PREAMBLE
5.3
THE EFFECTIVENESS AND SUITABILITY OF RCD VIRUS AS A BIOCONTROL AGENT IN NEW ZEALAND
5.4
THE INTENDED PROGRAMME FOR WHICH RCD VIRUS IS PROPOSED TO BE USED
5.5 THE
EFFECTS OF THE RCD VIRUS ON NON-TARGET AND TARGET SPECIES.
5.6
THE LIKELY SUCCESS AND COSTS OF MEASURES WHICH CAN BE EMPLOYED TO AMELIORATE NEGATIVE
IMPACTS SUCH AS THOSE ON DOMESTICATED AND LABORATORY RABBITS
5.7
CHARACTERISTICS OF THE VIRUS ITSELF AND ITS METHOD OF PRODUCTION AND DISPERSION.
5.8 OTHER RELEVANT MATTERS
PROPOSAL FOR THE IMPORTATION OF THE RABBIT CALICIVIRUS DISEASE VIRUS (RCD VIRUS) AS A BIOCONTROL AGENT FOR RABBITS
REPORT PREPARED FOR THE CHIEF VETERINARY OFFICER
by
LEKTOS CONSULTING LIMITED
Palmerston North
February 1997
In preparing this review Lektos Consulting Limited has found it necessary to address some issues as if it is inevitable that permission to import the virus will be given. It is important that such issues are addressed as part of the overall reviewing process but the form or wording of our comments should not be interpreted as implying that Lektos is thereby in favour or otherwise of the importation of RCD virus (RCDV) into New
This review is specifically targeted upon those areas of the subject on which opinion has been sought from us by the Chief Veterinary Officer and are within our area of expertise. It is also our understanding that this review is likely to become a public document. For the last reason we have tried to avoid abstruse and overtechnical arguments or discussions in order that the interested public are informed and are able to assimilate easily the incorporated information and use it as they think fit.
5.3 THE EFFECTIVENESS AND SUITABILITY OF RCD VIRUS AS A BIOCONTROL AGENT IN NEW ZEALAND
The success or otherwise of RCD in achieving stated goals is dependent first of all upon the likely behaviour of the virus/disease in the field. Although the Applicants correctly state that it is not possible to give "A definitive forecast of how RCD might spread and recur in New Zealand......." There is an implied suggestion by the applicants which is echoed by many of the supporting submissions that RCD will behave in New Zealand (and especially in the rabbit prone areas of the South Island) in a manner close to the Australian experience.
The pattern of spread in Australia has some unusual features. The vastness of Australia probably makes detailed epidemiological observations more difficult than in other situations but two notable features appear to be:-
(1) an apparent ability to travel rapidly over long distances (one cynic has said as fast as a graziers four wheel drive!) and :
(2) a somewhat slower "infilling".
The epidemiological components which might explain these phenomena have not been adequately determined although "bush flies" are currently in favour as important vectors. The questions of "vectors" and survival of the virus outside of the rabbit in varying local and seasonal environments and in/on any carrier animals are not fully resolved for Australia and remain open for the New Zealand environment. Another unknown relates to the immune status and its degree of homogeneity in the New Zealand rabbit population. Although a small sample of rabbits from this country have been shown to be susceptible to RCD we are not aware of any statistically based sampling being carried out which would exclude the possibility of subpopulations having a degree of immunity. A theoretical possibility exists that some populations might be infected with a non-pathogenic calicivirus which confers cross-immunity to RCD. The uncertain history of foundation of various rabbit populations in New Zealand make this a possibility (although remote). More recent introductions of domesticated/laboratory rabbits may also represent a means whereby non pathogenic strains of calicivirus may have become introduced into this country.
The Decision Maker would be wise to consider the above matters with special care because they are fundamental issues when balances are made between the chances of success versus the prospects of adverse consequences.
