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

Key text

What is biological control?

Biological control is a form of pest control that uses one organism to control the numbers of another (Box 1: Biological control). It is most often used against introduced species that have become pests – most indigenous organisms are kept under control by parasites or predators that have evolved alongside them.

Some Australian examples

An early recorded attempt at biological control in Australia was the release in the 1890s of three hundred cats. They were released at Eyre, on the coast of the Great Australian Bight, in an attempt to stop rabbits spreading further into Western Australia. Many of the cats starved and the rabbits were hardly affected.

Since then biological control has been successfully used in Australia many times, most commonly for the control of insect pests or weeds. The best known examples are the introduction of the Cactoblastis moth to control prickly pear and the use of the myxoma virus to control rabbit numbers (Box 2: The history of myxoma virus in Australia).

The control agent must be thoroughly investigated before release

Nowadays biologists are required to carry out extensive research before a control organism is released because it is important to find out whether it will attack species other than the pest species. Once a control organism has been selected and found not to be harmful to other species, it is produced in large numbers and released from quarantine. It is then tested in the field and subjected to careful monitoring.

Biological control agents can get out of control

Past attempts at biological control, where the testing was not rigorous enough, have sometimes caused more harm than good. The most famous (or infamous!) example of this is the cane toad. Now a dreadful pest in many areas of northern Australia, the toad was deliberately brought here to control the beetles that were attacking sugar-cane plants in Queensland. It wasn't particularly effective at that job, but it was a great survivor and soon started moving out beyond the canefields, poisoning any birds, mammals or snakes that tried to eat it.

Rabbit calicivirus disease: history

Rabbit calicivirus disease was first noticed by scientists in 1984 when rabbits in China started dying in large numbers. The virus has since spread to Europe and Mexico.

In 1991, the Australian Animal Health Laboratory imported samples of the virus to test at its laboratory in Geelong, Victoria. After three years of testing in many species, there was no evidence that the virus could infect other hosts.

To test the potential effectiveness of the virus in the wild, the Australian and New Zealand Rabbit Calicivirus Disease Program began field trials at a high-security quarantined area on Wardang Island off the coast of South Australia in 1995.

At first the virus spread very slowly in the rabbit population, but when spring arrived it spread much more quickly. In October 1995 the virus escaped from the quarantine area and then spread to the mainland. It moved quickly through South Australia and on to western New South Wales, killing rabbits in its wake. Before long, infected rabbits were found in parts of Victoria, Queensland, New South Wales, the Northern Territory and Western Australia.

Further tests were required before a national release of the calicivirus was permitted in Australia

In April 1996 the Federal Minister for Primary Industries and Energy, John Anderson, announced that before a coordinated national release of rabbit calicivirus could occur, koalas, wombats and echidnas should be tested for possible susceptibility to infection and that there should be further study to confirm there is no effect on humans.

The official release of the virus began in October 1996

At the end of August 1996, the Minister for Primary Industries and Energy announced that the studies had been completed to his satisfaction and recommended a coordinated release of the virus.

By September all State and Territory governments had agreed to the release of the virus. New South Wales was the first State to act, releasing 20 infected rabbits on a property south of Wagga Wagga in October 1996.

Since the release of the virus, scientists have been monitoring the number of rabbits, native plants and native animals by taking random samples from a number of areas and then estimating the size of the populations and determining population densities (Box 3: Estimating population size and density). They have seen a dramatic reduction in rabbit numbers in inland Australia. This reduction in the rabbit population has allowed the regeneration of many arid-zone shrubs and an increase in the numbers of native animals.

Box 1. Biological control

Biological control is the use of one living organism to control another. For example, the moth Cactoblastis cactorum was imported to Australia from South America to control prickly pear. This resulted in about 250,000 square kilometres of agricultural land being cleared of prickly pear.

