Australian Academy
of Science

The introduction of rabbit calicivirus disease (RCD) into Australia has generated a great deal of controversy. Initially, the debate centred on the use of a virus to control a mammal such as the rabbit. More recently, concern has been expressed because the virus has not been universally effective in all parts of Australia. As the old adage goes 'It's never possible to please everyone'.

In reading about RCD in the press it would be easy to conclude that the release of RCD had not been as successful as scientists anticipated. However, that would be a total misunderstanding.

Following the successful release of myxoma virus as a biological control in 1950, rabbits have been held at relatively low numbers for nearly 50 years. Nevertheless, some parts of Australia continued to be plagued by rabbits and this was particularly so in arid areas where mosquitoes, the major vector of myxomatosis, were rare. Rabbits continued to cause erosion and gnaw away at tree seedlings, adding to the desertification of Australia's heartland.

RCD has effectively turned that problem around. It is now an additional force helping to control rabbits in inland Australia. Experimental sites such as the Nullarbor Plain in Western Australia, Erldunda in the Northern Territory, the Flinders Ranges in South Australia, Hattah-Kulkyne National Park in northwest Victoria and Muncoonie Lakes in southwest Queensland have seen remarkable declines in rabbit numbers. At many of these sites, the rabbit population apparently fell by over 90 per cent when the disease first struck. Furthermore, there is good evidence at many of these sites that the virus is persisting and breaking out regularly, keeping rabbits at about 10 to 20 per cent of their original numbers.

This reduction in the rabbit population has been enough to allow significant regeneration of many arid zone shrubs. Shrubs which were declining are now increasing again and a few rare species, considered to have disappeared over wide areas, are being recognised as they re-sprout from deformed, overgrazed stumps.

Other potential benefits from the spread of RCD are becoming apparent too. A good example is the reduced use of '1080' poison for rabbit control, which in some States has fallen by about two-thirds. If this is a real result, it will not only mean lower costs for farmers trying to control rabbits but also a reduced risk to other wildlife species during rabbit poisoning. Of course, more information is needed about the use of '1080' in the longer term. Initial information of this type could be a little misleading if some farmers are holding back on poisoning in the hope that the calicivirus will resolve all their problems. On the other hand, farmers are also being encouraged to capitalise on the initial impact of RCD so, in some areas at least, rabbit control efforts have actually increased.

The true benefits of RCD in terms of reducing the costs of rabbit control in farming areas can only be estimated precisely over time. Similarly, economic benefits such as better wool or beef production and better land management will also take some time to unravel from all those other factors such as drought, changing markets and production costs which also influence productivity.

For those areas where RCD has not been so effective, more work needs to be done to determine the reasons for the apparently poor performance. Is it because the virus does not persist? Are rabbits simply so productive in some areas that they can effectively re-coup their losses to RCD? Are other factors involved?

If the problem is poor persistence of the virus, it may be possible to remedy this. At present it is possible to release more virus, but to improve its likely effectiveness this needs to be done with a full understanding of the epidemiology of the disease. For example, it is important to know that the virus has not been recently active in a particular area and that the majority of rabbits are susceptible to the disease. There is little point in trying to introduce the virus into a population consisting of immune survivors from an earlier outbreak. The timing of releases is also important as RCD spreads best at moderate day-time temperatures of between 15°C and 31°C (usually in spring and autumn), possibly because insect vectors are most active under those conditions too.

It is generally recommended that releases of the virus should be in autumn, when conditions for spread are good and there are few young rabbits in the population. Young rabbits are often doubly protected against RCD as they have a better chance of overcoming the disease than adult rabbits and furthermore may carry maternal antibodies which protect them for the first 8 weeks of their life. However, by the time they have reached about 10 weeks of age, they have generally lost this protection and are fully susceptible to RCD.

Beyond simple re-introduction of RCD, it will be necessary to have a very good understanding of how RCD spreads in rabbit populations if it is to be more closely integrated with existing rabbit control measures. It will be some time before clear recommendations can be made. In the meantime, normal rabbit control measures should be continued: even if RCD does not reduce rabbit numbers in some areas, it will certainly slow down the rate of recovery of rabbit populations after other forms of control have been imposed. If rabbit control can be more effective, or if it only needs to be carried out once every 2 to 3 years, this is still a major saving.

To conclude, it is clear that RCD has been an immediate success, particularly in arid Australia where rabbits were a major problem and the costs of removing them were often too high to be justified in terms of the economic returns from cattle and sheep production. There have also been clear conservation benefits, particularly in terms of shrubland vegetation.

RCD appears to complement myxomatosis, and the effects of both diseases appear to be additive so that we should see rabbits held at a lower level than myxomatosis was able to achieve alone. Because the RCD virus can be introduced on contaminated food and is spread by social contact or by biting and non-biting insects, it has greater potential for use as part of a coordinated control strategy than the myxoma virus, which must be injected and relies on the presence of fleas or mosquitoes for further spread.

Back to A plague on the pest – rabbit calicivirus disease and biological control.