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Managing populations of certain species such as free-ranging horses (Equus caballus) is a significant challenge, with animal-welfare, environmental, sociological, public policy, and economic dimensions. Immunocontraception is a useful tool, but vaccines must have multi-year efficacy to be technically feasible and cost-effective. SpayVac® has demonstrated single-dose, long-term contraceptive efficacy in several species. Grey seals (Halichoerus grypus) experienced an 80-85% decrease in fertility for at least 10 years post-treatment. No fallow deer (Dama dama) treated with SpayVac became pregnant for 3 years following vaccination. In a field trial with white-tailed deer (Odocoileus virginianus), no treated does became pregnant during the first 2 years post-inoculation, and 13% became pregnant during the year 3 compared to 78-100% of control does. A captive trial with white-tailed deer demonstrated contraceptive efficacy of 80–100% for 5 years with a single injection of SpayVac. Mares exhibited pregnancy rates of 0, 17, 17, and 17%, respectively, 1–4 years after treatment with SpayVac, compared to 75-100% for controls. In another study, the fertility rate for treated mares was 13, 47, 43, and 43% compared to 100, 98, 100, and 83% in controls for 1–4 years post-vaccination, respectively. Horses have seven different IgG isotypes, and we recently demonstrated that SpayVac preferentially stimulates production of IgG4/7 antibodies, which may contribute to its long-term immunocontraceptive efficacy. Vaccination methods, long-term population effects of immunocontraception, herd health and nutritional status, and population modeling scenarios should all be considered, when developing vaccination strategies for different species.

There are currently >75,000 feral horses (Equus ferus caballus) and burros (E. asinus ) on U.S. public lands, yet the Appropriate Management Level (AML) is set at just under 27,000. Wildlife managers, conservation biologists, and livestock ranchers are concerned about the impacts that these free-ranging horses have on shared rangelands. Immunocontraceptive vaccines may have the greatest potential to regulate horse population numbers once AML is reached; however, the vaccine must have multi-year efficacy to be both technically feasible and cost-effective. Immunocontraception based on porcine zona pellucida (PZP)-specific antigens is highly tissue-specific, targeting the ova, and blocking sperm binding through antibody occupation of ZP receptors on the ova. ZonaStat-H and PZP-22 are PZP-based vaccines; however, their contraceptive efficacy does not last long enough to meet management needs. SpayVac® achieves multi-year efficacy with a single dose, without boosters, because the PZP antigens are encapsulated within liposomes (multi-layered, submicroscopic vesicles), which gradually release PZP glycoproteins to antigen-presenting cells over an extended period of time. We review results from SpayVac trials in horses and other species to deepen our understanding of how the vaccine works and how it may best be applied to regulate free-ranging horse populations at AML. We examine 3 studies in horses, which used different SpayVac formulations and delivery methods, to learn more about the relationship between antibody titers and contraceptive efficacy, as well as potential mechanisms of action (e.g., preferential stimulation of IgG4/7 antibody isotypes). Additional research to explore possible effects of injection site location, antigen purity, role of PZP antibodies, and different formulations (e.g., dose, adjuvant) is needed for efficacious application to free-ranging herds.