Assessing recombinant vaccinia virus as a delivery system for fertility control vaccines in the brushtail possum (Trichosurus vulpecula)

The introduced brushtail possum (Trichosurus vulpecula) is a major threat to native biodiversity and agricultural production in New Zealand. Research on non-lethal management methods is focussed on fertility control, and aims to develop zona pellucida (ZP) vaccines suitable for bait delivery to free-living possums. Vaccine delivery remains a challenge. One highly successful oral wildlife vaccine which has been widely used to control rabies in wildlife in the US and Europe, is based on a replication-limited recombinant vaccinia virus (rVV). The potential of rVV as a vaccine delivery system has yet to be tested in any Australian marsupial species. In the present study we evaluated the infectivity of rVV, as well as cell-mediated and antibody immune responses, in the marsupial brushtail possum. Possums were exposed to a model recombinant vaccinia construct (rVV399, which expresses the Eg95 antigen of the hydatid disease parasite Echinococcus granulosus) or the parent vaccinia virus strain (Lister) by applying 108 pfu of virus, drop wise onto the external surface of the nose and into the oral cavity. Both the recombinant and parent viruses persisted in the mucosal epithelium around the palatine tonsils for up to 2 weeks post-exposure. The parent vaccinia and the rVV399 construct induced peripheral blood lymphocyte reactivity against viral antigens in possums by 4 weeks post-exposure, and were still detectable at 4 months post-exposure. Serum antibody reactivity to the antigen Eg95 was recorded in 7/8 possums which received a single dose of the rVV399 construct and in 7/7 animals which received tripledose delivery, with titre end-points in the latter case exceeding 1/4000 dilution. This study demonstrates that vaccinia virus will readily infect possums via an oronasal route and is capable of generating immune reactivity against both viral and heterologous antigens. This highlights the potential of recombinant vaccinia as a wildlife delivery system for fertility control vaccines for the brushtail possum and potentially other marsupial species.