Proceedings of the Vertebrate Pest Conference

Fertility control vaccines are under development for a number of pest wildlife species, mostly based on whole zona pellucida (ZP) or individual ZP proteins. Such vaccines must be effective, long lasting, cheap and readily deployed. One approach to deployment is oral delivery in baits. This is one strategy being taken in New Zealand for control of brushtail possums, 2- to 4-kg marsupials introduced from Australia in the 1850s, and now major pests of both conservation and agricultural production. New Zealand has highly effective aerial and ground systems for delivery of toxic baits to possums that could be adapted readily to deliver vaccine baits. Recent trials in captivity where female possums were immunised with recombinant possum ZP3 and ZP2 proteins demonstrated 70-75% reductions in fertility in natural and assisted breeding trials. Immunisation with possum-specific epitopes of the ZP2 and ZP3 proteins has also proved effective at reducing the numbers of fertilised eggs recovered from immunised females. For field delivery of an oral vaccine, we are investigating the use of bacterial “ghosts”. These are the empty cell walls of bacteria that have been modified to express possum ZP proteins in their cell walls. 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 proteins, causing a contraceptive effect. In a recent proof of concept trial, female possums immunised with a possum ZP2-bacterial ghost vaccine by nasal spray showed a significant reduction in fertility. Oral delivery may require protection of vaccine ghosts from degradation. We have developed a protective system and are currently repeating this trial using oral delivery of the ZP2 ghost vaccine. Our future priority is increasing the vaccine efficacy and longevity ahead of limited field trials in 2009.