In the absence of more specific information on vectors from Australia it seems reasonable to take note that the range and number and quantitative presence of potential vectors in New Zealand is likely to be less in comparison to Australia with its much greater vertebrate and invertebrate biodiversity. This may therefore mean that spread in New Zealand will be less spectacular and the option of using the virus as a "biocide" referred to by the applicants may become a necessity. If this were to prove to be so, the field costs of its use and virus production costs would be much higher than if natural transmission proves to be the predominant epidemiological feature of the disease in this country. However, poor natural (vector) transmission would allow more precision in the use of RCDV and could reduce the risk of exposure of domesticated rabbits to the virus and might even allow a restriction of the virus to certain parts of the country.
Given the uncertainty associated with the role of vectors in New Zealand and the physical and other environmental differences between the general landscapes of Australia and New Zealand it is our opinion that in the short term the performance of RCDV as a rabbit control agent could well be less spectacular in New Zealand as compared to Australia but used wisely it may have a longer useful life.
The actual techniques and feasibility of use of RCDV as a biocide need to be studied in more detail and approved parameters for such use should be incorporated in the conditions associated with any approval to import the virus.
The option of preliminary trials on an island off New Zealand have not been canvassed in the application although we assume they have been considered. Because of the Wardang Island experience it should not be assumed that an off shore island study under the environmental conditions prevailing in New Zealand necessarily make an escape of the virus to the mainland a certainty. The reason for the escape from Wardang remain unknown. Although about 24 New Zealand offshore and outlying islands are said to have rabbits very few of these would appear to be suitable for such trial work. Nevertheless submissions have emphasised the need to closely monitor the events associated with any introduction of RCD. The Australian experience has indicated the difficulty of monitoring an outbreak as it spreads erratically through varying and vast landscapes. A suitable trial on an offshore island would enhance the chances of useful information flowing from associated and well planned monitoring over the restricted area such a study site would provide.
It is difficult not to conclude that the release programme intended by the applicants is an unfortunate compromise between a desire to use the virus as widely and as soon as possible and a more measured need to study the characteristics of the disease under New Zealand conditions before a more effective management strategy can be formulated.
Two alternative assumptions must be made in answering this question:
That the virus/disease would behave in New Zealand in a manner broadly similar to that shown in Australia.
OR
That the virus/disease spreads poorly in New Zealand and requires repeated re- introduction at target sites.
If the virus behaves in "Australian Fashion", for basic biological reasons, it should be assumed that there will be natural selection among rabbits for resistance to clinical disease and some attenuation of the RCD virus so that it does not completely destroy its rabbit host. The critical questions are which of these phenomena will occur first and when. Because of the confounding variable of endemic immunity in rabbit populations in Europe epidemiological data from that source is of very limited applicability to countries such as Australia (and New Zealand). It is therefore most likely that the first hard evidence on the emergence of resistant rabbit populations and/or attenuated strains of RCDV will come from Australia. Only subjective opinions can be given as to the time scale. For what it is worth we suspect that the most useful period for use of the virus as a biological control agent would be in the first ten years. Thereafter there will need to be an increasing reliance on other techniques of rabbit control.
If the virus/disease were to spread poorly in New Zealand and require frequent and targeted reintroductions into specific areas we believe a strategic plan should be employed whereby the useful life of the virus as a biological control agent could be enhanced. As in the alternative scenario no exact forecast can be given of the time scale relating to the emergence of resistant rabbit populations or attenuated virus but our subjective view is that under the circumstances of poor natural spread and assuming an effective and planned scheme of biocidal use of the virus, its useful life would be very many times greater than that under the scenario of rapid "natural" spread.
Quality assurance measures on the virus used for reintroduction should be designed to ensure continued virulence of the virus. If an integrated control strategy can be specifically applied to target areas the chance of resistant rabbits emerging could be reduced. However, a caveat regarding the use of the virus in some form of bait must be given. Present production methods for replicating virus are by harvesting livers from infected rabbits. For use in baits it would clearly be prudent and necessary to purify the virus preparations so that infected baits cannot be detected by rabbits by olfactory signals relating to the primary tissues or other materials in the virus preparations. It should also be noted that until other methods of producing virus are found the large quantities of virus that might be required for use in baits presents major logistical problems.
