Feature

Myxomatosis, a Continuing Debate

Does myxomatosis hold the answer for the high country rabbit problem?

By Janine Griffin

Myxomatosis has, once again, been proposed as a solution to the high country rabbit problem. The South Island Land Protection Co-ordinating Committee, a coalition of high country farmers, recently applied for MAF permission to import the virus and a carrier flea for introduction to areas in central Otago and Marlborough where rabbits occur in high numbers.

If approved, the introduction could take as long as three to five years, at an estimated cost of around $2 million. There is no guarantee, however, that the virus will reduce rabbit numbers.

Efficient Vector Needed

In 1953-54, the myxoma virus was introduced to New Zealand. Local outbreaks occurred, but the virus failed to spread because of the lack of an efficient vector, or carrier, of the disease. It was clear from this that a vector would be needed if the virus is to spread.

The current application proposes the introduction of the rabbit flea, Spilopsyllus cuniculi, as a vector. Jim Bell, of MAF's Rabbit and Land Management Programme, believes the flea will establish itself in sufficient numbers to allow myxomatosis to spread. Conditions in the Mackenzie Country are similar to those in Spain and other parts of Europe where the rabbit flea is present and is the primary carrier of myxomatosis.

An initial myxomatosis outbreak would, says Bell, result in a high kill rate in the Mackenzie Country. Other management strategies would still be required, however, to control rabbits once they adapted to the disease.

In Britain and Europe, where the main vector is the rabbit flea, myxomatosis spreads at much lower rates than in Australia, which has a mosquito vector. The benefits of rapid spread in Australia are offset by attenuation of the virus and the development of resistance by rabbits.

Rabbit Resistance

That rabbits adapt to myxomatosis is well-known from Australian studies. In the months after the first myxomatosis outbreak in 1950, the disease killed 99% of rabbits. Since then, however, the virus has become progressively less effective, and some strains have appeared which kill less than 50% of infected rabbits.

Changes in the genetic makeup of the virus, known as attenuation, result in less virulent strains becoming more common. In addition, the rabbits themselves seem to become increasingly resistant. This apparently genetic resistance to the disease, combined with the virus' attenuation, reduces the effectiveness of myxomatosis.

In Britain, myxomatosis spreads more slowly and attenuation is not as rapid as in Australia. There is some evidence that the rabbit populations are slowly developing their own resistance ot the disease. The slow spread of the flea-borne disease seems to maintain more virulent strains and slows down the development of resistance.

Both Australian and British introductions of myxomatosis were not well-managed, says Bell. The release of the myxoma virus in Australia was erratic, with the first "release" an escape. In Britain, myxomatosis is thought to have been illegally introduced. It is difficult, therefore, to compare the New Zealand situation with overseas experiences.

Other Options

Research in Australia is now focused on developing a non-virulent strain of the myxoma virus which carries a gene that will sterilise rabbits. Such a strain could be developed by 1995.

It is hoped that this form of virus will slow rabbit reproduction to a manageable level. But if a recombinant virus were introduced to an area where myxomatosis was already present, the recombinant version could die out, or the rabbits could become immune to infection.

The rabbit flea is highly specific to rabbits, as it requires rabbit hormones to breed. Worries that it could infect native birds, and perhaps carry diseases, are probably unfounded. The risk of the fleas carrying diseases potentially threatening to humans is low. A 1986 disease risk assessment reported no evidence of the flea as a carrier of any disease other than myxomatosis. Screening of imported fleas will minimise the risk of other diseases entering the country.

Despite 40 years of myxomatosis, Australia still has a rabbit problem, and it appears that Britain and Europe will also have problems in the future. Myxomatosis has not been effectively managed overseas, and a deliberate, well-managed local introduction could prove successful. But how long it would remain successful is unknown. If myxomatosis is successfully introduced and fails to control rabbits, it could present New Zealand with both a continuing rabbit problem, and a new, uncontrolled flea and virus.

At present, myxomatosis is only a temporary solution to the rabbit problem. If used in New Zealand, it must be used in conjunction with other solutions, such as changes in land management practices.

Furthermore, an introduction of myxomatosis at present could jeopardise the future use of a more permanent alternative, a recombinant myxoma virus which spreads sterility among rabbits.

Janine Griffin is a freelance journalist specialising in science issues.