Rabbit Biocontrol Advisory Group
A Hundred Years of Rabbit Impacts,
and Future Control Options
Over the past 1000 years the natural environment of New
Zealand has been changed completely. Two waves of human
settlement, the first from neighbouring Polynesian islands, and
the second from the other side of the world, have seen to that.
The balance of species that had evolved in the absence of
humans and other mammals was thrown into disarray. Compared with
what was to come, the impact of the first Polynesian settlers was
slight. The real upheavals began with mass migrations from the
British Isles after 1840. In a concerted effort to recreate
England in the South Seas, new settlers introduced a vast array
of animals and plants, both commercial and recreational. They
felled or burned great native forests to make way for sheep and
The impact on the existing ecological systems was devastating.
The plan to transplant rural England turned sour. Some animals,
once free of their natural predators, bred uncontrollably. Not
only did they wreck existing ecosystems but worse, as far as the
farmers were concerned, they soon came to dominate much of the
land earmarked for farming.
The rabbit was one such creature. Ever since, farmers,
politicians and latterly scientists, have been battling to gain
the upper hand. Ferrets, stoats, cats and other natural enemies
have all been used to hunt down the wily rabbit. So have poisons,
guns and traps. Hundreds of millions of dollars were spent in the
fight; 50-80% of it from the taxpayers purse from 1950
until 1989. Now, all costs must be met by landowners.
With the aid of assorted methods of control, a manageable
rabbit population equilibrium was achieved across most of
developed pastoral New Zealand. Only in the high risk semi-arid
zone of the South Island did rabbits continue to be a serious
threat to the economy and conservation.
But the problem may not be contained within the tussock lands,
as rabbits occupy 55% of New Zealand and their "problem
status" appears to be widening once more.
In recent years it has been discovered that the ferret
population, the rabbits main natural predator, is widely
infected with tuberculosis. They have now been officially
declared Tb vectors. This potentially changes the whole focus of
"the rabbit problem."
Over much of rural New Zealand, the rabbit population has been
kept at an acceptable level by ferret and cat predation, rainfall
in association with soil type, and 1080.
Ferrets have been identified as a major problem themselves -
as spreaders of Tb - and rabbits are part of that problem as the
ferrets major food source. Instead of being a localised
problem, it becomes one of major concern for all cattle, dairy
and deer farmers.
The use of the poison 1080, a mainstay of rabbit control
activities since the mid-1950s, is also being questioned. Some
rabbit populations are bait and poison-shy. Despite the
poisons excellent safety record, some ask should a country
trading on its clean green image risk tarnishing this valuable
image by being the worlds largest user of 1080?
Rabbit Calicivirus Disease
A possible new element in this complex web could be a
naturally occurring biological control, the Rabbit Calicivirus
Disease. RCD first appeared in China in 1984 and is now present
in 41 countries. It is highly infectious and kills rabbits
quickly and quietly. There is no evidence to suggest it infects
other animals. It has been trialed by an Australia - New Zealand
consortium of science, farming, conservation and Government
agency interests for the past four years. New Zealand contributes
via the Ministry of Agriculture on behalf of the Ministries of
Agriculture, Conservation, Environment and Science.
The first known release in New Zealand was two couples set
down on Motuara Island in Queen Charlotte Sound by Captain James
Cook in 1777. They died.
Domestic rabbits of French origin were carried regularly on
early ships sailing the southern oceans. Along with pigs and
goats they were released on remote islands as emergency rations
The earliest settlers brought rabbits with them as a food
source. Doubtless some escaped. They were being sold as breeding
pairs in the Hutt Valley in 1842 and appear as a prize in a
Nelson ploughing contest in 1843. Acclimatisation societies took
great trouble to import breeding stock and distribute the
offspring for sporting purposes. Most domestic breeds failed to
become established in the wild, but some did. Eventually they
were supplanted by the ordinary wild European rabbit.
The Plague Begins
The rabbits released in 1858 on Rangitata Island, just inland
of the mouth of the Rangitata River, 120 km southwest of
Christchurch, were probably wild-type and may have been the first
to escape and cause serious damage.
By 1876 a Parliamentary Select Committee was investigating the
rabbit nuisance and reporting that "The mischief already
done is most serious, is increasing, and, unless some effectual
remedy be adopted, is likely to increase."
