7.0 Characteristics of the Virus
The history of the emergence of RCD in China in 1984 is well known but there are significant gaps in our knowledge of its origin. Professor Alvin Smith has concluded that the RCD virus originated in another species. The epidemic pattern of the disease is consistent with that of a novel host-parasite relationship. He concludes by inference from his knowledge of other caliciviruses that we do not yet know the entire host range of the RCD virus.
Coupled with the uncertainty of the origin of the RCD virus and its identification as a calicivirus responsible for the disease RCD is the question of whether a picarnavirus or a parvovirus is also involved. I believe that notwithstanding that Koch’s postulates have not been met in their entirety, the evidence summarised in the CVO’s report that the calicivirus is the causative agent is compelling and the evidence supporting the involvement by other viruses is not compelling.
Host Range
The host specificity of the RCD virus was a matter of considerable debate during the consultation process. Apart from the testing of 28 species other than the rabbit undertaken by CSIRO, the Australian Bureau of Rural Resources summarised 14 scientific publications reporting the results of tests on 19 other species of animals and birds.
The Australian testing programme was subject to considerable criticism but a conclusion that the virus did not induce disease or produce compelling evidence that the virus multiplied in the tissues of the test animals seems inescapable.
| I have concluded that while the possibility that the virus might infect some species other than the European rabbit has not been absolutely ruled out, and there is evidence of subclinical infection in dogs and foxes, the extensive testing programme and the absence of field evidence indicates that the probability of the virus producing clinical disease or pathological change in species other than the rabbit is low. Smith’s contention that | ||
| "Host specificity has to be proven before rabbit haemorrhagic disease agent can be approved as a biological control agent" | ||
is an unachievable standard.
I am satisfied that the tests on the kiwi and native bat did not produce evidence of infection or of disease attributable to the RCD virus. I am satisfied that seroconversion was in response to the virus material injected and not due to virus multiplication.
Stability of the Virus
The CVO’s report provides a summary of the characteristics of RNA viruses generally, caliciviruses and the RCD virus. Like all RNA viruses, the RCD has an inherently unstable genome. They lack a mechanism to correct errors in the assembly of their nucleic acid. It can be inferred that this fundamental instability has the potential to result in an adverse evolution of the virus such as a change in host specificity or a change in virulence.
The RCD virus may change as the result of "errors" in the assembly of its genome. Recombination is theoretically possible but has not been recorded in caliciviruses.
The frequency with which RNA viruses undergo change which results in an adverse event is very low. The likely frequency with RCD virus is low. It is more likely that natural selection will favour changes which result in reduced virulence.
I have concluded that the evidence of inherent instability of the genome of the RCD virus is incontrovertible but the likelihood of such changes causing an adverse event is very low.
Epidemiology of RCD
RCD has infected rabbits in 40 countries in a period of 12 years but the means of spread is by no means clear. What is clear is that in most of these countries, rabbits are raised for meat and/or fibre and the scale of operation can vary from "backyard" to sophisticated "broiler" facilities. It is highly probable that human activity is a common factor in its spread.
More relevant to its use in New Zealand is the means by which the virus might spread from a release focus. The CVO’s report highlights the fragmentary nature of the information and speculation as to the mechanisms of spread.
The questions which Dr Mo Salman believes should be answered are set out in the CVO’s report (section 7.4).
Taking into account
- current unknowns regarding the epidemiology of RCD;
- relative absence in New Zealand of the vector insects which are
- to be important in spreading the disease in Australia;
- April 1997 results from Australia (CVO’s report, section 3.1),
I am unable to be confident that we can predict the behaviour of the virus in New Zealand. To agree to its introduction would require agreement to a period of trialing the use of RCD as a biocontrol agent. This is a fundamentally different proposal to that proposed in the Application.
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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