5.33 Humaneness
Animal welfare scientists have long debated the means whereby pain and distress in any animal can be best measured. There are those who champion the detection of stress related substances (e.g cortisol) in the blood as an important measurement and those who believe that behavioural observations are a more accurate and useful methodology. We suspect that both sets of observations have their contribution to make.
At least one submission and the Applicants refer to some rabbits squealing/groaning in the terminal stages of RCD. Personal observations tend to link shrill vocalisation in rabbits with fear (e.g impending predator attack) rather than pain. That possibility would need to be excluded. The applicants suggest that the vocalisation may be the result of microthrombi in the brain as a terminal event. Unless further evidence can be provided we find that explanation unlikely. It should also be noted that a "freezing response" is also a part of behavioural repertoire of the rabbit.
Because of the great difficulty of objectively determining whether or not an animal is in pain the common approach is anthropomorphic whereby we use our own human experience to project ourselves into a similar situation. Nevertheless, it has been stated that "Science can never prove that an animal is, or is not, suffering because we can never really access the private world of another’s mind" (Mason,G and Mendl,M in "Why is there no simple way of measuring animal welfare?" Animal Welfare 1993, 2:301-319)
The period from infection with RCDV to death is very short in comparison to that which is characteristic many other infectious diseases. This is a very desirable feature from the point of view of humaneness.
The main pathological feature of fatal RCD is widespread blood clot formation in small blood vessels (so called disseminated intravascular coagulation or DIC). Although any illness in human beings or animals has unpleasant features we are not aware that DIC per se is a particularly painful condition in human beings and this may also be true for rabbits.
Reports on the clinical/behavioural reactions of rabbits affected and dying from RCD do not lead one to believe that the illness and death of rabbits from RCD is particularly inhumane. Given the range of traumatic consequences following shooting of rabbits it is reasonable to suggest that the summation of effects from shooting would be far less humane that those arising from RCD.
The death of suckling young from starvation subsequent to the death of does is a feature of any system of rabbit control which kills does in the breeding season. Deaths of suckling young associated with rabbit control should however be seen in the context of reported high "natural" mortality in young rabbits e.g 90% mortality to 6 months of age in rabbits born in the Wairarapa between May and November and even higher mortality among those born at other times of the year. A large proportion of such mortality is likely to be due to the activity of predators. Predation is of course associated with considerable pain and stress.
5.4 THE INTENDED PROGRAMME FOR WHICH RCD VIRUS IS PROPOSED TO BE USED
5.43 Details of how RCD virus will be deployed.
Until the likely epidemiology of RCD under New Zealand conditions can be more precisely determined the best method of deployment cannot be determined with any accuracy. The Applicants have pointed out that "No (regional) council can be compelled to release rabbit calicivirus.......". Thus, even if an ideal longer term strategy could be formulated, there appear to be administrative difficulties in carrying it through on a national or even regional basis.
An "adaptive" management programme as proposed by the Applicants may be a theoretical rather than practical option. If the result of introduction of RCDV was to be an explosive epidemic, we suspect that the collection and analysis of information would be overtaken by events and the ability to positively "refine and modify" would be extremely limited. If this scope to refine and modify on the basis of experience is important, there would be greater wisdom in making a single release (preferably on a suitable island site) and subjecting the outcome to a very intensive study before taking the next step of further release(s).
The proposals on deployment given by the applicants are not entirely clear to us. The initial release sites (N.B. plural) are proposed as "likely to be located in a monitored area of about 1 square kilometre." Some (but presumably not all) sites "will be in locations where knowledge can be gained about possible impediments to RCD spread, such as rivers and ranges." We believe that having a multiplicity of primary sites of release (up to 12 ?) would be unwise and has the potential to make collection and interpretation of data much more difficult. It appears to us that the public interest in this whole matter would make it very difficult to conceal the exact location of any first releases on mainland New Zealand and unofficial facilitation of spread of RCD cannot be excluded.
As indicated above, if RCD spreads poorly in New Zealand and its persistence requires reintroduction there is a good opportunity to extend the potential life of RCD as a control agent. We have in mind a policy which would use RCD for a single reduction attempt in a specified area and thereafter control would be by other conventional means. Reintroduction to a previously infected area should not follow until the lapse of a specified period of say five years. There is a good chance that such a tactic may delay the emergence of RCD-resistant rabbit populations.
