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Review of the Project: The Development of a Cane Toad Biological Control (February 2003)

D. Hazell, R. Nott and M. F. Shannon
Department of the Environment and Heritage, August 2003


Term of Reference 3

Any matters that may impact on the potential for this project to meet the overall objective of developing an effective self-disseminating viral vector to disrupt the development of cane toads in a manner that will reduce the threat of cane toads on the survival of native Australian fauna.

The Likelihood Of Obtaining A Gene That Will Disrupt Cane Toad But Not Frog Metamorphosis

Discussion with both stakeholders and with the project team strongly suggests that this is possible as there is sufficient developmental difference between cane toads and native frogs. The micro array technology is now in place and should allow the identification of potential genes. The ability to clone these genes, generate protein and/or clone them into the virus, and deliver the protein or the virus to cane toads is also in place. This part of the project is feasible.

Potential genes should be obtained and some tested within the 3-year timeframe. It is important, however, that screening for species-specific genes is guided by a detailed plan to ensure that native species are comprehensively tested. It is unlikely to be feasible (and is probably unnecessary) to screen all of the 200 plus species of Australian frogs. Determination of species to be screened and rationale behind their selection needs to be presented. This would require consideration of phylogenetics, which is not well established for Australian frogs.

The Feasibility Of Generating An Attenuated Virus

Preliminary experiments suggest that the team have already obtained an attenuated virus but this needs to be verified and the infectivity of these viruses also verified. The focus on 'rational' attenuation is very appropriate.

Comprehensive testing is required to ensure that Australia's native species are not at risk through the release of the attenuated, recombinant ranavirus. This is a major task that will require substantial expertise from outside the project team. Considerable planning needs to go into determining which species are to be tested.

Ecology (eg. habitat preferences and behaviour in the environment) may differ substantially between species that are closely related taxonomically. Inter-specific variability in time spent basking, for example, can influence susceptibility to pathogens.

The costs and feasibility of comprehensively testing indicator species should be considered carefully as well as associated ethical issues. For example, there are threatened frog species with very few remaining populations like the Southern Corroboree Frog. Questions remain as to how their susceptibility will be determined.

Species, such as the Gastric Brooding Frogs Rheobatrachus silus and R. vitellinus, are now considered extinct in the wild. Is this assumed to be the case or would the susceptibility of such species (that have unique physiology) be considered?

Ranaviruses are also known to infect other major animal groups, namely reptiles and fish. These animal groups would also require testing. Susceptibility to the virus may vary across different life stages of an amphibian species. Effects of exposure during the egg, tadpole, metamorph and adult stage should all be considered.

Susceptibility to disease from the attenuated ranavirus is likely to differ with environmental conditions and the health of the exposed individual. These should be considerations for testing.

The objective of testing the recombinant/attenuated virus in a number of indicator species should be preceded by a more specific objective of developing a detailed strategy for comprehensively testing Australian frogs, fish and reptiles for successful attenuation. Such a strategy should demonstrate that expertise has been sought from frog, fish and reptile biologists, ecologists and virologists.

Selection Of A Viral Vector That Will Be Effective And Self-Disseminating

The ecology of ranaviruses remains virtually unknown (eg. its survival, mobility and pathogenicity in the wild). The capacity for ranaviruses to provide an appropriate viral vector will be influenced by these characteristics. Research is required addressing these issues to ensure that the considerable effort being invested into the attenuation and recombination of a ranavirus is not wasted.

Specifically, further information is required on the following issues:

To date only one ranavirus has been isolated only once in Australia from a native frog species - Limnodynastes ornatus. This is useful foundational information. However, the prevalence of ranaviruses in wild cane toad populations needs to be determined, particularly during the tadpole stage (as this is the proposed stage of the life cycle that is to be targeted). It is understood that there is a ranavirus present throughout the range of the wild adult cane toad population (shown in data presented by the research team during review process).

