Present status (5.1)
Assessment of current control (5.2)
Summary of rabbit problem
2. RCV
classification, history and origins
3. Epidemiology of RCV
4. RCV as a biocontrol agent
5. Host range of the virus
6. Risk of Human Infection
7.
Probability of RCV to mutate and consequences of mutation of RCV
8. The effects of
introduction on New Zealand flora and fauna
9. Programme to release the virus
Report on the Application to Import RCD into New Zealand
By Brian Boag
25 February 1997
Before giving my detailed response, mainly on the ecological implications of the possible introduction of RCD, I should like to state that because of the categorical statements made in the letter of 31 January 1997 concerning 1) The fact that vaccines will be available for use in New Zealand if RCD is introduced and 2) the timetable of the Biosecurity Act which means it will not be enforced until early 1998 no comments on these subjects will be made in this
The rabbit problem historically has unquestionably been considerably greater and more widespread than it is at present and this has been well documented. The introduction of ferrets and cats and application of the range of control techniques available, particularly poisoning, has been responsible for the decrease in the rabbit problem to the status it is in today. However, it is quite evident that New Zealand still has a very serious problem in the high rabbit prone areas especially in Central Otago and the Mackenzie Basin. Within these areas it is clear from many of the submission from farmers and other organisations intimately concerned with farming that rabbits have a severe impact of pastoral agriculture.
The environmental impact of rabbits is more difficult to assess both quantitatively and qualitatively. It is obvious that, in rabbit prone areas, where rabbits have been excluded the growth of grass is considerably enhanced. However, the effect rabbits have on the plant community structure, especially the native plants, is not known to any extent. From what little information there is e.g. Norbury, D. 1996 (DOC Science for Conservation 34, 1-32) it would seem that rabbits have a net harmful effect on the native flora. Similarly little is known of the direct or indirect effect on native vertebrates or invertebrates although there are cases e.g. Cronwell Chafer beetle where rabbits have been implicated in endangering their survival.
I would therefore suggest that at present most of the evidence suggest that rabbits have a significant harmful impact on New Zealand's native flora and fauna but more work is needed on quality and to quantify this especially in the rabbit prone areas.
The impact of the present rabbit problem on other economic values is extremely difficult to access. Evidence given in a considerable number of the public responses would suggest that some rural communities are at threat if farmers and their families are forced off the land by the rabbit problem. It would seem that even if the rabbit problem did not get worse some families might still have to move our due to the decrease in wool prices.
A more detailed study of the impact of pests including rabbits and possums on rural economies should be undertaken by an appropriately qualified person.
Assessment of current control (5.2)
To comment upon the present status of the control of rabbits etc. I shall briefly go through the different control options and say which I believe are important and when I see the problems or opportunities to incorporate them into an integrated control strategy. Much of this information is available in the Rabbit Manages Fact Pack (Foran, B., 1991).
a) Poisoning
Over most of the land mass of New Zealand rabbits either do not exist or are kept in control below economic threshold levels mainly by predators. In the areas where rabbits are a problem, these problems can be severe and the most widely used control measure seems to be either 1080 or Pindone poison. Poisons are not used against rabbits in the British Isles but from my very limited experience of seeing how poisoning works in New Zealand it would seem that although they are expensive they are also very effective. Bait avoidance or neophobia would seem to be associated with the inappropriate use of 1080. In theory, at least, 1080 and Pindone could, if used properly, continue to be used into the foreseeable future. However, my impression is that as a) there is no antidote to 1080 and b) dogs and other non-target creatures can and have been killed by its use, public pressure will increase for at least restrictions to be put upon the use of 1080 or even its withdrawal. This point must be considered when weighing up the pros and cons of the introduction of RCD to New Zealand.
b) Fencing
Traditional net fencing is used extensively throughout New Zealand and is effective. Its use could be extended if the costs were not so great. Although occasionally rabbits can climb over it, especially when there is deep snow on the ground, it can last for years and needs little upkeep although routine inspection is critical.
The use of electric netting or fencing specifically designed for rabbit control is used widely in the UK to protect crops. As an easily moved, temporary fence it might have its place as a part of a control strategy in stopping or slowing down the spread of RCD infected animals into uninfected areas i.e. if RCD became established in New Zealand. This would probably not stop the eventual spread from infected areas but it might allow time for predators to be controlled.
c) Habitat destruction
Evidence from many parts of the world have shown that this can have a long term effect but is not always practical. Warren ripping can only be undertaken when the soil is not too stoney and the terrain relatively flat. I feel there is little scope to increase this control technique further in the worse infected areas of New Zealand.
d) Fumigants
Little was said in the admissions from the general public on the use of fumigants as an additional or alternative technique to RCD. I believe carbon monoxide, phosphine and chloropicrin have been used in New Zealand while cyanide is used extensively in Great Britain. Where large proportions of the rabbit population live in burrows these chemicals can, if used correctly, be very effective. However, they are expensive, can be very dangerous in inexperienced hands and the terrain needs to lend itself for their use.