For greatest impact on rabbits, any release of RCDV should be at a time when numbers of young rabbits are at a minimum.
5.44 The technical and financial underpinning of the programme.
Source and production of the virus are key technical issues. If data derived from field and laboratory experience with RCDV in Australia and Laboratory work in New Zealand are to be a fundamental information base for the decision, it is absolutely vital that the RCDV which might be used in New Zealand is identical to that from which these data were derived.
Any permission to import RCDV must therefore be strictly limited to the Chechoslovakian V-351 strain sourced from AAHL and warranted as "pure" according to MAF specified criteria. Full production specifications are not given in the application but it must be emphasised that the purity of any RCDV product is only as pure as it is in the last replication in the rabbits used for this purpose. It should be produced only in specific pathogen free (SPF) rabbits from an adequately monitored laboratory colony and, especially if produced in Australia, the rabbits must be kept throughout their lives in adequately insect screened conditions. Adequate (in Australia) is taken to mean suitable screening for exclusion of all insects including Culicoides. There must be strict quality assurance protocols which are monitored by New Zealand regulatory authorities. Biological products which are based on animal tissue derived from a numbers of animals require particular care in production.
There are great safety advantages in having the virus used in any rabbit control programme, produced in New Zealand. This is very strongly recommended.
There is insufficient hard information on which to base an analysis of the financial underpinning of the programme. For example the costs of production of virus and the costs of field use of RCDV would be very different if biocidal as opposed to biocontrol use is required. Furthermore expenditure options or costs for integrated use of current control methods are not given in any detail.
5.45 Integration of RCD virus with other control measures.
If the experience of any release of RCDV in New Zealand indicates that it must be used as a "biocide" we are in agreement with the Applicants that a time-out period between successive introductions of virus in any given area should be mandatory. However, it is our opinion that a period of 5 rather than the suggested 2 years time-out should be enforced. A time-out of 2 years is insufficient to allow a thorough mop up operation using current control measures and importantly at 2 years a reasonable sized population of surviving resistant "young" rabbits should be anticipated. At 5 years virtually all of the resistant survivors will be dead from other causes.
The applicants refer to the possibility of the benefit of any time-out period being "thwarted by private harvest and release of feral rabbit calicivirus if the public felt that official processes were too slow". The Decision Maker should assess the gravity or otherwise of this concern because it has the potential to seriously erode the benefits of the use of RCDV as a rabbit control agent in this country.
There are insufficient details in the Application to assess the degree, quality or form of any integration of other control measures. It is said that there will be integration with "current" methods. Much more consideration of the details of integrated control tactics appears to be required. In particular, it is necessary to plan the organisational and financial structures will underpin the integrated use of current methods. If as suggested, bait (as opposed to toxin) shyness is a serious emerging problem and if it becomes appropriate/necessary to use RCDV as a biocide then continued use of baits other than for RCDV might be unwise.
We are concerned that the applicants might be too committed to the so called "adaptive management programme". We do not suggest that management should not be adapted in the light of experience but unless options are well thought through beforehand there may be insufficient planning in place to allow successful adaptive management changes when required.
5.46 Limitations on the control of RCD virus in a programme
Our major concern regarding the control of use of RCDV in any rabbit control programme is directed towards the need to ensure that the virus at the point of use is safe ( i.e. carries no contaminants and is of specified titre. Use of inappropriately low titre batches would increase the chances and rapidity with which RCD resistant rabbits may emerge. As the Decision Maker will know, extremely good production, packaging, transport and handling protocols are essential to ensure that the product at point of use is fit for the intended purpose. An important component of such protocols is that all those involved in the use of the product are sufficiently well trained in its handling storage and use. Furthermore unless users are accountable for use within a specified control plan, the common good may be subverted.