The prevalence of specific ranaviruses have not yet been determined. Ranaviruses are considered extremely robust in nature and readily transported through the environment. For this reason no specific host for transmitting the virus around the environment has been considered necessary.

Given the high densities at which cane toad populations occur (and the robust nature of the ranavirus) it seems logical that if ranaviruses are present in the wild population, the effects should have been considerable. Either ranaviruses are not present, or they are not as robust, virulent or transportable in the natural environment as thought. This issue must be resolved.

If ranaviruses are found to be present in the wild cane toad population but prevalence is low then another means of delivery for the virus may be required.

Factors driving prevalence in the natural environment will need to be explored and understood if ranaviruses are to be used as the viral vector. This may relate to variability in mobility or capacity to infect under natural conditions. This is a major challenge, requiring substantial fieldwork and monitoring of cane toad populations (if ranaviruses are present in wild populations). It is important to note that any ranavirus in its wild form may not reflect the nature of the attenuated virus. Similarity between behaviour of wild type ranavirus and the attenuated recombinant ranavirus will need to be demonstrated.

Concern Regarding The General Issue Of Releasing A Recombinant Form Of Ranaviruses

The potential for virus mutation is naturally a major issue of concern in releasing a form of ranavirus into the natural environment. Ranaviruses are considered a stable type of virus and the probability of it reverting back to a pathogenic form is considered by the project team to be close to zero. However, the broader scientific community does not share this confidence.

Evidence of the stability of ranaviruses or other similar viruses must be presented (ie. scientific literature and detailed rationale) so that the scientific and lay community may be confident that the issue has been properly considered. Personal assurance is unlikely to be sufficient. Confidence from the broader community will be essential if the virus is eventually to be released.

Containment Of The Recombinant Virus Within Australia

Ranaviruses can survive in the environment (ie. not in a host) in a moist or desiccated state for extended periods. For example, data presented by the project team showed that in its desiccated form (at 27 degrees Celsius) the virus could survive up to eight weeks.

It is therefore conceivable that someone could transport the virus unknowingly (eg. on their boots or fishing gear) to another country. The implications of such an event are two fold. Firstly, the virus could be transferred to a country where native species of amphibians, fish or reptiles were susceptible (ie. the virus was not sufficiently attenuated for these species) resulting in mortality and or sickness. Secondly, the virus could be transferred to a country where cane toads were native, resulting in the disruption of cane toad metamorphosis and subsequent loss of the species from their natural ecosystem.

Risk of disease transfer between countries is clearly an issue for quarantine authorities. However, a recombinant form of a ranavirus that can disrupt cane toad metamorphosis should not be released unless it is feasible that a ranavirus could be contained within Australia. The outcomes of this feasibility exercise will influence the future directions and continuation of this project.

It should not be the responsibility of the project team to undertake this feasibility exercise. However, it would be appropriate for the project team to consider how the virus may be manipulated to reduce risk of spreading to other countries. For example, it may be possible to manipulate the virus and reduce its survival time in a desiccated state, without reducing its effectiveness as a biological control. If the adult cane toad is able to carry the virus then survival of the virus in a desiccated state will be less crucial for virus mobility.

Demonstration Of Proof Of Concept To The Scientific And Broader Community

The project team has clearly been proactive through the media in providing the lay community with information on the research. However, it is just as important that the broader scientific community is informed of the progress of the research, through forums such as scientific conferences, workshops and journals. The project team is no doubt engaged in such activities, however it is important that evidence is presented in future progress reports, alongside media coverage.

This project is clearly of interest to a diverse range of scientists and practitioners from amateur herpetologists to ecologists and virologists. Scientists and other practitioners are likely to be able to assist the project in some way if they are kept informed of its progress. Keeping the entire community informed of project progress is in the best interests of the project, as concern will only build over time.

