Two viral biological control agents, Myxomatosis and Rabbit Haemorrhagic Disease Virus or RHDV (also known as Rabbit Calicivirus or RCD) have been introduced to Australia to help reduce the impact of rabbits on agricultural production and the natural environment.
The myxoma virus was introduced into eastern Australia in 1950 and to Western Australia in 1951. Despite some less virulent strains evolving, outbreaks (epizootics) of myxomatosis (the disease caused by the myxoma virus) are still important in helping to keep rabbit numbers under control.
The virus is transmitted on the mouth-parts of biting insects, particularly mosquitoes and fleas. Mosquitoes were the only efficient spreaders of the myxoma virus in Australia until the European Rabbit Flea and the Spanish Rabbit Flea were introduced in 1969 and 1996. Outbreaks of myxomatosis during winter are mainly spread by European rabbit fleas and often tend to kill more rabbits due to the added stress imposed by lower temperatures at this time. Mosquito-borne outbreaks generally occur in summer.
Studies by research staff from the Department of Agriculture and Food, Western Australia (DAFWA) have shown that myxomatosis alone will often not reduce rabbit numbers to an acceptable level. Death rates for this disease can vary from 30% to up to 90%, but are typically around 50%. In some years, high mortality may occur but the rabbit population can recover quickly the following year because most surviving rabbits are immune. Following an outbreak of myxomatosis, infected rabbits that survive acquire lifelong immunity. As well, young born to immune mothers are protected from the disease during the first six weeks of their lives by maternal antibodies. Exposure to the virus during this period will convert the temporary immunity into life-long immunity.
Because of this, an outbreak of myxomatosis does not usually occur in two consecutive years because there are too few susceptible rabbits. By the third year, natural attrition, and the birth of new susceptible rabbits, reduces the proportion of immune individuals and the population again becomes susceptible to the disease.
RHDV was introduced to Australia in October 1996. This virus, which is specific to rabbits, was accepted as a biological control agent against rabbits in Australia after undergoing rigorous testing on livestock and native species. The virus was deliberately released into selected rabbit populations at about the same time it began to arrive in Western Australia from other parts of the country.
The initial impact of RHDV was dramatic, causing drops of up to 90% in some rabbit populations, but there were other areas where little or no apparent effect was seen. The greatest impact occurred in the drier parts of the country (less than 300mm annual rainfall). A new strain of the virus which will have greater impact on rabbits in higher rainfal areas is planned for release in WA in 2017.
RHDV is spread by direct contact between rabbits and by some biting insects. It is also transmitted by some species of blowflies. The virus can persist in the environment for several weeks under mild conditions but its survival is shorter than this at higher temperatures.
For reasons not well understood, young rabbits up to about eight weeks of age are less susceptible to RHDV than are older rabbits. In addition, maternal antibodies temporarily protect rabbits born to immune mothers. If exposed to the RHDV virus at this time, these kittens are likely to survive and develop antibodies, which give them life-long immunity to the disease.
The timing of RHDV outbreaks is therefore important in the long-term impact of RHDV on rabbit populations. If outbreaks occur, either by deliberate release or naturally, when there are many young rabbits present, sufficient rabbits are likely to survive, allowing the population to recover.
The interaction between myxomatosis and RHDV and their combined impact on rabbit populations requires further study. To date, RHDV does not appear to have lessened the occurrence or impact of myxomatosis on Australian rabbits. Outbreaks of both diseases continue to cycle through many rabbit populations, occasionally simultaneously at some sites.
It is not possible to control outbreaks or the impact of biological control agents such as the myxoma virus and RHDV. The susceptibility of rabbit populations to these diseases will depend upon a number of factors including the number of susceptible rabbits (those without antibodies), and the availability of suitable vectors for transmitting each disease. These diseases should be viewed as an aid in controlling rabbit numbers and should be supported with other control methods such as poisoning, refuge removal, ripping, fencing and shooting to ensure greater long-term effect. The addition of follow-up control is vital to maximise the benefits from these diseases.