In their natural environment, most organisms are kept under control by their natural enemies (parasites, predators and diseases that have evolved with them over a long period of time). But when an animal or plant is transferred, either accidentally or deliberately, to a new environment, its natural enemies are often left behind. Free of restraint, it may increase out of control and become a pest. Australia has been particularly vulnerable in the last 200 years because immigrants did not see any value in Australian flora and fauna and so imported plants and animals for food, ornamental purposes, hunting or just to make Australia look more like their homelands. Most Australian insect pests and weeds have been introduced from other countries, where they are controlled by naturally occurring predators or parasites.

In a typical program of biological control in Australia, scientists attempt to find a pest's natural enemies in its original habitat. If a promising predator or disease is found, it is tested to ensure that it does not attack other species. If after these tests it is considered safe, it is released as a biological control organism.

When successful, biological control has important advantages over other methods. It is specific to the pest, and so does not affect other organisms or the environment. It is self-perpetuating and involves minimal cost after the initial research.

Related sites

• Control of introduced species (Questacon, Australia)

• Classical biological control (CSIRO, Australia)

• Biocontrol (University of Adelaide, Australia)

• Killer water weed (Australia advances, CSIRO)

Box 2. The history of myxoma virus in Australia

The ancestors of the rabbits in Australia lived in North Africa, whereas the myxoma virus evolved in South America where it has kept the rabbit population under control. The history of the interaction of rabbits and the myxoma virus has become the most completely documented example of the interaction of a host animal species and a disease organism.

Wild rabbits were introduced into Australia in 1859, and by the 1880s they had become a major pest. In 1919 Dr Aragao, a South American researcher, suggested the introduction into Australia of the myxoma virus. However, in spite of the devastation rabbits were causing to farming land, Australian authorities were reluctant to damage the thriving rabbit meat and fur industries.

Laboratory experiments with myxoma virus were finally carried out in Australia in 1924, but were not promising. It was not realised that the spread of the virus depended on insect vectors, such as mosquitoes.

In 1933 Dr Jean Macnamara, a medical specialist from Melbourne, visited New York to study poliomyelitis. She met Dr Richard Shope, who was investigating myxomatosis in domestic rabbits on Californian fur farms. She realised the potential of myxomatosis for controlling the Australian rabbit population and had samples of the virus sent to Australia. However, authorities would not let the samples into the country.

Jean Macnamara then persuaded Stanley Bruce, Australian High Commissioner in London, to help. Sir Charles Martin, who had been chief of the Division of Animal Nutrition at CSIR (Council for Scientific and Industrial Research, the precursor of CSIRO), had moved to England and was working on myxomatosis at Cambrige University on behalf of the CSIR. In 1936 he reported that the virus was specific to rabbits and thus safe to import into Australia, although he questioned how well it would spread throughout the rabbit population.

Experiments in Australia in 1936 confirmed Martin's results. It was difficult to obtain sites for field tests because of the vigorous protests of rabbiters and rabbit merchants. Eventually sites were chosen in South Australia, but the trials were not a success. The area was drought-stricken and there were few insect vectors to spread the disease.

Work on the myxoma virus ceased in 1943 and Jean Macnamara publicly criticised the work as 'a pathetically limited inquiry'. CSIRO resumed work on myxomatosis in 1950, with trials at five sites in the Murray Valley, all of which appeared to be failures. However, at the end of 1950 and early in 1951 myxomatosis swept through the Murray-Darling river systems. Heavy rains at the time meant that mosquitoes bred and then carried the virus from infected to uninfected rabbits, sometimes several hundred kilometres away. Within 3 years, the disease had been carried to all parts of Australia and rabbit numbers were drastically reduced.

However, as early as 1953 scientists studying the virus and rabbits noticed that the virulence of the virus had changed from being 99.9 per cent effective to 95 per cent, a small but significant drop. The less-virulent virus took 3 to 4 weeks to kill a rabbit instead of 6 to 10 days, so that sick rabbits could be bitten by mosquitoes and fleas for 3 to 5 times as long as a rabbit suffering from the highly virulent strain. The milder strain was therefore more successful in infecting rabbits, and it spread rapidly. Through this selection the virus evolved to a less-virulent form.