In Central Otago and the Mackenzie Basin there is probably little scope for the use of fumigants to be extended due to the number of rabbits which I believe do not live in burrows and the stoney undulating nature of the terrain.
e) Shooting
A number of the submissions from the general public mentioned shooting as an environmentally acceptable humane way of controlling rabbits which could also give employment to people who were out of work. On the face of it this looks feasible but there are many practical problems associated with its widespread implementation, not least the fact that many who are out of work live a long way from where rabbits are a problem. Under certain circumstances i.e. lamping at night with a .22 rifle where rabbit numbers are low, e.g. after poisoning them can make sense.
I would therefore suggest that shooting cannot be considered a practical alternative to the use of poisoning or possibly the introduction of RCD.
f) Trapping
I believe leg hold or gin traps are allowed in New Zealand but they are labour intensive as are the use of snares which are widely used in Britain. However these traps can be considered by some as inhumane and catch unwanted animals.
More recently "box traps" set along fences have been enjoying a considerable revival in Britain where rabbit numbers are high and these I believe could be exploited in New Zealand. These traps, acting on a trap door principle, catch the animals alive and unharmed. Rabbits caught in them can be despatched humanely and sold for food while any other animals or birds e.g. hedgehogs can be released unharmed. However digging the boxes into the terrain where rabbits are a problem could be difficult and expensive.
g) Repellents
The use of repellents has been used experimentally to protect high value crops. These repellents are usually based on egg extracts or on predator odours but their cost has meant they have not been used to manage rabbit populations in an agricultural context. They have also been shown, when used down rabbit burrows, to significantly reduce rabbit numbers fora number of months (Boag & Mlotkiewicz (1994) J. Chem. Ecol. 20, 631-637).
Within the context of the possible introduction of RCD into New Zealand, repellents could have a part to play in acting as a temporary invisible barrier to keep infected and uninfected populations apart but only for a limited period as their effectiveness would probably disappear after a few weeks/months.
h) Biological control
Leaving aside myxomatosis and RCD there are two other possible developments which are worth mentioning. The first is the use of contraceptive techniques which are at present being investigated by researchers in Australia and New Zealand. If successful these would have a major impact on the rabbit problem. The second is the possibility of genetically engineering certain crops to protect them from rabbit damage e.g. rabbits eat tulip bulbs but not daffodils, and will eat most trees but not elder. However, both of these techniques depend upon genetic engineering in one form or another and it is by no means certain that the general public will accept the perceived risk associated with these techniques. It is also likely that it will take many years before these techniques could be practically exploited.
New Zealand has a serious rabbit problem which is confined to a relatively small area of its land mass. There is a range of conventional control measures but these have not managed to reduce the rabbit populations in the worse areas to below economic threshold levels or levels where damage to the environment is minimal. The cost of increasing the use of conventional control measures to that which might control the rabbit populations cannot be carried by the existing farming enterprises. If nothing is done some farms will become bankrupt and the environment will continue to suffer from soil erosion and invasion of weeds e.g. hieracium.
2. RCV classification, history and origins
I do not feel competent to comment on this.
I do not know the methods of transmission of RCV but should like to make the following observations.
a) It would seem from what has happened in Great Britain and Australia that the virus can "jump" considerable distances and does not necessarily spread gradually on a continuous front. This means that it is probable that if RCV is introduced into New Zealand at the 12 recommended sites it will eventually escape outside these test areas. If this does occur then contingency plans for the reduction of predators close to areas where rare fauna exist must be in place and acted upon swiftly.
b) From my recent experience with the New Zealand flatworm (an exotic pest introduced from New Zealand into Great Britain) I believe that man either deliberately or quite innocently could be responsible for the spread of RCV into and around New Zealand sometime in the foreseeable future. I realise that its introduction into Australia was because man had brought it purposely to an inshore island but without man's deliberate help it had previously spread to 40 countries throughout the world including islands e.g. the British Isles.
The effectiveness and suitability of RCD virus as a biocontrol agent (5.3)
I have read all of the submissions including the analysis undertaken by Taylor Baines and Associates. It would seem that there is a great deal of conflicting "expert" opinion on this matter. Notwithstanding these I feel RCD virus could be considered as a suitable candidate as a biocontrol agent.