In the North Island, a few domesticated rabbits were seen in
Wairarapa from the mid-1850s, but it was the release of wild grey
rabbits by Mr Carter near Carterton around 1870 that started the
plague in this area.
Driven Off Land
Evidence from the South Island is skimpy, but in Marlborough
the first release was by George and Charles Keene in 1861 on
their Swyncombe Estate near Kaikoura. By 1882 theyd become
so numerous that the Keenes were forced to abandon the property.
In Southland and Central Otago the first documented
liberations were in 1848, but appear to have died out. Wild
rabbits were commonly sold into the mid-1860s. Within ten years
the game animals had become a problem of "immense
proportions." In the eight years 1877 - 84, 77 sheep runs
totalling 627,935 hectares were abandoned in Otago, mainly
because of rabbits.
Rabbit numbers peaked around 1890. In 1891 about 120,000 were
taken off the Molesworth and Tarndale runs in Marlborough by 40
men dogging and digging.
If there was a silver lining, it was in the burgeoning rabbit
skin export trade. In 1873, 33,000 skins were exported; in 1877,
nearly a million. By 1882 more than nine million were dispatched
and by 1894, 17 million.
The most suitable rabbit country in the South Island was
colonised by rabbits by 1900. The forested North Island took
longer to clear and rabbits spread more slowly. They thrived in
the drier east coast from Wairarapa to East Cape. The far north
was not colonised by rabbits until the 1950s.
Rabbits evolved in the Spanish peninsula with its hot dry
summers and warm moist winters. In New Zealand most of the
habitat was not suitable despite the lack of natural enemies. The
dense damp forests and tall tussock grasslands did not suit a
species adapted to short, dry forage conditions.
It was the land development practices of last century -
turning forests into pasture, and heavily grazing tall tussock -
that rapidly converted many areas into rabbit-friendly
Nowhere was this more so than in the tussock grasslands
occupying millions of hectares east of the Main Mountain Divide,
which runs up the centre of the South Island into the lower North
Between 1850 and 1880 these virgin tussock grasslands were
first occupied by sheep. In Otago, for instance, sheep numbers
increased from 59,000 in 1855 to 694,000 in 1861. Sheep numbers
doubled every three or four years. Grazing licences were issued
for this Crown-owned land on the condition that properties were
Fire and grazing
Fire was used as a basic management tool to create short
grassland more suited for sheep. Firing also created very
attractive conditions for the rabbits; especially when it was
also heavily grazed by sheep. By the mid-1870s rabbits had spread
throughout the tussock lands. The Rabbit Nuisance Act was passed
in 1876. Thirty-two tussock land properties totalling 300,000
hectares were abandoned as uneconomic between 1876-1879, having
suffered from the combined impact of pastoralism and rabbit
depredation. The native grasslands evolved in the absence of
1880 to 1950 has been labelled as the years of
"exploitative pastoralism", as farmers "sought to
make use of existing plant resources with virtually no
application of energy directly to their augmentation, maintenance
Over this time stock numbers on the hill and high country
tussock lands steadily declined. By the early 1950s, when numbers
levelled out, only 10% of the sheep numbers of the 1880s
Overgrazing became acute in periods of low wool and mutton
prices, when stock were held over for another season in the hope
of better prices. These lows often coincided with low rabbit skin
prices, thus fewer rabbits were killed. As a consequence the
deterioration of the grasslands accelerated. Overgrazing led to a
process of desertification in some areas.
Apart from the native falcon, there were no natural enemies to
slow the rabbit explosion. In the need to achieve balance,
European solutions were sought. Foxes were already banned because
of their taste for young lambs, but ferrets, weasels and stoats
were rapidly introduced; despite fears by some that they might
endanger native fauna.
Ferrets arrived in 1882 and the Department of Agriculture bred
up large numbers for release. The Department also advertised
widely in the British press for live stoats and weasels. Between
1884 and 1886, 4000 ferrets, 3099 weasels and 137 stoats were
liberated. Runholders joined in the breeding frenzy.
Domestic cats had already been pressed into service. In
November 1867 the Lyttelton Times reported their being released
to control rabbits. In 1885, Mr Acton-Adams brought 200 by wagon
from Christchurch to Tarndale and announced plans to run a
regular "four-horse express with cats." Despite one
load overturning and drowning in the Acheron River, he was to
turn out 1000 cats a year on Richmond Dale at 10 to 15 cents a
Similar stories abound in the North Island. One farmer posted
a man with a hatchet at the wagons exit shoot to chop off
the tail of each cat as it was released. This was to ensure they
would not be recaptured and sold again.
