Fertility control vaccines for possums: progress, challenges and prospects
Worldwide there is a lot of interest in developing humane non-lethal methods of pest control. In New Zealand research is focused on fertility control and aims to develop publicly-acceptable immunocontraceptive vaccines suitable for delivery in bait to free-living possums.
Since 2000, Janine Duckworth and her team at Landcare Research, in collaboration with Karen Mate and Carmen McCartney from the University of Newcastle Australia, have tested a range of injectable vaccines targeting the possum egg coat or zona pellucida (ZP). They have identified several marsupial-specific ZP proteins that prevent the fertilisation of eggs in possums (Table) but which have no effect on the fertility of bird and non-marsupial mammal species such as chickens and mice. Janine’s team have also assisted Lynne Selwood from Melbourne University to identify molecules that play a key role in the development of possum embryos. Some of the molecules from the early embryo’s appear to be unique to marsupials and injectable vaccines targeting two of them (proteins CP4 and VAP1) cause long-term infertility in treated female possums.
Table. Effect of vaccination by injection with possum-derived zona pellucida (ZP) antigens on the fertility of female possums following superovulation and artificial insemination.
|Vaccine antigen||Reduction in number of possum embryos relative to controls|
|possum ZP3 protein||80% ↓
|possum ZP2 C-terminal protein||72% ↓
|possum ZP2 N-terminal protein||75% ↓
|possum ZP3-peptide||60% ↓
|possum ZP2- peptide||64% ↓
Vaccine delivery to free-living possums has been a major challenge. Two delivery systems have been evaluated. First, in collaboration with Petra and Werner Lubitz at the University of Austria, bacterial ghost vaccines (BGs – particulate vaccines derived from non-living empty cell envelopes of gram-negative bacteria) engineered to express possum ZP molecules have been shown to significantly reduce both the fertilisation rate of artificially inseminated possums and the conception rates of naturally bred possums when the BG vaccine is delivered via oral or eye/nose routes. However, the initial promise of this work has not been fulfilled, as the team has been unable to sufficiently improve the intensity and longevity of the immune response to make the BG vaccines practical for field application. This is despite developing new forms of BGs capable of expressing the ZP antigen at higher levels, and encapsulated formulations to prevent the breakdown of proteins by enzyme and gastric acid degradation in the gastrointestinal tract.
Recently, therefore, the team reviewed potential delivery systems for fertility control vaccines in possums, and identified replication-limited poxviruses (such as recombinant vaccinia virus) as a potentially promising approach to developing an oral vaccine for possums. This choice was based on the highly successful oral rabies vaccine used to control rabies in wildlife in the USA and Europe for the last 20 years. As the first step in evaluating this live vaccine approach, in collaboration with Steve Fleming from the University of Otago, possums were exposed to a recombinanvaccinia virus expressing a model protein. The virus was applied to the external surface of the nose and into the mouth; a route of delivery designed to simulate the natural feeding behaviour of possums. The recombinant vaccinia virus established a short-term infection, and 14 of 15 treated possums developed antibody responses to the model protein. This is the first report of an Australian marsupial demonstrating an immune response to a recombinant antigen in a vaccinia virus.
The potency and longevity of vaccinia-based vaccines expressing an immunocontraceptive antigen in possums are yet to be confirmed. However, these initial results, and the extensive safety and efficacy precedents set by the commercial oral rabies vaccines used overseas, highlight the promise of recombinant vaccinia as a vaccine delivery system for fertility and disease control in possums and other marsupials. Janine’s team now has two pieces of the possum control puzzle: antigens that reduce possum fertility plus a potential means of delivering these proteins immunologically to possums via an oral route. The key to advancing the live vaccine concept for possum fertility control research to reality will be adequate funding to put these two components together. Following the closure of the Possum Biocontrol Outcome Based Investment funding stream (OBI) last year, this proof of concept research has been funded by Landcare Research. The challenge is to secure ongoing funding to support the research needed to move the fertility control vaccine from concept to reality.