At the same time, evolutionary selection processes were working in the rabbit population. If one rabbit in a thousand had a natural resistance to the myxoma virus, it alone would survive to leave offspring and their chance of surviving would be greatly increased by lack of competition from other rabbits. They would also inherit their parents' resistance. Thus in a few generations the proportion of resistant rabbits would increase.

The two factors, attenuation of the virus and inherited immunity to the virus, have led to the situation where, today, the myxoma virus may kill only 50 per cent of the rabbit population during an epidemic.

Box 3. Estimating population size and density

One of the first things to do when studying a population is to find some way of working out the number of individuals in it, for without some estimate of the numbers you cannot begin to record any changes in the population.

Counting organisms that are stationary

With fixed organisms such as trees or barnacles, one can count the individuals in an area if the area is small. However, if you need to estimate the number of trees in a 1000 hectare forest or of barnacles on a rocky shore, it is only necessary to count several sample areas. From the numbers in the sample areas you can estimate the total number or you can work out the average population density of the trees per hectare or barnacles per square metre. You can follow this procedure:

1. Define the whole area (A) in which the population is to be estimated.

2. Choose a small sampling unit (area a). This is an area in which you expect to be able to see and count all individuals. For example, your sampling unit might be a rectangle with an area of 1 square metre. Ecologists call these sampling units quadrats.

3. Choose a sample size (n). This is the number of quadrats that you will select in area A. The selection of quadrats must either be random (if you can select a lot of quadrats) or representative (if you are restricted to only a few).

4. Count the number of individuals in each quadrat.

5. Find the average number of individuals per quadrat. To do this, divide the total number of individuals by the number of quadrats.

6. Calculate the estimated number of individuals (N) in the whole area, as follows:

   Total population = Average number per quadrat × Total area/Area of quadrat.

Of course, your estimated number of individuals (N) is not the actual number of animals in the area, but a reasonable approximation. How close it is to the real number will depend on how large a and n   are. The more quadrats you select, and the larger each is, the better the estimate will be, but more (and larger) quadrats require more time and effort.

Calculating the population density is simple once you know the total number of individuals in a population and the total area. Divide the total number of individuals by the total area and express the result as number of individuals per unit area.

Figure 1. In this example the study area is 100 × 100 metres. Therefore A = 10,000 square metres. Each cross represents a shrub. Each quadrat measures 10 × 10 metres. Therefore a = 100 square metres. There are 12 quadrats so n = 12. Use the steps given for this method to estimate the population illustrated in the figure. How accurate was your estimation?

Counting organisms that move around

For many organisms that move around, such as small mammals and fish, carrying out a census is not easy. Animals can be surveyed (eg, red kangaroos are large enough to be counted from the air) but the technique must be used very carefully and systematically by counting in several locations and at different times. Another method is called 'mark, release and recapture'. The animals to be counted are captured, marked in some way so that they can be recognised later, and then released. With a little calculation, you can estimate the total population size from the proportion of marked and unmarked animals in the second trapping session, as the following procedure shows.

1. Define the area in which the population is to be estimated.

2. Decide how to catch and mark the species in question.

3. Catch a reasonably large number of animals within the study area, all during a relatively short time (usually minutes to hours).

4. Mark all the animals caught and release them. (The method of marking must not harm the animals nor make them conspicuous to predators.) The number caught, marked and released is M animals.

5. Leave the area for a while to allow the marked animals to mix completely with the rest of the population.

6. Repeat the capture procedure. Sort the animals caught into marked and unmarked. The number of marked animals is m and the total number of animals (marked + unmarked) is n.