In 1882 the Governor was given the power to proclaim any
animal, such as a wild cat or stoat, to be the natural enemy of
the rabbit and to forbid the capturing or killing of such an
Weasels never became abundant, but stoats and ferrets
prospered. As feared by some, the stoats went into the bush and
killed many birds - as well as some rabbits. Ferrets were still
being bred for rabbit control in the 1920s. Cats spread into all
kinds of habitats with a varied diet that included rats, rabbits,
birds and lizards.
The release of so many carnivores failed to hold down the
rabbit population. Many predators were accidentally killed during
the trapping, gassing or poisoning rabbits. In 1876 at the
instigation of the powerful South Island landholders most
affected, the Rabbit Nuisance Act was passed. Rabbit boards were
to be set up and paid for by landowners, who also had to do
rabbit control work. In 1886, Government subsidies were
introduced, but it was not until 1920 that the board concept
began to grow.
Various controls methods were used. In some places fences were
erected. Up until the 1940s the main control methods were
poisoning with phosphorus or strychnine, shooting and trapping,
and fumigation of burrows with chloropicrin or smoke.
Killer Policy Begun
By 1947 the rabbit problem had again become serious. During
the Second World War, farms had become run-down and rabbit
control had been weak. Favourable climate during the 1940s may
also have contributed. With the Rabbit Nuisance Amendment Act
1947, total war was declared on the rabbit. A Rabbit Destruction
Council was established with an avowed "killer" policy.
Chairman from 1948 to 1965 was South Island farmer Bart Baker
whose mission of "getting the last rabbit" was
prosecuted with missionary zeal. Farmer levies were matched by a
Government subsidy. Rabbits and rabbit products were gradually
decommercialised by a progressive levy on all sales. By 1953 the
levy rose to 66.6% and in 1956, trading in rabbit products was
By 1960 live rabbits could only be kept by hospitals, zoos and
research centres. It is said the Rabbit Destruction Council even
wished to ban the sale of stuffed toy rabbits.
After rabbits were devalued the most commonly used poison was
arsenic. Labour intensive methods like trapping, shooting and
dogging remained popular, but were never cost-effective as a sole
method of control. In the mid-1950s sodium monofluoroacetate
(compound 1080) was trialled successfully and thereafter became
the standard poison used, usually with chopped carrot as bait.
Aerial Battle Begins
Aerial poisoning revolutionised the battle on remote country.
About 2800 tonnes of bait was dropped over nearly 5000 hours in
Night-shooting was used in the late 1950s after poisoning
drops, in order to bag the elusive "last rabbit." While
night-shooting tallies were impressive, theres little
evidence it really reduced already sparse populations. Its main
value was as a follow-up to poisoning.
In the early 1950s, before control had been achieved by other
means, attempts were made to introduce the controversial virus
that causes myxomatosis. Encouraged by successful Australian
releases, infected rabbits were released at 21 sites in the North
and South Islands. The virus died out in the winter of 1952.
Further trials in the summer of 1952-53 also failed for lack of
an insect vector to spread the disease.
ELIMINATION OR CONTROL
At the peak of the crisis, when the hillsides seemed to move
with rabbits, farmers flocked to join the "Kill all
rabbits" crusade. The first battles were won astonishingly
rapidly. In March 1948 nearly 20% of the land controlled by
Rabbit Boards was "heavily infested." Five years later
it was three per cent. Farmers practiced improved farming
techniques by spelling, seeding, top-dressing, growing crops and
The early successes were so spectacular that the Rabbit
Destruction Council adopted a total eradication policy in 1959.
They thought the last rabbit was in sight. But complacency was
growing amongst farmers, despite strident annual reports from the
Rabbit Destruction Council calling for greater effort. In 1965
the Rabbit Destruction Council warned
"Too many Rabbit Boards have departed from a
sound killer policy ... [and] are quite prepared to live
with some rabbits, so long as they are not doing any harm to
The rabbit never remains static; if complacency sets in
rabbits will soon increase."
By then, rabbits no longer threatened agricultural production.
In 1966 Bart Baker retired and by 1968 most boards were just
cropping the rabbits which remained at low density except in a
few hot spots such as Central Otago, parts of inland Canterbury,
Marlborough and East Coast North Island.
