WAR AGAINST RABBITS

Press, Volume XCVI, Issue 28420, 29 October 1957, Page 20

WAR AGAINST RABBITS

Australia Buys Tinned Fleas (By a Reuter Correspondent) MELBOURNE.—On the principle of setting a thief to catch a thief, Australia is planning to use the imported rabbit flea to help fight the country’s worst imported pest—the rabbit. Australia’s millions of destructive rabbits stemmed from a small consignment brought from Europe in the early colonial days. But the rabbit flea, found in nearly all other countries where the European rabbit has migrated, either did not arrive with those first rabbits or did not survive. Now hundreds of rabbit fleas are being brought in tins by airliner from England and soon they will be in the rabbit war. Recently arrived fleas are in the care of the biological control department of the Commonwealth Scientific and Industrial Research Organisation in Canberra. The department is establishing a colony of them and making tests to ensure they are harmless to native animals. Next year, if the fleas pass all tests, they will 1 be released to spread myxomatosis i among Australia’s rabbits.

It is expected that they will play a big role in spreading the; rabbit-killing disease which is' saving Australia millions of 1 pounds each year by reducing the : rabbit population—and the dam-! age it does. Since myxomatosis was first re-1 leased in Australia in 1950.! scientists estimate that it has I cut the number of rabbits, then in j plague proportions, by 80 or 90' per cent. But, due to the rate; at which rabbits breed, those remaining are still a serious problem and the battle against them I must be waged relentlessly by i the various government rabbit control authorities, farmers’ organisations and the land-owners l themselves. Each year a wave of myxomatosis is sent through Australia by the release of thousands of rabbits inoculated with the myxoma virus. But the disease, which affects only rabbits, will not spread by itself. It. must be carried from a sick rabbit to a healthy rabbit by an insect of some kind. So far the mosquito has been the main means of transmitting myxomatosis from the affected rabbits to others—and has done a wonderful job. However, it has been found that the mosquito alone is not the complete answer as a vector—or spreader—of the disease. There are several factors involved. One is that the mosquito does not reach some high country. Another is that its usefulness is seasonal as it must be brought into contact with infected rabbits during its breeding time, its period of greatest activity each year, to be fully effective. Then too, its breding is affected by unseasonal weather behaviour, a dry period with little surface

v ater about tending to reduce its numbers at the crucial period. “Becoming Immunp” Yet another—and probably the most important aspect of the myxomatosis campaign—is that a proportion of Australia’s rabbits are becoming immune to the disease—and are breeding more immune rabbits. Rabbit control authorities have discovered that a ‘-field’’ strain of the myxoma virus—that is a strain which has been kept going for some time by the rabbit to mosquito, mosquito to rabbit . sas5 as become weaker than the laboratory strain and has dropped in killing power. Ten to 15 per cent of the rabbits infected with this weaker strain recover and. having gained immunity through infection, will not die of myxomatosis. Norwill their offspring. This strain infects rabbits for about 25 days, during which time the sick rabbits are easy prey for the mosquitos which bite them and then carry the weaker virus on to other rabbits. In this way immunity is spread. This is the reason why Australia is no longer using a French strain of the myxoma virus. It was found that it had a high killing power but quickly drooped in virulence in the field and instead of helping to wipe out the rabbit it became an immunising , agent. The Australian standard laboratory strain, which has been re-1 leased each year since 1950. has a killing power of over 99 per | cent, of fully susceptible rabbits.| It takes about 12 days to kill a i rabbit. Control authorities hope that j the introduction of the rabbit j flea will help in spreading the stronger virus. Unlike the mns-! quite, the rabbit flea leaves the \ rabbit only when the rabbit dies. : This means that if the flea is on ! a rabbit infected with the less virulent virus it will stav -with that rabbit while it lives and ' the wea ker strain. ! rabbit dies of virus of i power the flea wUI leave the dead rabbit and infect;

another, thereby spreading the more lethal myxoma “bug.” The flea is also expected to penetrate those areas not normally visited by mosquitos. Will myxomatosis exterminate Australia’s rabbits? Authorities do not think so, according to Mr Geoffrey Douglas. Victorias coordinator of rabbit control. “Point of Equilibrium” “We think that a time will come when myxomatosis and the rabbits’ immunity will reach a point of equilibrium.” he said. “Then we will have to get to work with other methods, including the orthodox ones which have been improved lately. “But the general impression is that even without further control the rabbit would never get back to the plague proportions of 1950. With continued control, the time of equilibrium will be put off. “Myxomatosis isn’t the complete answer to the rabbit problem and ‘follow-up’ methods of killing, such as poison, must be used as j well—especially to kill the im- , munised rabbits. But myxomatosis has been a wonderful inI vestment for Australia. It saves [crops, puts more land into pro- ; duction and helps combat erosion ]ty keeping the land clad. ! “The total amount spent on the ' myxomatosis campaign would not ■be £250.000, but the returns to ' the country from it have been j estimated conservatively at £lOO j million since 1950. Some people ‘estimate that it saves £5O million |a year.”