7. The estimated total number of animals (the population size N) in the area is calculated as follows:

   Total number = Number marked × Total number caught/Number of marked ones caught.

Can you see why this is so? It is a prediction of the number of animals that would have to be caught to be sure of catching all the marked animals.

Figure 2. The proportion of marked to unmarked animals captured at the second sampling can be used to give an indication of the total population size. Use the steps given for this method to estimate the population illustrated in the figure. How accurate was your estimation?

Of course, N is only an estimate and how close it is to the real population size will depend on a number of factors: the number of animals you manage to catch and mark as a percentage of the total, whether the marked animals mix properly with the unmarked ones, and whether any lose their marks or are affected by being marked. Migration, births and deaths that take place between the two sampling events can also result in inaccurate estimates.

Related site

• Kangaroo counting (Box 1 of Nova: Science in the news topic, Is Australian wildlife fair game?)

Activities

• Northern Tablelands Dung Beetle Express, Australia
  • The dung beetle monitor – a monitoring guide for schools, landholders and Landcare groups.

• Lab Notes (Australian Broadcasting Corporation)
  • Dung Beetle Part 1 – some simple activities for students about dung beetles based on an article by Karl Kruszelnicki. Activities and teacher notes available.
  • Dung Beetle Part 2 – some more activities for students about dung beetles based on an another article by Karl Kruszelnicki.

• Biotechnology Online (Biotechnology Australia)
  • Some introduced pests in Australia – students work in groups or individually to research a pest and present information about pest control.

Activity 1. Graphing and interpreting variation in rabbit numbers

Year	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Rabbits per kilometre	40	60	80	20	10	15	25	10	15	30	10	30	60	80	100
Rabbits per hectare	4	6	8	2	1	1.5	2.5	1	1.5	3	1	3	6	8	10

1. Graph the rabbit density per hectare. How do you explain the apparent existence of half rabbits in years 6, 7 and 9?

2. In which year do you think a biological control agent was released?

3. Why did the farmer and his daughter count the rabbits at the same time every year? Suggest reasons why the results might have been different if they had counted rabbits in early summer rather than early autumn.

4. How do you explain the fluctuations in rabbit numbers between years 4 and 11?

5. How do you explain the increase in rabbit numbers in years 13, 14 and 15?

6. A new biological control agent is introduced after year 15 and rabbits become scarce. What do you think the farmer can do to make sure that rabbit numbers stay low?

Teachers notes

1. Some students may not know how the number of rabbits counted per kilometre are converted to rabbits per hectare. The width of the observed strip is about 100 metres. The reported values are rabbits seen per kilometre, so each reported value applies to 100,000 square metres or 10 hectares. Therefore the approximate densities of rabbits per hectare are one-tenth of the number seen per kilometre.

   Although there are many methods of determining population size, this method is useful in this instance because it is relatively easy to do and the object is to compare relative numbers of rabbits between years not to determine the absolute number of rabbits.

2. A biological control agent (such as a strain of myxomatosis) was introduced towards the end of year 3.

3. To make a valid comparison of rabbit numbers, counts must be made at the same time each year. Any counts made in early summer are likely to be high because rabbits in southern Australia usually breed in spring. However, numbers then decrease, because young rabbits are particularly vulnerable to predation, food shortages and disease.

4. The fluctuations could be caused by factors such as:
   • the availability of food and water;
     
   • predation;
     
   • irregular outbreaks of disease;
     
   • irregular outbreaks of the biological control agent.

   Often a combination of these factors is important in causing the fluctuations in numbers. If the biological control agent introduced at the end of year 3 is transmitted by another organism (ie, a vector, such as mosquitoes transmitting myxomatosis), the abundance of the vector will also affect rabbit numbers.

5. Factors that could explain the increase in rabbit numbers:
   • resistance to the biological control agent released in year 3 could build up in the rabbit population;
     
   • changes in the agent could make it less effective in killing the rabbits;
     
   • a very good season with plentiful food produces more breeding than usual, and more rabbits are produced than are eaten by the predators present.