Total Eradication Abandoned
In 1971 the total eradication policy was officially abandoned.
By then the RDC had been replaced by a broader Agricultural Pests
Destruction Council, so as to accommodate other introduced pests
such as possums and rooks. From 1975 to 1980 research in the
Ministry of Agriculture concentrated on the need to develop
cost-effective management practices which reflected the actual
threat of rabbits in different habitats.
In the early 1980s the Government abandoned the rabbit
rate-link subsidy which had been basic to rabbit destruction
since the 1880s.
In 1983 a review committee reported that most vertebrate pests
were at satisfactory levels and recommended that:
- rabbits should not be considered differently from other
- land should be classified according to its suitability
- government funding should be phased out,
- land unsuitable for rabbits should not be
- rabbit rates.
Eventually a separate review of regional government and
resource law overtook the reorganisation of pest destruction. In
1989, it became a responsibility of the newly-formed regional
councils. Rabbit management had to compete for funds with other
demands of the community.
Only Central Otago and parts of inland Canterbury and
Marlborough, where the rabbit problem remained chronic, continued
to receive government support. This came via the Rabbit and Land
Management Programme, set up in 1989 following the
recommendations of a task force investigation.
Since the early 1970s some populations have remained stable,
controlled by a combination of climate, soil, predators and an
annual shoot. The exception was in the dry tussock grasslands of
Central Otago and Canterbury, where a resurgence of rabbits began
in the mid-1980s. This coincided with a run of dry years, the
emergence of bait and poison-shy rabbits, insufficient control
and over- reliance on poison. With the on-going reduction in
Government input to rabbit control this led to renewed calls from
farmers for the introduction of myxomatosis.
THE RABBIT NOW
By 1970 it was accepted that with improved farming practices,
populations of rabbits living on the developed pasture lands of
New Zealand rarely increased at all when control was relaxed.
Only on free draining soils in drier years, did rabbit
populations increase markedly.
Most rabbits live in small groups in scattered burrows or
above ground in thick scrub. On improved pasture more than 50% of
the adult does are pregnant for at least nine months of the year.
On average each doe produces around 45 young per year.
Young born in spring can themselves be breeding by late
summer. Average litter sizes are from four to six.
In some districts many young die in flooded nests or are
killed by ferrets. Cats and stoats kill many that do leave the
nest. Of adults, fewer than 50% survive for more than a year.
Wild populations are often stable for years on end.
THE CURRENT PROBLEM
The 1989 Rabbit and Land Management Task Force and the 1987
Parliamentary Commission summarised the situation as it stood in
the late 1980s.
- Rabbit numbers had been reduced by control measures (1080
mostly), and by pasture improvement and land management
to about 5% of the 1940-50 population.
- Rabbits remained a serious problem in the high risk
semi-arid tussock lands of the South Island, covering
about 400,000 hectares on 100 properties.
- Continual poisoning had resulted in bait and poison-shy
rabbits in some areas.
- Returns from conventional pastoralism could not finance
current methods in the long term.
- External funding, changes in land uses and additional
controls could all be needed.
- Rabbits remain an intermittent and expensive problem in
significant areas of both the North and South
Islands hill and high country.
The Parliamentary Commissioner for the Environment Helen
Hughes, in her 1987 report on the introduction of myxomatosis for
rabbit control, observed that paying for rabbit control from the
earnings from extensive pastoral production on high rabbit risk
lands did not, over the long term, appear to be economic.
Total Management: A Focus on Sustainability
In 1987 the Government, adopting the Parliamentary
Commissioner for the Environments recommendations that the
full complexity of land use and rabbit control be examined,
established a task force to accomplish this. In 1988 the task
force recommended a five year Rabbit and Land Management
This programme was to develop an integrated approach to rabbit
control. This involved landholders taking charge of controlling
the pest on their own land - something they had not been able to
do since 1947 - and integrating it with overall land management.
At the same time a new research and monitoring programme set
out to unravel the ecological, economic, institutional and social
elements of sustainable land use in the dry tussock grasslands.
There was growing recognition that the recent spread of
introduced weeds, such as hieracium, and high rabbit numbers are
partly symptoms of a 150-year decline in the carrying capacity of
the dry tussock lands as well as, in the case of the rabbit and
in some cases the sheep, a cause of the decline.
