Efficacy and safety of recombinant zona pellucida vaccines in domestic horse mares and current application of native porcine zona pellucida vaccines in African elephant cows

Abstract

The use of immunocontraceptives in veterinary population management and wildlife fertility control is well-established. The porcine zona pellucida (pZP) vaccine has been successfully used in approximately 85 species. The successful application of pZP vaccine over several decades in domestic and free-roaming horses (Equus ferus caballus) has served to inform the development of population control management programmes for other species including the African elephant (Loxodonta africana). Elephant populations in South Africa, predominantly within fenced game reserves, have been humanely managed with the pZP vaccine since 2000. Whilst the pZP vaccine meets many of the gold standard requirements for population management, it has not been without its caveats. These are partially due to the potential complications associated with both native derived ZP proteins from pig ovaries and their formulation with Freund’s adjuvants. With increased demand for effective and humane population management, there is a need for improvements to current vaccine formulations for economical, regulatory and animal welfare reasons. This research investigated the ovarian effects, immunoreactivity and safety of a novel recombinant zona pellucida (reZP) vaccine for use in horses, as both a model and target species. Additionally, alternative commercially available, effective adjuvants for inclusion in ZP-based immunocontraceptive vaccines were tested. Finally, the current status of a pZP immunocontraception programme in managed populations of elephants in South Africa was examined. This work investigated a potential alternative to native pZP immunocontraception in mares supported by ovarian effects, antibody titre responses and safety profiles. A reZP vaccine prepared with promiscuous T-cell epitopes of tetanus toxoid and bovine RNase formulated with Montanide™ Pet Gel A and Poly (I:C) adjuvants was shown to be a suitable alternative for immunocontraception in the horse. Additionally, the measurement of anti-Müllerian hormone (AMH) was associated with measurements of ovarian activity subsequent to immunocontraception. This may facilitate monitoring of ovarian function following immunocontraception in horses under extensive conditions which may be similarly applicable in other species. Finally, this work showed increasing and successful application of the pZP vaccine in South African elephant populations. An effective, safe reZP vaccine holds promise for commercialisation with its associated higher yields and reduced costs in vaccine production. This will increase availability of immunocontraception for managing South African elephant populations and its potential application in other species. Overall this work reported novel and improved alternatives to currently-employed methods for veterinary population management of horses with the potential for similar applications in elephants and other species.