Myxomatosis and calicivirus

Myxomatosis is a disease which only affects rabbits and possibly hares. When it was first released in the Murray-Darling Basin in the 1950s it had a 99% mortality rate, reducing the numbers of rabbits from more than 600 million to less than 100 million over two years.

Initially, myxomatosis was spread by mosquitoes, but as Lynette Gillam recalls, other blood-sucking insects soon carried the disease: "A neighbour next to us ran racehorses ... they were stabled in Geraldton for many months during the racing season. He then brought them home to his farm, and their stables adjoined the shearing shed, under which there was a huge rabbit warren. All of a sudden the rabbits there were dying of myxomatosis, and he had done nothing to bring myxo near his place so he couldn't understand it. But everyone else began to drop on to what was really the problem - these horses had brought home fleas, which had got into the rabbits, and that's what started the spread - and it gradually spread up the river."

Over the following years, rabbit populations developed some resistance to the disease, but even today, myxomatosis is still effective in keeping numbers down.

Rabbit Hemorrhagic Disease, better known as calicivirus disease, is a naturally occuring virus which only affects the European rabbit. The disease was officially released in Australia on 18 October 1996 in Cranbrook, Western Australia, although it had also escaped a year earlier from Wardang Island off the coast of South Australia. It is highly infectious and is spread in several ways - through contact between rabbits, through droppings, mosquitoes, fleas and other insects. However, as Dr Twigg explains: "It's not been as effective as we thought in all areas, such as high rainfall areas because there was another similar virus there, which appears to impact upon the ability of the calicivirus to spread across the country in those wetter areas."