The worst-affected land is now of limited productive value for
pasture. Other uses such as plantation forestry are being
considered but it will be necessary first to reduce rabbit
numbers to provide a useful window of opportunity for change.
There will be daunting social as well as financial and
ecological difficulties in attempting to replace pastoralism as
the dominant land use. Rabbits, despite the gains made by the
Rabbit and Land Management Programme, are still a significant
foe. The spread of hieracium, declining product prices, rising
land values, increasing climatic variability and the constraints
of current pastoral leases all pose considerable challenges.
In a mid-1995 report for MAF, Manaaki Whenua Landcare
Research, Lincoln, updated the picture.
Rabbits, it concluded, contribute to many unresolved
land management issues. On the most rabbit-prone land they
are a major factor in determining the profitable and sustainable
use of the land. Rabbits are also pests on conservation
values, both directly by eating native plants and
indirectly by sustaining high densities of predators.
In 1992/93, a survey of 18 properties in the Rabbit and Land
Management Programme showed the costs of rabbit control exceeded
income in 10%, 75% and 90% of land in the moderate, high and
extreme rabbit-prone classes, respectively.
It concluded that while rabbit numbers had been reduced in all
but some intractable areas within the R&LMP areas in Otago
and Canterbury, a taxpayer and ratepayer subsidy of over 70% of
costs had been needed to achieve this.
The national costs of rabbits are at least $22 million
annually for the direct costs of control ($7.8 million from
landowners, $5.7 million from ratepayers and $450,000 from the
Department of Conservation), plus the unremitted costs to
production of $8 million.
Despite some change in farming practices as a result of
R&LMP, the economic viability of farming on problem land
remains doubtful. Of 18 R&LMP farms monitored in 1992/93, 13
were expected to run with a cash deficit.
This rather gloomy picture was not confined to the R&LMP
areas. A 1994 survey showed that large areas of South Island farm
lands with rabbit problems remained economically unviable in
current land uses.
From a conservation viewpoint, rabbits remain a pest. Not only
have they eliminated many small native inter-tussock herbs and
grasses, but indirectly they have had an adverse impact on
numerous native animals by reducing vegetative cover and
supporting predators. Our methods of trying to control rabbits
also increase the risks to non-target animals.
Ferret- Friend or Foe?
For several years now, rabbits have not been a significant
pastoral pest over much of the country. Much of the credit for
this must go to the large ferret and cat population that has
developed and kept rabbit numbers in check.
However, the ferrets days as the farmers friend
seem numbered. Recent research reveals that not only does its
diet include such endangered fauna as the yellow-eyed penguin,
but it carries bovine tuberculosis, which is a major threat to
the dairy, cattle and deer industries.
Surveys show up to 30% of ferrets are Tb carriers. To remove
this potential source of infection to New Zealands cattle
herds, the feral ferret population now needs to be reduced. This
could be done directly by a labour intensive programme of
poisoning and trapping, or indirectly, by cutting back its main
food source in many areas, the rabbit.
Whichever option is chosen, what has until now been considered
an economically acceptable level of rabbit population nation-wide
may have to be revised downwards. Also, if a concerted assault is
launched on the rabbits main predator, then new control
measures will have to be found to prevent a rapid explosion in
With constant herd testing and the pasteurisation of all milk
products, the risk of human infection from bovine Tb has been
removed. However, a reservoir of the disease remains alive in the
forests and scrublands surrounding our farms, carried by possums,
wild deer, possibly wild pigs and ferrets.
Cattle and farmed deer become infected when they lick dying or
dead possums - 2%-5% of which are infected - which stagger out of
the bush in the last stages of the disease.
Ferrets are a particular threat in South Canterbury and Otago,
where high rabbit populations have resulted in high ferret
Scare Tactics Fear
The very high standards of slaughterhouse hygiene in New
Zealand and the pasteurisation of all dairy products means there
is no risk of becoming infected with Tb from New Zealand exports.
The meat and dairy industry fear, however, that unscrupulous
rivals could use scare tactics against our products in our
Currently New Zealand does not meet the international bovine
Tb standard which is having no more than the equivalent of 0.2%
of herds infected. New Zealands score is between 1.5% and
2%. Amongst our EC rivals, only Ireland and Spain rate worse.
Australia, the USA and most Western European countries are well
Cattle and dairy farmers now want control programmes against
ferrets. That, of course, would risk triggering an explosion in
WHICH CONTROL METHOD?
