“Parasites Of No Help When Rabbits Are Few ”

Press, Volume CI, Issue 29871, 11 July 1962, Page 10

“Parasites Of No Help When Rabbits Are Few ”

“Parasites are unlikely to be of much help in getting rid of rabbits in New Zealand, particularly now that rabbit numbers are generally low,’’ said Mr P. C. Bull, of the Animal Ecology Division, Department of Scientific and Industrial Research. Mr Bull has been studying rabbit parasites since 1950. “As the number of rabbits decreases, the number of parasites they contain also falls, because there are fewer rabbits to spread infection and the rabbits have fewer contacts with each other,’ 1 said Mr Bull.

Mr Bull has investigated 21 species of rabbit parasites, including two mites, a louse, a dog flea, a rat flea, a liverfluke, two larval stages of dog tapeworms, five nematodes (roundworms), and several protozoa (one-celled animals).

The European rabbit flea, which was important in spreading myxomatosis in rabbits in Europe, had never become established here, and the other external parasites which infest New Zealand rabbits were apparently never successful in propagating the disease or in otherwise controlling the animals, said Mr Bull. In Australia, myxomatosis had been spread by mosquitoes, but the New Zealand mosquitoes did not seem to bite rabbits enough to help much in carrying the disease. There seemed little prospect of myxomatosis being helpful in killing off the remaining rabbits in New Zealand, even if the European rabbit-flea was introduced, because there were too few contacts between rabbits in present sparse populations. There was a possible advantage in introducing the flea now. however, in that if it could be established in rabbits while they were at a low ebb it would spread with them if for any reason (such as adverse economic conditions) there was a reduction in the present efficiency of control. Myxomatosis should not be introduced at this stage, as to do so would result in the emergence of attenuated strains of the organism. This had happened in Australia, where it now killed fewer of the rabbits it infected. New Zealand was in a much more enviable position than Australia with regard to rabbit control, because whereas in Australia myxomatosis was becoming less effective. New Zealand still had her poisoning and other proved techniques administered by well-organised rabbit boards under the guidance of the Rabbit Destruction Council. Among the internal para-

sites examined were the coccidia, protozoans which lived inside the living cells of the host. One species caused serious damage to the liver in young rabbits and was sometimes mistakenly referred to as hydatids. The disease killed off rabbits up to about four months old, and was most successful ■when the young were present in large numbers. With the diminished population of rabbits, this parasite was now less effective than formerly. Tapeworms Two species of dog tapeworms whose laryae occurred in rabbits were, unfortunately. expelled from dogs by drugs used in the control of hydatids. Most of the rabbits infected were adults, indicating that control of these worms was in any case unlikely to be of much good in checking any swift expansion of the rabbit population The weights of infected and uninfected rabbits were similar, and the percentage

of females pregnant was about the same.

The pin worm, a roundworm, similarly . had no measurable effect on weight or fertility, although infections were sometimes very heavy. This worm was absent from the southern half of the North Island, though common elsewhere.

A second roundworm, the small intestinal worm, had more effect, often reducing the weight and fertility of heavily-infected rabbits. It seemed most effective, however. when the resistance of the rabbits was already reduced by some other factor, such as overcrowding. The stomach worm, another roundworm, was absent in the north of the North Island, although it was advancing into that area. Comparisons between the thrift of rabbits in areas where the pin and stomach worms were absent and where these worms were present did not indicate any control by the worms.