I believe that if RCV is introduced it is likely that the New Zealand experience will be similar to that found in Australia and 90% or more of the rabbits will be killed in the rabbit prone areas of Central Otago and the Mackenzie Basin but that where rabbit densities are less then proportionally fewer rabbits will be killed. The main pastoral and environmental goals would possibly be achieved in the short term without conventional control but I very strongly suggest that to exploit the use of the introduction of the virus to the full, follow up control measures e.g. poisoning then possibly shooting or other appropriate techniques should be employed (see previous comments).
The ability of RCD virus to maintain the pastoral/environment goal over time is problematic and would need skilful management. I feel it is imperative that the "window of opportunity" afforded by any introduction of the virus is used to change the type of farming enterprise in the rabbit prone areas. It would be folly to increase the stocking rate of sheep to take the place of the rabbits which had been killed. This would leave conditions in which we know rabbits could thrive and they could return to. It would be better to, where possible, irreversibly change the habitat so rabbits could not return e.g. introduce trees. The search for other alternative land uses to sheep rearing should be a high priority and receive possible government support.
(An overall assessment of the ability of RCD virus to maintain the stated pastoral goal over time also depends upon how the RCD virus is to be deployed and is given in my reply to topic 9).
The humaneness of the virus is a subjective point and has been covered in the initial submission. However, while I should like to see any rabbit that has to die do so instantaneously and without pain e.g. shooting, the death from RCV is to all accounts preferable to that occurring when rabbits are infected with myxomatosis, (I have post-mortemed hundreds suffering from myxomatosis) coccidiosis or broken teeth.
In hindsight I believe the testing of other species of animals with RCV could have been more rigorously undertaken. In particular I should have liked larger samples of animals and the tests to have run for a longer period.
I am not an expert in this field but have attended European Union meetings in Brussels where a similar calicivurus (European Brown Hare Syndrome) was discussed. The consensus of opinion was that this calicivirus was restricted to hares and had not "jumped species" to the rabbit and to date I believe most of the evidence would suggest that RCV is specific to the rabbit. The fact that antibodies have been found in a range of animals artificially injected with the virus does not surprise me. Plant viruses are injected into animals to produce antibodies but no one suggests these animals are necessarily infected or adversely harmed by the plant viruses.
The fact that RCD occurs in many countries throughout the world with, so far, no proven switch to another animal or bird would suggest that it is unlikely to occur in New Zealand i.e. why should it occur in New Zealand when it has not occurred anywhere else? However, it is important to point out that it would be wrong to suggest that it could never happen, but I believe the chances are very low and there is a negligible risk to native New Zealand wildlife especially birds.
This does not worry me personally but I can understand the concern expressed by the general public especially with examples like BSE so much in the news. However the numbers of contacts between people and rabbits and hares infected with either the pathogenic or non-pathogenic strains of RCV and European Brown Hare Syndrome in Europe and the rest of the world must be many times greater than that likely to occur in New Zealand. There have been no proven connections between RCV and humans although there are suggestions that it may have occurred e.g. "The Bailey Case" (submission 38). To answer some of the concerns expressed by Dr A Smith and others I should like to see tests sensitive enough to detect RCV in humans and animals undertaken in Australia. I also do not see why RCV trials should not be undertaken on human volunteers.
7. Probability of RCV to mutate and consequences of mutation of RCV
Rabbit calicivirus has possibly mutated in the relatively recent past either from another animal "host switching" or from a non-virulent strain to a virulent strain and there is no reason to suggest it could not happen again. However, if myxomatosis is taken as a model, then, in the rabbit, the virus is likely to become less virulent with time and the rabbit will also develop resistance to the virus. How fast this happens, if it does, is a moot point and answers will only become evident with time. The chances of caliciviruses mutating and changing host range has it seems possibly occurred already but lack of knowledge makes an objective assessment of the risk associated with RCD virtually impossible.
8. The effects of introduction on New Zealand flora and fauna
The effects, positive and negative on RCD virus (5.5)
The effect of RCD on non-target species is difficult to assess but as outlined in the original submission are probably beneficial. If the native flora can compete with introduced weeds and become widely established then perceived wisdom would suggest that much of the associated fauna would also return and increase. However there is an obvious problem with the rabbit predators. In the short term i.e. up to 1 year after an introduction of RCD, then it is likely that prey switching will increase the predatory pressure on other prey species especially where rabbit populations were high. For some of the rabbit prone areas this will probably not be much of a problem but it could be very important where there are low populations of rare native fauna at risk (see 5.6 under topic 9).
The effect of the introduction of RCD on rabbit populations will depend upon rabbit densities. Where they are high probably 90% of rabbits could be killed (c.f. the Australian experience). This in turn might lead to the virus dying out and not maintaining itself. In low densities rabbit to rabbit contact may normally occur only occasionally and the infection would not become established or spread. To test this I would recommend that if RCD is to be introduced at least some of the release sites are in low density rabbit areas. Results from these sites would have relevance for large areas of New Zealand.