Over the past 130 years many control methods have been
employed. Everything from the release of carriage loads of cats
to poisoning and gassing and ripping up burrows.
In recent times it has been recognised that rabbit management
must be an integrated part of total land management. However,
greater sophistication of battle techniques alone will not
suffice. New methods of controls are needed.
Existing tools are becoming too expensive to apply or, like
1080 and myxomatosis, are declining in their effectiveness. The
environmental concerns of New Zealanders, increasingly focused on
issues like animal welfare, chemical pollution, pure water and
food, and the reliability of scientific advice, are also
affecting the decision making process.
Pest controllers also have to battle the "cuddly
bunny" image held by most urban New Zealanders. As one
agricultural observer wrote in 1981, "No creature has had a
better press historically than rabbit. Except maybe fairies.
Although it is acknowledged there are bad fairies."
Consumer concerns in our distant markets and the preservation
of New Zealands valuable clean green marketing image, are
also influencing what we can do on-shore.
For forty years, 1080 poison has been the major, and most
successful weapon in the on-going battle against introduced pests
like rabbits and possums.
New Zealand currently uses nearly 4000kg of 1080 a year,
mostly on aerial possum control. In 1993/94 Regional Councils
used about 500kg on rabbit control. Australia, in comparison,
uses 250kg a year to control rabbits, foxes and dingoes, while in
the USA, a minute 0.5kg per year is used.
Until an acceptable form of biological control becomes
available, 1080 will remain the toxin of choice for large-scale
feral pest control. Abandoning its use would raise major
questions about New Zealands ability to preserve the
remains of its unique ecological heritage. It would also threaten
the acceptance of exported primary produce.
Like all poisons used in rabbit control, 1080 is lethal to
other species. Farm dogs are particularly susceptible. A small
range of insects may be susceptible and in the early days, birds
died after eating 1080 carrot bait and raspberry jam baits.
However, since the 1970s, baits have been dyed green to deter
birds and the small pieces screened out before application.
In the bush 1080 is biodegradable, broken down by
micro-organisms in the soil into non-toxic compounds. It has only
been detected at extremely low levels, and then only once in New
Zealand, in streams draining poisoned areas. Despite the
excellent safety and environmental record of 1080, there is
increasing public pressure to reduce its use. The disquiet
involves a mix of concerns for human safety, for the effects on
non-target species and for the environment as a whole. It is also
linked with a growing distrust of chemical and scientific
These concerns could translate into market access barriers in
the future if the protests are taken up overseas by consumer
groups or rival producers. Regular use of 1080 is also leading to
control failures, with bait and toxin-avoidance behaviour now
developing in rabbit populations in several areas in the South
This anticoagulant poison is commonly used on rats and is also
used by some farmers for rabbit control. There is increasing
evidence that other wild life (particularly predatory birds) are
being killed by pindone.
Since 1993 it has been legal to commercially process wild
rabbits. Critics are sceptical of its worth as a control measure.
They argue that the high rabbit population densities needed to
make harvesting cost-effective would seriously damage the land.
While there is an opportunity to develop businesses harvesting
and marketing wild rabbits, the value of carcasses, cost of
hunting and other market factors will probably limit the
contribution harvesting can make to rabbit control.
The success of the myxoma virus in killing rabbits after its
introduction in Australia in 1950 led to several attempts by
South Island high country farmers to have it released here. The
virus was officially released here in the early 1950s but, while
it killed the treated rabbits, it failed, seemingly through lack
of a host insect to spread the virus.
While all rabbit control methods cause some pain to the
victim, myxomatosis is a particularly drawn out ( 8 to 30 days)
process, accompanied by skin lesions, pus discharges, swollen
head and genitals, blindness and emaciation. Much of the
opposition to its release in New Zealand has concentrated on its
excessive inhumanity. There have also been fears that the rabbit
flea that would have to be brought in to spread the disease,
might spread to native species such as the kiwi.
In 1976,1987 and 1993 the Government rejected applications for
its introduction. By the time of the last request, a joint New
Zealand-Australian scientific team was trialling rabbit
calicivirus, a deadly disease first reported in domestic rabbits
in China in 1984.