5.5 THE EFFECTS OF THE RCD VIRUS ON NON-TARGET AND TARGET SPECIES.
5.51 Non target species
The potential for effect on non target species is perhaps one of the major issues of concern among those who made submissions. It may not be generally understood that absolute guarantees can rarely be given in biology. It may not always be appreciated that proof of some negatives can be logistically impossible to achieve. Consideration of the human disease known as smallpox or variola might be a helpful illustration. It is generally agreed that smallpox is eradicated from the world and national and international agencies have translated this belief into policy. No longer is vaccination against small pox required or believed to be necessary. However, although the last recorded case was in October 1977 it can not be absolutely excluded that smallpox or a closely similar pox disease affecting human beings could not at some time emerge. It is impossible to prove or disprove this. In such circumstances judgements, as in the smallpox example, are therefore made on the basis of a summation of current epidemiological evidence and more general biological knowledge.
Submitters opposed to the application of RCDV have appropriately focussed on phrases which in the application which use the term "there is no evidence that". The fact there is no evidence that a particular event will or will not occur is not the same as saying that a particular event will or cannot occur. The Decision Maker therefore has to take special note of the quality of evidence which forms the basis on which such statements are made.
A number of criticisms have been made of the tests carried out in Australia and New Zealand to determine whether or not a range of not target animal species might be affected by exposure to the virus should it be liberated. Some of these criticisms are valid. To use the New Zealand tests on kiwi as an example it is immediately clear that had more animals been available for tests a number of variables could have been included in the design. Route of inoculation is one such. If for instance a minimum of 5 kiwi per group were used and a number of different routes of inoculation and various titres of virus and killed virus controls were used for each route, and kiwi were killed at different times after inoculation, something of the order of 200 or more kiwi would be required. It is clear that a decision was made to limit the number of animals used to what was regarded as a reasonable minimum. I suspect that a judgement was made that if parenteral inoculation of a high titre of virus did not produce disease it was most unlikely for disease to follow natural exposure or inoculation by other routes. In New Zealand and Australia there is an ethical requirement placed upon all those who carry out experiments on animals to, firstly explore alternatives to animal use and secondly, if their use cannot be avoided, to use the minimum required to answer the question(s) posed. We presume the primary question posed in the kiwi and other laboratory experiments on animals was:-
If we expose animals to what might be considered to be a high dose of virus, inoculated by the route most likely to induce disease, will disease result?
The answer for all non target species tested in Australia and New Zealand was no.
The question could be raised:- Should additional species have been tested?
This is a decision which has to be made on the basis of previous knowledge. For example such decisions would consider the probability of New Zealand reptiles becoming affected with RCD taking into account that other reptiles tested in Australia did not.
Laboratory experiments must be viewed in conjunction with field experience and in the light of general knowledge of infectious diseases.
No laboratory experiment can take into account all the variables which might apply in the field. The Decision Maker is extremely fortunate that as a background to his decision there is now available much valuable information from Australia on the behaviour of the virus in a country in which the disease is likely to behave in a similar (but possibly less florid) manner to that which might occur in New Zealand.
In this context we strongly recommend that great weight is placed by the Decision Maker on the information available on the field behaviour of RCD in Australia.
5.52 Target species
The pathological effects observed in rabbits with RCD have already been referred to above when discussing the humaneness or otherwise of RCD. We have examined pathological preparations from rabbits affected by RCD and can confirm that widespread intravascular coagulation is an outstanding feature of the disease and it is this phenomenon which is likely to be the common cause of death in affected rabbits. There will be surviving rabbits in any infected population. Some will survive because they did not become exposed to infection, others will survive because the disease did not prove serious enough to cause death in particular individuals. Young rabbits have been shown to be refractory to the disease and they will also survive. Anecdotal reports from Australia indicate that release of the virus into rabbit populations in which there is a high proportion of young rabbits may give kill rates below 50 per cent.