6. Measures to ensure rabbit numbers stay low could include shooting, trapping, poisoning and ripping up warrens.

   When more data on the spread of the calicivirus become available students could compare the rate of the spread and the killing capacity with that of myxomatosis. To prepare for this, students could record the spread of the calicivirus on a map of Australia and collect data from reports in the media as they become available.

Activity 2. A hypothetical situation involving biological control

In the area sprayed with insecticide, the local people were upset about their cats becoming sick and dying. There was an outbreak of mice and rats in the villages. Furthermore, there was a dramatic decline in the freshwater fish that the villagers rely on for the protein in their diet. Mosquito numbers fell for a few years and then started to rise again.

In the drained area the rice harvest decreased after a few years, mosquito numbers fell (although not as much as in the insecticide-treated region) and remained at their lower level. Tourists were disappointed as several exotic bird species became rarer.

1. Write a report for the government of Ruritasia that explains the causes of these events and clearly sets out the interconnections involved.

2. The Ruritasian government wants to try a form of biological control against mosquitoes, by introducing either a predatory fish species with an enormous appetite for mosquito larvae, or an insect virus that kills mosquitoes.
   • What would you advise about the advantages and disadvantages of biological control and the type of procedures that would give the best results?

Teachers notes

1. The following points are relevant to an explanation of the causes of the events in Ruritasia:
   • the need to control mosquito numbers because they are the vector for the malarial parasite;
     
   • the insecticide used was not specific to mosquitoes and affected other organisms as well;
     
   • the build-up of resistance to the insecticide in the mosquito population;
     
   • the possibility that mosquito larvae are part of the food chain for fish;
     
   • the possibility that the exotic birds nested on or beside the waterways that were drained;
     
   • the fact that the insecticide persists in the food chain.

2. Advantages of using a form of biological control include:
   • pesticides, which are harmful to all parts of the food chain, are not needed;
     
   • biological control is self-perpetuating;
     
   • suitable biological control organisms do not attack other species;
     
   • usually a large proportion of the pest population is destroyed.

   Disadvantages of using a form of biological control include:

   • disruption of the food chains that include mosquito larvae;
     
   • the need for environmentally unfriendly follow-up operations to ensure that the mosquito population does not build up resistance to the biological control agent.

Activity 3. Food webs

Teachers notes

Encourage students to consider a variety of plant types (eg, trees, shrubs and herbaceous flowering plants, as well as grasses) and to include invertebrate animals, native and feral birds, and mammals. When students list the effects of removing rabbits from the web, encourage them to consider effects several steps along a chain and not just direct predator-prey relationships.

Activity 4. Learning from the myxomatosis experiment

1. Explain why, almost 50 years later, we still have a rabbit problem.

2. What could be done to prevent the same thing happening with the calicivirus?

Teachers notes

1. There is still a rabbit problem in Australia because the effectiveness of the myxoma virus decreased. There are two main reasons for this: the ability of the virus to kill rabbits decreased and the proportion of resistant rabbits increased. (See Box 1, The use of the myxoma virus to control rabbit numbers.) In addition, the impact of the myxoma virus in Central Australia was reduced because there were few flies or mosquitoes there to help spread the virus.

2. After the introduction of the calicivirus, farmers should reduce the number of surviving rabbits by using other methods to kill the survivors (eg, poison baits, shooting, ripping up warrens). The eradication is more effective when farmers coordinate their efforts with their neighbours.

The effectiveness of calicivirus as an effective biological control agent could be prolonged by correctly timing releases. Factors to be considered are:

• the virus can be spread by rabbit-rabbit contact (high rabbit numbers enhance this method of transmission);
• cool, moist conditions are ideal for the survival of the virus and possibly for the survival of vectors;
• if rabbit kittens under about 8 weeks old are infected by the virus and develop a life-long immunity to it.