Capacity Of The Project Team To Address Overall Objective

If the overall objective of this project is to develop a biological control then the issues raised through this review should be incorporated into a comprehensive plan. For example:

The project team would probably incorporate these issues into the program after the next three years. Understanding how the virus behaves in the wild is likely to require a long-term study and should therefore receive attention within the next three to five years. If the project team does not intend to address these issues, which are pertinent to the overall objective, then this must be made clear and EA should assess the implications of these gaps in achieving the overall aim.

Generation of a fully tested biological control is a long-term solution and should not be the only method for addressing the cane toad problem

This is a long-term research project that is unlikely to yield results (ie. a reduction in cane toad numbers in the wild) within the next decade. Meanwhile, the cane toad problem continues to spread geographically. This issue is not a criticism or limitation of the project in question, but an outcome of the review panel examining the overarching issue of the need for cane toad control.

Research to date has identified potential to reduce cane toad numbers in targeted areas. Specifically, cane toad preference for breeding in open areas offers opportunities for decreasing habitat quality through revegetation of riparian zones and wetlands. In addition, cane toads differ from native amphibians in that they have a well-developed sense of smell that is used to locate food. This trait could potentially be used in baiting to attract cane toads to certain areas, or to deter them.

While such approaches will not reduce the numbers of cane toads overall, they are likely to provide benefits to specific areas where cane toads are likely to have their greatest impacts. For example, areas such as Kakadu, or localised environments inhabited by rare or threatened species could be targeted, especially in the short to medium term.

There are areas within Australia that have not yet been invaded by cane toads. Currently, there are no obvious barriers to cane toad spread on the Australian mainland. However, islands may be effectively protected against cane toads through the development of a strategy and support framework for local communities managing these areas. This should be a high priority for the conservation of island biodiversity.

Findings from Term of Reference 3

  1. Identifying a gene that will be specific for toad development/metamorphosis is achievable. A plan for screening for specificity needs to be developed.
  2. It would appear feasible to generate an attenuated virus. Once again a comprehensive plan for the testing phase, as detailed above, needs to be established. The strategy has to take into account issue raised above.
  3. It is unclear if ranaviruses will self-disseminate to the extent necessary for a successful biological control. This issue requires research on the ecology, survival, mobility and pathogenicity of ranaviruses as a long-term goal. Specifically the team needs to develop an assay to isolate ranaviruses or to detect 'species' specific antibodies in cane toads. It appears some of this work has been undertaken (see Whittington O'Rourke, Hyatt and Chisholm 2002 MOLECULAR AND CELLULAR PROBES 16 (2): 137-151). EA and the research team need to investigate sources of sick toads that could have ranaviruses. This could be facilitated through existing networks (eg The Northern Territory Frog Database - http://www.frogwatch.org.au/website; Double Helix) or through the development of a new network. Resource constraints may limit the research team's ability to fully achieve this (see point 7 below).
  4. From a scientific perspective it is reasonable to assume that mutation(s) to alter the attenuated virus to a more virulent form are very unlikely to occur if the virus is released. However, this needs to be communicated to the public so that concerns are covered. Stakeholder interviews implied that some in the scientific community also held concerns about the release of a virus. If the scientists are not in full agreement then this is likely to lead to conflict in wider community.
  5. EA and AQIS need to plan for the possibility of a released virus escaping from Australia. Such issues as illegal export of live frogs and physical transfer of the virus (eg. on fishing equipment) need to be investigated.
  6. Notwithstanding intellectual property issues, good communication with the scientific community as well as media needs to be undertaken. A communication strategy that takes into account scientific conferences, scientific publications, and media releases needs to be put in place by the research team.
  7. The project team is already fully committed and does not have the capacity to engage in additional activities without significantly more resources. EA needs to address this issue.
  8. While the project is achieving its stated objectives, cane toads continue to spread. EA needs to investigate short to medium-term strategies to manage this issue. EA needs to engage a wider range of stakeholders and expertise to develop and execute this strategy.