RABBIT CALICIVIRUS DISEASE - (RCD)
RCD (initially named rabbit haemorrhagic disease - RHD) first
appeared in China in 1984 and quickly spread to Europe and
Mexico. In ten years it has spread to 41 countries on four
Chinese scientists claim that the original outbreak in China
followed the introduction of Angora rabbit breeding stock from
The disease spread via the international trade in domestic
rabbits. In Europe it first erupted in Italy in 1986, where it
killed 64 million farmed rabbits. It spread to France in 1987,
Spain in 1988, Germany in 1989 and Denmark and Sweden in 1990.
It quickly spread into the wild rabbit populations of Europe
and North Africa. An outbreak of RCD in Mexico was associated
with a shipment of rabbit meat from China in January 1989. Wild
Mexican rabbits - cottontail, jack-rabbit and volcano rabbit -
are not susceptible, so a wild reservoir of the disease was not
established. RCD was successfully eradicated from Mexico by
Impact on wild rabbits
In Europe, RCD has spread rapidly over wide areas, killing up
to 90% of adult wild rabbits. It now follows a two-year cycle.
Young rabbits up to 5-8 weeks old do not usually die from the
disease, but they develop antibodies and become immune.
They survive to become the breeding population for the next
year. However, the immunity passed on by the mothers is only
temporary. The next generation of young become susceptible and
RCD can spread through the population once again. This suggests
that if it is introduced into New Zealand it will be necessary to
combine RCD with other methods to provide the most effective
method of rabbit control.
RCD is transmitted by direct contact with other young rabbits
or via insects and possibly predators. It is not transmitted as
an aerosol on the wind. In contrast, myxomatosis can only be
transmitted by a biting or sucking insect.
Vaccines to protect domestic rabbits from RCD have been
developed in Europe. One is now registered in New Zealand as a
precaution against accidental importation of RCD from Australia.
If required it would be sold through normal veterinary outlets.
JOINT NEW ZEALAND & AUSTRALIAN RESEARCH PROGRAMME
In 1991, following a two-year investigation in Europe, New
Zealand and Australian scientists began a jointly-funded
three-year project at the high security CSIRO Australian Animal
Health Laboratories, Geelong, Victoria.
A strain of rabbit calicivirus was imported from the Czech
Republic and a breeding colony of wild rabbits established.
The research showed that RCD kills laboratory and wild rabbits
quickly and quietly. The virus proved highly lethal to Australian
and New Zealand adult wild rabbits, killing 99 out of 100.
Twenty-four hours after infection, the animals appear
increasingly lethargic and their temperature rises. They die 30
to 40 hours after infection. The disease causes the rapid
development of blood clots in major organs such as lungs, hearts
and kidney. These clots block blood vessels and result in death
from heart and respiratory failure. RCD does not cause
Rabbits up to 5-8 weeks old are less susceptible to the virus
than adults; however, the survivors excrete enough virus to kill
adult rabbits. There was no evidence that New Zealand and
Australian rabbits have had any prior exposure to any similar
viruses. This means they are unlikely to have any resistance to
No harm to other animals
The European rabbit, Oryctolagus cuniculus, is the only
species known to be susceptible to RCD. Rabbit calicivirus is
related to European Brown Hare Syndrome (EBHS) and is likely to
have evolved from a common source. However, laboratory studies
show it is not possible to infect rabbits with EBHS virus, or
hares - including Australian and New Zealand wild examples - with
The Australian trials confirm the evidence from 40 countries
that RCD is so specific that no other animals are susceptible.
None of the 28 non-target species tested was infected by the
virus. All animals were exposed to the equivalent of a thousand
The animals chosen for testing included a representative range
of New Zealand and Australia fauna:
Domestic animals: horses, cattle, sheep, deer, goats,
pigs, dogs, cats and fowls.
Feral animal: foxes, hares - including New Zealand
examples, ferrets, rats and mice.
Australian native mammals: bush rats, spinifex hopping
mice, plains rats, fat-tailed dunnarts, northern brown
bandicoots, brush-tailed bettongs, tammar wallabies and
Birds: long-billed corellas, feral pigeons, silver
gulls, brown falcons and emus.
Reptile: common blue-tongue lizards.
Emus and lizards were tested at New Zealands request.