To date, the best spread of the virus and most effective killing of the rabbits have been recorded during the breeding season of rabbits.

This question could lead to an interesting discussion about whether every last rabbit has to be killed in order to wipe out rabbits in Australia and, if this happens, whether we can then stop being vigilant about possible new outbreaks. At this stage it appears that the rabbit will remain in Australia for some time.

Activity 5. The calicivirus controversy: a role play

Teachers notes

The following 'position statements' might help students get started.

Grazier: I have 1.5 million rabbits on my property and soon I will be unable to make a living. I have tried many control methods but the problem is too big. Shooting only gets rid of a small number of rabbits; many rabbits are now immune to myxomatosis; ripping up or bulldozing a warren costs $20 to $40 per warren; poisons are expensive. If I could get hold of an infected carcass, I would get it onto my property as soon as possible.

Botanist: Rabbits graze on small shrubs and trees as well as grass. Trees and shrubs in inland Australia have only been able to grow and survive to reproduce during two periods in Australia's history: once before rabbits were introduced and once just after myxomatosis was introduced. Scientists are now worried that the remaining seeds are close to their limit of viability.

Zoologist: Over 25 per cent of the native mammals have disappeared in areas that are heavily infested with rabbits. Rabbits are very efficient grazers and also destroy habitats that may be necessary for the survival of some small mammals. Zoologists are concerned about the viability of these small mammals.

Politician: The Akubra hat industry forms part of my constituency. It takes fur from 13 rabbits to make one Akubra hat. If the Australian rabbit population is decimated, the Akubra industry will have to import rabbit fur from elsewhere. However, as a Senator, I also represent graziers who are demanding the release of the virus to save their properties from being denuded by rabbits.

Environmentalist: My organisation is very concerned that if the rabbit population is decimated, foxes will prey on small native mammals instead, thus endangering the survival of the native mammals. On the other hand I also appreciate that rabbits have a destructive effect – on plants and on species of animals that depend on the plants for food and shelter.

Geneticist: Is it possible that the rabbit calicivirus disease is capable of infecting animals other than the rabbit? Viruses are known to mutate quite readily, and a mutation might allow the virus to jump between species.

Veterinarian: There are vaccines currently available in Australia that will protect inoculated rabbits from contracting the disease. We need to run an advertising campaign to tell people that they can have their pet rabbits inoculated against the virus. The government should pay for this campaign and for the cost of the inoculations out of the money that is made from the increased productivity of the farms.

Animal liberationist: My organisation is opposed to the human species modifying the environment to suit itself. Other species in this world are equally important and we should not destroy the rabbit population of Australia.

To make the best use of role-play activities:

• have a clear idea of the desired outcomes;
• help students by ensuring that they have adequate information and that they understand the likely attitudes of the people they are representing.

Always leave time at the end of the lesson for debriefing. Debriefing can include questions that help students to distinguish between the simulation and reality, and between their own attitudes and those they acted out during the role play. In addition, you could tell students the desired outcomes and then they could discuss how effective they think the activity was. Debriefing helps to ensure that any antagonisms developed during the role-play do not continue after the lesson.

Activity 6. Spreading the calicivirus: a controversial issue

• What do you think the government should have done?

Teachers notes

Encourage students to consider the following points:

• risks that might arise from using the virus;
  
• the economic effect of rabbits on graziers and pastoralists;
  
• economic uses of rabbits;
  
• the effect of rabbits on native vegetation and small native mammals.

Governments and individuals often have to make decisions about controversial issues. Decisions can be based on emotion, impulse, random choice, habit, policy, precedent, or on the careful consideration of all the facts and options available. Although consideration requires time and effort, this process helps to identify more options.

Making thoughtful decisions should include considering the constraints related to an issue or a problem. Constraints limit the number of possible options for solving a problem.