The effect of the RCD virus on indigenous flora and fauna and natural communities and the integrity of indigenous ecosystems occurring in the most of New Zealand will be negligible. Only in areas where rabbit populations are high is any effect probably going to be significant and in these areas the overall effect is likely to be beneficial. Similarly any effect on valued introduced flora and fauna e.g. hares could be positive (Hare numbers increased after myxomatosis reduced rabbit numbers in the British Isles). However the effect on other plans and animals could be negative it depends upon what the "valued" introduced flora and fauna is.
As far as the effect of RCD on the intrinsic value of ecosystems and ecosystem stability in the Central Otago and Mackenzie Basin is concerned I believe the removal of rabbits would, in general, be a good thing. While some introduced weeds may increase soil erosion and other indigenous plants would increase.
The effect of RCD virus on social well being of the people and indirectly on their health is difficult to assess. I would hope that the type of farming enterprise i.e. sheep farming would change after any introduction of RCD because if it remains I believe the rabbit problem will return. However, if rural communities can be maintained under alternatives, e.g. forestry, tourism etc. then in the long term the introduction of RCD may have a beneficial effect on the social well being of the people.
The prey switch problem
Much of the answer to this has been given in response to Topic 7 above. However my comments on the prey switch problem were not given and are as follows. Rabbits can be a major prey items of feral cats and ferrets and if rabbits are removed then there will be an increased pressure on other prey items, including indigenous numbers of the New Zealand fauna. Where these predator pressures are likely to result in the decrease of the indigenous fauna to below sustainable levels it is imperative that there is a concerted drive, financed by the government, for control of the predators. This would have to cover a large enough area to stop predators from neighbouring areas taking over from those killed. Predator control would not have to be continued indefinitely as lack of prey would probably reduce the reproductive capacity of the feral cats and ferrets. Recent evidence (since the submissions were made) supports this hypothesis e.g. Villafuerte et al. (1966) (Effect on red fox litter size and diet after RHD in north-eastern Spain. J. Zool. 240, 764-767) found litter size in semi-arid areas of Spain to be significantly reduced after RHD. However, due to the lack of knowledge of the food webs occurring in nature there maybe unexpected developments e.g. if the reduction in rabbits leads to a reduction in cat and ferret predators, rat numbers could increase and these in turn adversely affect indigenous
9. Programme to release the virus
The intended programme in which RCD virus is proposed to be used (5.4)
The outlined plan in the original submission was generally sound but I would have like some of the 12 proposed sites to be in areas where rabbit populations were low. This would allow models to be produced which would be applicable throughout most of New Zealand and not just to a relatively small amount of its land mass.
Ideally the chosen release sites should be isolated from surrounding rabbit infested land. The virus should be deployed throughout any one chosen site simultaneously via carrot bait or something similar. The timing is critical and the New Zealand winter has been recommended in the original submission as the rabbit population will be made up of older animals which will be susceptible to the virus. I agree in general with this suggestion but believe it could possibly be further refined depending on how it was introduced. My experience in observing the epidemiology of myxomatosis over a 20 year period would suggest that the best time for the spread of the disease would be when rabbits are most mobile i.e. early winter. However if infected bait is spread throughout the area as recommended then late winter/early spring would be preferred as, in theory, the whole of the population should be susceptible. However in areas where rabbits breed all the year round it is possible that a different optimum strategy should be developed.
Approximately 1 week after the introduction of RCD (i.e. enough time for initially infected rabbits to die, and infect and kill other rabbits) poisoning and then shooting or other conventional appropriate control programmes should be introduced to maximise the kill.
A very valid point made during the submissions was that if RCD does die out it should probably not be left to individual farmers to determine when it will be reintroduced as ad hoc applications might increase the rate at which resistance to the virus builds up. The release of the virus should be controlled by the local council and probably be used not more than once every two years.
The financial underpinning of the RCD release should be done by central government as should a monitoring programme which could produce data to help stop the build up of any resistance etc.
The likely success and costs employed to ameliorate negative impacts (5.6)
The major cost of ameliorating the negative impacts of the introduction of RCD will probably be controlling the predators especially around sensitive areas where rare indigenous fauna are present. However, the cost of supplying vaccines to domesticated rabbits and laboratory colonies may also be considerable and should in the short-term be carried by central government.
Contact for Enquiries
Manager, Strategic Science Team
MAF Biosecurity New Zealand
PO Box 2526
Wellington
NEW ZEALAND
Phone: +64 4 894 0115
Fax: +64 4 894 0731
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