5.57 The health and social well being of people.
There appears to be concern among some submitters that RCDV might cause disease in human beings. To illustrate the difficulty in proving the negative of this proposition it might be said that even if 100 people ( or anthropoid apes ) could be experimental exposed to RCDV and none developed disease this would not be an absolute guarantee that under some circumstances it might not cause disease. Of course experimentation on human beings is not ethically possible but in any case much more valuable information can be obtained from field experience and the most relevant information must be that derived from epidemiological and clinical experience in countries in which RCD is or has been present This evidence is particularly valuable because within a large exposed population it is reasonable to expect that there will be some individuals with reduced immunity to infectious disease in general e.g those infected with HIV/AIDS, those with certain cancers and those under treatment with immunosuppressive drugs. If RCDV were to cause disease in human beings it might be expected to appear in such people. Observations on large populations are also valuable because they also take into account other variables such as age, sex, intercurrent disease and various degrees and routes of exposure.
We are not aware of any official national or international health reporting agency in the world having reported human disease caused by RCDV. It can, of course be said that if human disease caused by the virus did occur it might not be detected because of lack of awareness or because its clinical signs might be close or identical to a disease which is already known and defined. Exposure of human beings is most likely to be by oral routes or by inhalation or conjunctival routes. If RCDV were to cause human disease we believe the chances of heavy exposure by such routes would have been highest in China. The disease in that country was reported in Angora type rabbits and it appears that their was considerable contact between rabbits and harvesters of rabbit wool. If the outbreak of RCD in rabbits in Mexico was indeed associated with imported infected rabbit meat it would suggest that domestic Chinese rabbit meat was also infected. On theoretical grounds harvesting and handling of rabbit wool would be a potent means of exposure by creation of dusts which become deposited on the human conjunctiva (c.f. human transitory conjunctivitis associated with exposure to detritus from Newcastle Disease infected chickens). We are not aware of any reports from China which might lead one to believe that RCDV caused human disease in that country. Italy is another country in which there has been considerable contact with RCD infected domesticated rabbits and people without reported human disease due to RCD. Because of the vast number of infected rabbits now in the environment of Australia special note should also be taken of their field experience.
One submission makes particular critical mention of the 1996 report "Rabbit calicivirus and human health" by the Rabbit Calicivirus Health Study Group in Australia. The submission is critical of the design of the work on which the report is based and questions the conclusion in the study that no significant association between exposure to RCDV and subsequent bouts of illness could be demonstrated.
As the Decision Maker will know, serological studies on any disease can present problems as to interpretation. Variables as to sensitivity, specificity, cross-reactions with related but distinct agents and agreement as to "cut-off points" between "positive" and "negative" in any test are among the many issues debated by scientists studying the use of serological (and other) tests in studying disease. Whatever the species under study resolution of such debates becomes much easier if there exists a distinct disease syndrome as the reference point..
Our general understanding of the pathology of infectious diseases leads us to advise the Decision Maker to decide whether or not there is any evidence that a distinct human clinicopathological entity exists which can be causally associated with exposure, in the field or laboratory, to RCDV. The answer to this question is paramount.
We are in some difficulty in making useful comments on the social well being of people subsequent to any release of RCDV. The potential positive aspects of successful rabbit control upon those engaged in agriculture and upon rural communities in rabbit prone areas have been detailed in submissions. Adverse effects on harvesting of rabbits for commercial and personal reasons have also been well canvassed in submissions. We recommend that these submissions be given appropriate weight.
5.62 Domesticated rabbits
The answer to this question is heavily dependent on the epidemiology of the disease after its introduction. If use of RCDV as a biocide becomes necessary it should be possible for any users of the biocide to take effective steps to reduce the exposure of domestic and laboratory rabbits. For example it might be imprudent to deploy the biocide in areas adjacent to urban centres. If a fulminating epidemic similar to that in Australia is an outcome of release of RCDV in New Zealand, the problems of amelioration become more difficult. Since the outcome is uncertain at this stage it is prudent that it be assumed that the latter will occur in order that measures are in place before the event. Potentially useful measures are prevention of entry of infection to premises where rabbits are kept and the use of RCDV vaccine. If insect vectors are proved to be important in New Zealand, under some circumstances it may be possible to take measures to exclude or reduce their entry to the area where the rabbits are kept. It is however appreciated that this may be expensive, difficult or impractical at many sites. The avoidance of the introduction to their premises of potentially contaminated food or fomites are practical steps available to any owner of domesticated rabbits.