Most decisions involve accepting one or more compromises, which economists term ‘trade-offs’. Accepting trade-offs means that one must reach an acceptable balance between the benefits and costs of various options. What is an acceptable balance is often the difficult part of the decision-making process.

Further reading

Useful sites

Rabbit calicivirus news (CSIRO, Australia)

This site has questions and answers about the rabbit calicivirus – a good starting point for information about this topic – and press releases from August 1995 monitoring the spread of the virus.
http://www.csiro.au/communication/rabbits/rabbits.htm

• The cactoblastis moth (The Science Show, 7 April 2007)
  Tracks the successes and failures of the use of cactoblastic moth as a biological control.
  http://www.abc.net.au/rn/scienceshow/stories/2007/1890832.htm

• Mulga miracle – calici aftermath (The Slab, 16 November 2000)
  Covers the effectiveness of rabbit calicivirus in reducing rabbit numbers and the impact of fewer rabbits in Australia's southern arid zone.
  http://www.abc.net.au/science/slab/calici/

• The feral outrage
  Transcript of Ockham's Razor, 17 November 1996. Professor Rob Morrison, Chair of the Anti-Rabbit Research Foundation, presents the benefits and perceived costs of introducing rabbit calicivirus into Australia. He also covers the concept of risk and risk perception.
  http://www.abc.net.au/rn/science/ockham/or171196.htm

• The bunny bug – friend or foe?
  Transcript of Background Briefing, 21 April 1996. Good discussion of the events surrounding the escape of the calicivirus from Wardang Island. Contributors from Australia and overseas express different points of view.
  http://www.abc.net.au/rn/talks/bbing/stories/s61.htm

This paper was prepared for the Prime Minister's Science and Engineering Council's meeting on 13 September 1996. It provides an overview of the problem of rabbits in Australia, existing methods for controlling this pest, the role for rabbit calicivirus disease and longer term prospects for other control measures including the use of fertility control agents.
http://www.dest.gov.au/archive/Science/pmsec/14meet/rcd1.html

Glossary

biological control. A strategy for the control of pests or disease-causing organisms that relies on the use of other living organisms rather than chemical pesticides.

Cactoblastis cactorum. A moth whose larval stage (caterpillars) feed on prickly pear.

calicivirus. A family of very small viruses, different species of which cause diseases in several animal species. One species causes haemorrhagic diseases in rabbits (called rabbit calicivirus disease or rabbit haemorrhagic disease). This disease rapidly kills mature but not young rabbits, but affects no other animal species.

habitat. 1. The place normally occupied by a particular organism or population.
2. The sum of all the factors that determine the existence of a community (eg, the freshwater habitat).

host. An organism on or in which a parasite lives.

immunity. A body's reaction to the introduction of foreign substances, through the production of defensive substances such as antibodies.

myxoma virus. (Also referred to as myxomatosis virus.) The virus that causes myxomatosis in rabbits.

myxomatosis. A disease in rabbits caused by the myxoma virus, transmitted by mosquitoes and fleas.

organism. Any living thing, whether single celled or many celled.

parasite. An organism that lives on or in an organism of a different species (the host) and gains some advantage at the host's expense.

population. All the organisms of one species that inhabit a given area.

population density. The total number of individuals of a species per unit area. Using density instead of total number gives a basis for comparison between numbers in different places or from time to time in the same place.

resistance (biological). The ability to withstand the effects of a disease-causing organism.

vector. An organism that transmits parasites, viruses or bacteria from one host to another.

virulence. The degree to which a disease-causing organism can affect the organism it attacks.

virus. A submicroscopic infectious agent consisting of a nucleic acid (DNA or RNA) molecule surrounded by a protein coat. Viruses cannot replicate outside a living cell. More information can be found at How viruses work (How Stuff Works, USA).

Rabbit calicivirus disease – a useful biological control
by Brian Cooke, CSIRO Division of Wildlife and Ecology, March 1998

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.