Establishing feasibility for porcine zona pellucida immunocontraception in a large population of free-roaming horses inhabiting the Virginia Range of Nevada, USA

Abstract

In North America, range constraints are increasingly encroaching on wild horse habitat, necessitating effective but humane reproductive management (Kirkpatrick and Turner. J. Zoo Wildl. Med. 1991; 23:445-452). The largest free-roaming wild horse fertility control program, both in terms of population (>4,000) and territory size (≈300 000-acres), inhabits Nevada's Virginia Range. This program, commenced in 2019, provided a unique opportunity for an extended temporal field study investigating feasibility of porcine zona pellucida (pZP) immunocontraception. Vaccine is administered via remote intra-muscular dart-delivery with an initial treatment of 0.5mL native pZP formulated with 0.5mL Freund's complete (primer) and incomplete (booster) adjuvant, respectively and thereafter at intervals ≈8-12 months. A database, continuously updated from field observations, provides individual animal records including date and type of intervention and of births, deaths and movements. This ongoing study aims to establish feasibility by evaluating vaccine efficacy measurable via annual birth rates and population demographics and concurrently analyze the effects of treatment frequency and season. For this reported initial phase, analyses included monthly data (January 2019 - December 2021; 36 months) for all females. This was characterized by:number of cumulative vaccinations subset into four further classifications considering vaccine having no efficacy loss or loss within a 6-, 12-, and 18- month period post vaccination; based on foaling data, likelihood of being in-foal; based on foaling data, likelihood of conceiving in that month; and based on age, considered as mature or immature (<1-year-old). Foaling rate and population size were descriptively presented over time at monthly intervals. Results described mean numbers of vaccinations per mature mare for any month, vaccination count in relation to conception rate (within mare) and population conception rates. A total of 1,501 individual mares were treated with administration of 4,795 vaccinations. Population data showed a decreased number of immature mares, declining pregnancy rate and a seasonal breeding peak that declined dramatically from 2020. The proportion of vaccinated mares increased steadily and by 2021 showed population coverage ≈40-80%. The most salient findings showed that the system reached stability, assuming a 12-month decay rate, at an average ≈1.0 vaccination/mare/year and can be recommended as a treatment frequency. As data collection and treatment is ongoing, future analyses will provide evidence-based and robust recommendations for best management practices.