Vaccination of rabbits is an effective prophylactic measure against RCD, and as in Australia, becomes the prime choice especially for rabbits which have high commercial or emotional value. It is fortunate that the commercially available vaccine(s) is/are inactivated i.e killed virus preparations. The balance of commercial parameters and risks of infection would doubtless be the decision making inputs on any individual’s decision as to whether or not to vaccinate.
There has been some disquiet in Australia about the cost of vaccination and whatever the price at the point of delivery in New Zealand might be, this is a cost not previously carried by rabbit owners and complaints should be expected.
5.63 Laboratory colonies
It is our belief that it would probably not be necessary to vaccinate laboratory rabbits kept (as they should be) under high quality barrier conditions. If owners of laboratory colonies to not have full confidence in their own systems or as insurance against system breakdown they may wish to vaccinate at least some potential foundation breeding stock.
5.7 CHARACTERISTICS OF THE VIRUS ITSELF AND ITS METHOD OF PRODUCTION AND DISPERSION.
5.71 Host range
The evidence available indicates that the European Rabbit Oryctolagus cuniculus cuniculus (Linnaeus 1758) is the only species established as suffering from a disease associated with RCDV.
5.72 Stability of the host relationship.
Stability of the host relationship with the virus is best measured by experience with this facet of its biology in overseas countries where the disease has occurred. For this purpose experience in Australia is potentially more valuable than that of Europe. In Europe the epidemiological picture can be confused by the presence of the European Brown Hare Syndrome Virus (EBHSV) and by apparently non pathogenic caliciviruses infecting rabbits. For example Chasey et al from the Central Veterinary Laboratory and Central Science Laboratory in Britain (Proceedings of the European Society for Veterinary Virology, International Symposium on Caliciviruses, 1996) have reported that antibody to RHD (RCDV) "has existed for many years in UK rabbits" and that 64% of a sample 900 adult wild rabbit sera showed a protective level of antibodies before the spread of clinical RCD.
Opponents of importation have strongly emphasised the fact that San Miguel Sea Lion Virus is an example of the ability of caliciviruses to infect more than one species. Others have quoted other caliciviruses with a very stable host relationship in spite of the acknowledged facility of this group of viruses to change genetically. The close contact of human beings with the very common caliciviruses of cats without any evidence of adverse effects on human beings is also noted. Another piece of field evidence for consideration is available in New Zealand where there is a large population of feral cats ( infected with feline caliciviruses) in close contact with wild rabbits.
We recommend to the Decision Maker that, while taking note of the theoretical virological and molecular biological issues before him, he should place greater weight upon what has actually happened in the field in those countries in which RCD virus has become established.
It is our view that experience to date indicates that a stable relationship exists between RCDV and Oryctolagus cuniculus.
5.74 Epidemiology
The likely epidemiology of RCD in New Zealand has been discussed elsewhere in this review. In the event of introduction of RCDV into this country it would be prudent in all planning to anticipate that it may well behave in a manner which is closely similar to that experienced in Australia although New Zealand has the option of properly planned release. As indicated elsewhere in this review, an alternative possibility exists for the behaviour of the virus and preceding any release, equal effort must be given to planning on the assumption that the infection will spread much slowly and the use of RCDV for rabbit control might be largely limited to biocidal methodology.
Because the pool of vector candidates in this country is much smaller than in Australia and because of other environmental differences which may impede spread of the disease or shorten field survival of the virus there is a clear possibility that spread within New Zealand may be less rapid than that experienced in Australia.
5.81 Environmental effects of rabbit carcases.
The potential effects of the presence of large numbers of rabbit carcases in RCD affected areas has been raised. In Australia we understand that in some rural areas the resulting smell has for a short time been unpleasant. Carcases also provide a short term substrate for the breeding of some species of fly. A balanced judgement should be made between the effects of numbers of poisoned rabbit carcases on the land as a result of current operations and the carcases which might result from RCD deaths.
If weight is to be put on this issue it is likely that any environmental adverse effects would be less with introduction of the virus in winter months.
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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