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Introduced diseases

Myxomatosis and, more recently, Rabbit Calicivirus Disease (RCD), are both viruses that have been deliberately released to kill rabbits. Myxomatosis in particular causes a slow and painful death.

Myxomatosis

The myxoma virus was introduced in 1950. Within 2 years the population was reduced from an estimated 600 million to 100 million (1, 2). At that time, the myxoma virus killed 99% of rabbits who came in contact with it, but today it kills less than 50% of infected rabbits (1). Other writers say it kills between 30% and 80% (2) or between 20% and 70% (3), depending on the strain of virus and the condition of the rabbits. What is certain is that some rabbits have resistance to the disease and have continued to multiply, to the point that there were an estimated 300 million rabbits when the rabbit calicivirus "escaped" in 1995.

The virus is spread by mosquitoes and European fleas as they bite rabbits. However, these insects can't survive in very dry conditions, so myxomatosis was not spread to arid central Australia. In 1992 the Spanish flea was introduced to spread the disease to the dry areas.

Myxomatosis is a very cruel disease. The following is a description of its progress once a rabbit has been infected (4):

" The virus begins to multiply in the rabbit's epidermis or dermis - the outer and inner layers of skin. It often produces an ugly bump on the site that looks like, but isn't, a tumour. By the second day, the virus has spread into the rabbit's lymph glands and by the fourth is located throughout the body tissues. By day five, the rabbit's eyes have begun to swell, as have other sensitive areas such as its genitals and anus.

Over the next three days, the infection becomes widespread and the tumour-like bumps appear over several places of the body. Sometimes, the swelling is so severe in a male that the scrotum is ruptured ...

A rabbit infected with the most virulent strain of myxomatosis is usually dead by the tenth day. If, however, the strain is not so lethal, or the rabbit has some immunity, it can live for three weeks. By this time, though, the eyes are closed, the ears hang pendulously, tumours cover the body, the animal's breathing is labored and pus pours from the nose. Death soon follows. "

Nothing can now be done to remove the virus from the environment - it will continue to be carried by wild rabbits. However, it is completely unacceptable that authorities are still supporting the spread of this dreadful disease, for example, by the release of the Spanish flea.

New Zealand, unlike Australia, rejected the introduction of myxomatosis.

Rabbit Calicivirus Disease

The Rabbit Calicivirus Disease (RCD) appeared in China in 1984 and killed millions of farmed rabbits. It spread into Europe and killed 64 million farmed rabbits in Italy alone (5). No-one knows why it suddenly became a problem in 1984. It is possible that the virus mutated into a stronger form, or that it jumped from another species.

There are different strains of the virus - the one imported into Australia came from a Czech laboratory. It was tested in a laboratory in Geelong, and then field trialed on Wardang Island off South Australia. Unlike myxomatosis, this virus doesn't depend on insects for its spread. Contact with an infected animal or its faeces, in addition to insects, will spread the disease.

The virus mysteriously escaped from the supposedly quarantined Wardang Island. It rapidly spread into various states. A year after the "escape" the virus was officially released.

Rabbits take about 2 days to die, the eventual cause of death being heart and respiratory failure. Here is a description of how they die (6):

" In the last 24 hours of their life they experience fever and an increased respiratory rate, and increasingly seem oblivious to their surroundings. They die from a slow deprivation of oxygen caused by a breakdown in the small blood vessels ".

It is hard to know how much the rabbits are suffering during this time. The fact that they don't squeal or struggle doesn't rule out the possibility of suffering. However, they are likely to suffer less than with myxomatosis.

RCD was tested on 28 species in Australia (9 domestic species, 5 feral species, 5 native bird species, 1 reptile, and 8 native mammal species). None of these animals developed antibodies to the virus, so scientists assumed that it would not affect any other species (7).

In New Zealand, antibodies to RCD were found in kiwis. Because they were uncertain if the virus would mutate into different forms or infect native species, and because they couldn't predict what species predators would kill if rabbits were wiped out, NZ authorities refused to allow the release of RCD. They said (8): " It is not possible to predict the indirect effects of the virus on indigenous flora and fauna because it is not known how the virus would behave in New Zealand ". Less than 2 months later RCD mysteriously appeared in New Zealand and has spread widely, no doubt with human assistance.

There are reports that RCD has killed up to 95% of rabbits in some areas of Australia (9). However, the problem with all diseases is now becoming apparent - as with myxomatosis, more and more animals become immune. This happens in two ways. First of all, some animals have a natural resistance to a disease. They live on and continue to breed, passing on resistance to their offspring, so that more and more of the population has this resistance.

Secondly, viruses become weaker over time. Very deadly strains kill their host before they have had a chance to spread the disease. Less virulent strains of the virus take longer to affect the host, who can still move around and spread the disease to other animals, ensuring its survival. Gradually the weaker strains will become more widespread than the more virulent strains. In Queensland, 67 of 110 rabbits tested at one site had resistance to RCD (9). So, just over 2 years after "escaping" from Wardang Island, the disease is already losing its effectiveness.

These examples highlight several problems with diseases to control populations:

• they can cause great suffering,
• the disease loses its effectiveness over time,
• as with any killing method, animals left alive continue to breed and before long the population will be back to where it started.

I would like to go to other documents on feral animals:

Feral animals in Australia: introduction
Poisons
Trapping
Mustering for slaughter and export
Other methods (shooting, warren ripping, dogging)
Alternatives to killing

References