Effects of immunocontraception on population, longevity and body condition in wild mares (Equus caballus).

The researchers examined the effects of contraceptive vaccine on wild horse populations, revealing it can successfully control population growth, reduce mortality rates, and possibly enhance longevity and overall health.

Objective of Study

• The study aims to explore the role and impact of contraception, particularly the use of porcine zona pellucida (PZP) vaccine, on wild mare populations. The researchers sought to understand the contraceptive’s effectiveness in controlling population growth, its impact on mortality rates, longevity, and the mares’ general health conditions.

Contraceptive Treatment and Its Effects on Population

• The contraceptive treatment of mares via PZP led to zero population growth in Assateague Island’s wild horse population within just one year of starting the treatment.
• The PZP vaccine showed high contraceptive efficacy—90% for mares treated twice in the year and annually thereafter, and 78% for mares given a single initial shot.
• The percentage of adult mares needing treatment decreased over the years. For the first three years of the study, 95%, 83%, and 84% of all adult mares received treatment. In the last two years, this figure dropped to 59% and 52%.

Change in Mortality Rates

• The study indicated that following the management-level treatment commencement, mare and foal mortality rates fell below the historic and pretreatment rates of approximately 5%, implying contraceptive treatment could actually decrease mortality rates.

Impact on Longevity and Body Condition

• The researchers noticed the emergence of two new age classes (21–25 and above 25 years) among the treated animals, suggesting an increase in their lifespan after treatment.
• The body condition scores of all horses, all adult mares, and non-lactating mares significantly improved between summer 1989 and autumn 1999. However, the scores of lactating mares did not show a significant change.

Research Implication

• The results from this study offer valuable data for creating realistic models for contraceptive management in free-roaming or captive ungulate populations. It serves as a confirmation of the effectiveness of immunocontraception vaccines in population control of wild species while maintaining their health and potentially increasing their longevity.