Tue, 19 Oct 2004

Breakthrough In Bid For Possum Birth Control

Researchers have made major progress toward developing a contraceptive for possums in the wild, with a new experimental vaccine that reduces possum fertility by a third.

Possums damage native forests, spread bovine Tb among cattle and deer, and eat the eggs and chicks of native birds. Trapping, shooting and especially poisoning are used to reduce possum numbers. Effective fertility control would reduce the need for poisons such as 1080, and further reduce risks to non-target species.

Landcare Research physiologist Dr Janine Duckworth and her team have developed a vaccine that works by immunising female possums so that their eggs are not fertilised. The technique uses "bacterial ghosts", which were developed overseas for medical vaccines to fight disease in humans. "We start with a harmless strain of the common E.coli bacterium. We then add a protein found in female possums' eggs into the cell wall of the bacteria. The bacteria are then killed and the shells of the empty bacterial cells or "ghosts" are given to the possum.

"The possum's immune system recognises the bacterial ghost as foreign, and produces antibodies against them. At the same time, it is tricked into developing antibodies against the possum egg protein as well, thus having a contraceptive effect." In trials held in containment, 20 possums were dosed with an inactive vaccine, and 20 with the vaccine containing the possum egg protein. The possums were artificially inseminated. Around 90% of the eggs of the possums given the inactive vaccine were fertilised. This figure dropped to 58% for the possums given the vaccine containing the egg coat protein.

"This is the proof of concept that we have been looking for. We are delighted it has worked so well," Dr Duckworth says. "Our next aim is to further reduce fertility, by more than 50%, and then to make the contraceptive effect last longer.

"If we can achieve this, the amount of poison used in New Zealand could be reduced by at least 50%, and potentially more as we improve the vaccine." Dr Duckworth and her team will continue working with collaborators at the University of Vienna, Austria, to refine the vaccine design.

"We will trial different preparation methods to improve the vaccine's effectiveness still further.

"Also, currently the vaccine is applied in a way that mimics a nasal spray, entering the possum's body through the mucosal surfaces of the eyes and nose. We want to develop an edible encapsulated bait for the vaccine, which would be easier to use in the field." All research on the vaccine so far has been held in containment. The researchers plan to apply for ERMA approval to run limited field trials in 2008.

ENDS

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