An experimental contraceptive vaccine developed by Landcare Research scientists may provide a long-term solution to New Zealand’s possum problem.

In trials at Lincoln vaccinated females were five times less likely to produce offspring than untreated females. This vaccine contained whole sperm but researchers are refining the vaccine by identifying specific proteins from the surface of the sperm critical for fertilisation. Possum eggs are also being investigated as a source of antigens.

The possum egg is surrounded by a protein layer called the Zona Pellucida (ZP), which is made up of three different different proteins ZP1-3. ZP3 has been used to develop effective contraceptive vaccines in species like pigs and horses. It is now being tested in possums where it reduces breeding by 70 per cent. Ideally the chosen proteins should not occur elsewhere in the body (so the vaccine will only work against sperm or eggs) and should only occur in possums.

Early tests

Although early tests have been successful they are only the beginning of the research programme. Once a vaccine is developed, the means of distributing it throughout the possum population has to be addressed. The contraceptive vaccine could be given to possums in bait as an oral contraceptive. Another possibility is to use a modified organism specific only to   possums, which carries an extra gene to produce the specific sperm or egg protein.

When the organism infects the possum the extra gene would cause it to produce copies of the sperm or egg proteins. The possum’s immune system would then produce antibodies against this foreign protein, sperm and eggs would be attacked in the reproductive tract and  the ovaries, and infected female possums would become infertile. A sperm-based contraceptive vaccine could also make male possums infertile.

The vaccine makes the possum infertile by tricking its immune system into treating the sperm or egg as a foreign body. Antibodies attack the sperm and eggs and prevent fertilisation. Possums sterilised by this modified organism could eventually regain their fertility as the  immune reaction declined, but possums would be susceptible to reinfection and consequent reimmunisation.

Contraceptive effect

The contraceptive effect could be longer lasting in females as each new mating would act as a booster vaccination with a new dose of sperm or egg proteins. Ideally the organism chosen to transmit the vaccine would be humane, possum specific and spread by close contact  between possums. It would also be one that would not survive long in the environment (to avoid spread of the vaccine across the Tasman).

The completed contraceptive vaccine may take 10 to 15 years to develop as it is essential that such a vaccine will only affect possums. A bait delivery system may be available within five to seven years.

The research is part of a long-term collaborative research project between scientists at Landcare Research, AgResearch and the Co-operative Research Centre for the Conservation and Management of Marsupials. It is supported by the Foundation for Research, Science and  Technology, the New Zealand Lotteries Grants Board, Animal Health Board, the Marsupial CRC and MAF Policy.