Tests were also conducted on the kiwi - which has a lower blood
temperature than other birds - and the short-tail bat, New
Zealands only native land mammal. While the bats were not
affected, the kiwis produced a low level of antibodies; a
reaction to the amount of foreign protein they were injected
with. They were not infected by the rabbit calicivirus and the
virus did not multiply in the kiwis. During the trials kiwis
continued to thrive and gain weight. The two surviving animals
will be returned to DoCs custody. Researchers concluded
that RCD was no threat to kiwis.
An area that is being researched in New Zealand and elsewhere
is the impact on the rabbits predators, and consequences
for other species if those predators no longer have plentiful
supplies of rabbits to hunt. Initial findings suggest that the
greatest impact on native species is likely to occur in the
period shortly after a successful introduction of RCD, and before
predator numbers begin to fall because of their diminished food
supply. There are unlikely to be any long- term impacts provided
any native species at risk during the transition period are
identified and predators are controlled in their habitats.
Ability to mutate?
Viruses are found in a vast array of animal and plant life and
it can be safely assumed that every species of animal and plant
will have its own particular range of viruses.
Viruses are infectious micro-organisms that are characterised
by their tiny size. They grow and reproduce inside living host
cells, taking them over and adapting them for their own purposes.
Usually a specific virus can only infect a single host - as
appears to be the case with RCD - or a group of related hosts.
This host specificity is often explained by proposing that
viruses have co-evolved with their particular hosts.
Some go further and suggest they are derived from the genetic
material of their particular host.
However some viruses have a particularly broad host range. The
rabies virus, which is infectious for all warm blooded animals,
is possibly the most notorious example. The calicivirus that
infects seals, fish and pigs is another.
For this reason it is important to comprehensively test a new
virus such as RCD before considering its release in the wild.
Viruses differ also in their ability to change.
The human influenza viruss ability to constantly throw
up new strains is well-known to all of us. However, the
Caliciviridae family of viruses, of which RCD is one, belongs to
a much more stable grouping of viruses.
See other material on viruses in the resource kit.
Island Quarantine trials
In March 1995, a two-year long trial of the virus under
natural conditions began on Wardang Island, 5 kilometres off the
South Australian coast. To begin, there was to be a year of
trials using high-security pens constructed around warrens. In
1996 more extensive field trials were programmed.
In September 1995, the virus escaped, first from the
quarantine pen to other parts of the island, then across to the
mainland. This coincided with the arrival on Wardang Island of
the Australian bashfully. The flies apparently acted as a vector
for the virus; although researchers have not proven this to date.
The virus has now spread over about one-third of south
Australia, east into NSW around Broken Hill and north into
southern Queensland. It has devastated rabbit populations,
killing over 90% in most areas. Australian authorities are now
considering whether to formally release RCD as a bio-control.
It is natural that present day New Zealanders are wary when it
comes to introducing new bio-controls. From an ecological and
crop damage perspective, a number of the plants and animals
introduced over the last 150 years have been major disasters. The
rabbit was certainly one of these.
19th Century attempts at biological control were a mixed
blessing. The importation of rabbit predators, for instance, did
ultimately have a major impact on rabbit numbers. But the cost to
native birds has been disastrous.
But, rabbit predators apart, most recent introductions or
proposed introductions of biological controls into New Zealand
have passed almost unnoticed.
Up until 1987, 230 biological agents had been introduced to
control insects and weeds, of which over 70 have become
established. These introductions were made following an
increasingly sophisticated check on the potential risk to
beneficial insects and native species.
The search for viral and bacterial controls for vertebrate
pests such as rabbits and possums is a more recent phenomenon. If
the trials currently underway prove successful, weapons such as
RCD have the potential to become greener answers to growing
public concerns about toxic compounds like 1080.
If New Zealand is to deliver on its commitments to sustainable
management, now and in the future, new ways of managing pests
will have to be found.
In Australia a long-term research programme is investigating
the possibility of genetically engineering myxoma virus to
control the fertility of rabbits.
Scientist believe it is possible to modify a less virulent
strain of the virus that causes myxomatosis to include genes that
will prevent conception in rabbits. This approach directly
attacks the feature which gives the rabbit its great advantage;
its high reproductive capacity. The aim is to trigger the
rabbits immune system to attack sperm as an enemy.
The researchers are also exploring ways this new technology
could be applied to other feral pests, especially foxes in
Australia and possums in New Zealand.
The application of these technologies in New Zealand will, as
for RCD, also require the introduction of a new organism (the
myxoma virus for rabbits) plus, in some cases, insects to spread