, too.

Cooke points out that much of the overseas opposition to RCV comes from
virologists and veterinarians. 鈥淭heir whole ethos is to get rid of viruses in
animal populations. I come from a different point of view, a conservation point
of view. I can鈥檛 sit back and watch whole ecosystems being destroyed,鈥� he
says.

鈥淎nything that increases the vegetation for local marsupials should be good
news,鈥� agrees Bell. But there are no guarantees. For instance, the good news
could sour if sheep, a decidedly non-indigenous species, end up replacing the
rabbits. And some Australian species鈥攊ncluding the wedge-tailed eagle, the
little eagle, the black kite and the swamp harrier鈥攈ave come to depend on
the introduced rabbits for food. So concerned is the Australasian Raptor
Society, it has begun a study into the effects of RCV on raptors in the
Strzelecki Desert of South Australia.

Future unknown

Marsupials may also suffer if feral dogs and cats and other predators opt to
eat them in the absence of abundant rabbit. On the other hand, marsupials may
flourish if the numbers of feral dogs and cats drop when deprived of an easy
food source. According to the Department of Environment and Natural Resources,
when RCV hit the Flinders Ranges in South Australia, the numbers of cats and
dogs fell. It remains to be seen whether control programmes will stop the
predator populations recovering, and so provide long-term protection for native
animals.

Over the next four years, the Agriculture and Resource Management Council of
Australia and New Zealand plans to solve some of the great unknowns about RCV
release and the Australian environment. It will monitor RCV鈥檚 impact on wild
populations of introduced animals, as well as native animals such as bandicoots,
wallabies and native mice, in ten areas around Australia. Those areas range from
the flat, treeless, Nullarbor Plain in the West to the forested slopes of the
Great Dividing Range in the east. It will be at least a year before the Council
will know for certain how animal populations are changing, longer for hard data
on changes in vegetation, says Mary Bomford, the biologist who is coordinating
the monitoring programme.

Meanwhile, the only point on which almost all experts agree, is that
Australia鈥檚 rabbits will develop some resistance to RCV. Even a population
which, Cooke says, may have already been reduced from 200 or 300 million animals
to 100 million could recover鈥攋ust as it did from myxomatosis. 鈥淚t will
give us some breathing space,鈥� says Cooke. But only, according to Lord, if the
Australian government supports additional rabbit control measures, such as
fumigating and ripping up burrows, while populations are at their most
vulnerable.

鈥淚t鈥檚 very frustrating. It was thought that myxo was the magic bullet. It
wasn鈥檛. The same mistake is being repeated with rabbit calicivirus disease,鈥�
says Lord. Some rabbit and calicivirus experts in Europe and the US might agree
with this sentiment鈥攂ut for rather different reasons.