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Rabbit Biocontrol Advisory Group

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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 cattle.

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.

New Equilibrium?

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 taxpayer’s 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 rabbit’s 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 ferret’s 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 poison’s excellent safety record, some ask should a country trading on its clean green image risk tarnishing this valuable image by being the world’s 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.

RABBITS GALORE

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 for castaways.

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 they’d 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.

ENVIRONMENTAL CATASTROPHE

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 environments.

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 Island.

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 quickly stocked.

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 herbivores.

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 or replacement."

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 remained.

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.

RABBIT CONTROL

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.

Wildcat Express

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 head.

Similar stories abound in the North Island. One farmer posted a man with a hatchet at the wagon’s 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 animal.

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.

Predators Fail

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 totally banned.

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 1953-54.

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, there’s 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 clearing scrub.

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 production.

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 farm pests,
  • land should be classified according to its suitability for rabbits,
  • government funding should be phased out,
  • land unsuitable for rabbits should not be
  • rabbit rates.

Regional Councils

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 Island’s 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 Environment’s 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 Programme.

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 ferret’s 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 Zealand’s 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 rabbit’s main predator, then new control measures will have to be found to prevent a rapid explosion in rabbit numbers.

BOVINE TUBERCULOSIS

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 numbers.

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 overseas markets.

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 Zealand’s 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 below.

Cattle and dairy farmers now want control programmes against ferrets. That, of course, would risk triggering an explosion in rabbit numbers.

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.

Cuddly Bunny

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 Zealand’s 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 Zealand’s ability to preserve the remains of its unique ecological heritage. It would also threaten the acceptance of exported primary produce.

Biodegradable

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 solutions.

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 Island.

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.

MYXOMATOSIS

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)

Origin

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 continents.

Chinese scientists claim that the original outbreak in China followed the introduction of Angora rabbit breeding stock from Germany.

Quickly Spread

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 September 1989.

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

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.

Effects

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 haemorrhaging (bleeding).

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 RCD.

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 RCD.

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 rabbit-lethal doses.

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 brushtailed possums.

Birds: long-billed corellas, feral pigeons, silver gulls, brown falcons and emus.

Reptile: common blue-tongue lizards.

Emus and lizards were tested at New Zealand’s request. Tests were also conducted on the kiwi - which has a lower blood temperature than other birds - and the short-tail bat, New Zealand’s 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 DoC’s 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 rabbit’s 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 virus’s 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.

THE FUTURE

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.

IMMUNOCONTRACEPTION

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 rabbit’s 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 them.

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