Contraceptive efficacy of priming and boosting doses of controlled-release PZP in wild horses

Abstract

Context At some sites, wild horse populations have been effectively and non-invasively regulated using remote darting with porcine zona pellucida (PZP) immunocontraceptive vaccines. However, this model has not been widely applied to wild horse herds in western USA, many of which are difficult to access because they roam large areas and are wary of people. Single-treatment, multi-year contraceptive vaccines would significantly broaden the scope for successful contraceptive management. Aims The aims of the present field studies were to (1) test the contraceptive effectiveness and longevity of primers incorporating PZP–adjuvant emulsions plus PZP and adjuvant in controlled-release pellets (‘PZP-22’); and (2) compare the contraceptive effectiveness and duration of PZP–adjuvant emulsion-only boosters with those of PZP-22 boosters when administered by hand or remote darting to PZP-22-primed mares. Methods Wild horses in herd management areas in Colorado and Utah, USA, were rounded up in 2008 and in Utah again in 2012. Previously untreated females received a single hand-injection of PZP–emulsion plus controlled-release PZP pellets (‘PZP-22’ vaccine) and were then released. In Autumn 2010, 50 Colorado females treated in 2008 were booster-darted with either the PZP-22 vaccine or the PZP emulsion alone. In Utah, 57 previously treated females rounded up in 2012 received hand-injections of either the PZP emulsion–pellet vaccine or the PZP emulsion alone. Maternity was established through field observation of nursing and other close associations. Key results Effectiveness of initial controlled-release PZP treatments over 2 years was low relative to previous trials. However, boosters delivered by dart or by hand to PZP-22-primed mares yielded high levels of contraceptive effectiveness for 3 years, with no consistent difference between simple PZP-emulsion boosters and boosters incorporating controlled-release pellets. Conclusions Priming mares with PZP-22 extends the efficacy of a subsequent PZP booster to at least 3 consecutive years. Implications Regulation of wild horse populations may be achieved with existing contraceptive agents by developing models and management plans that account for the increased longevity of repeated contraceptive treatments.