Reimmunization increases contraceptive effectiveness of gonadotropin-releasing hormone vaccine (GonaCon-Equine) in free-ranging horses (Equus caballus): Limitations and side effects.
Authors: Baker Dan L, Powers Jenny G, Ransom Jason I, McCann Blake E, Oehler Michael W, Bruemmer Jason E, Galloway Nathan L, Eckery Douglas C, Nett Terry M
Journal: PloS one
Summary
# Editorial Summary: GonaCon-Equine Reimmunization in Free-Ranging Horses Over an eight-year period (2009–2017), Baker and colleagues evaluated the contraceptive efficacy and safety profile of GonaCon-Equine, a gonadotropin-releasing hormone vaccine, in 57 free-ranging mares, administering either a single dose or a booster vaccination four years later and monitoring foaling rates, behavioural changes, and injection-site reactions. A single vaccination provided modest fertility suppression for two to three years (foaling rates 41–48% lower than controls in years two and three post-treatment) but efficacy waned by year four, demonstrating the reversible nature of the primary immunization. The critical finding was that reimmunization substantially increased contraceptive durability: mares receiving a booster vaccination showed foaling rates of just 0–16% over three consecutive years compared to controls, representing sustained suppression across multiple breeding seasons. The only observable adverse effect was intramuscular swelling at the injection site in approximately 62% of vaccinated mares; importantly, these reactions produced no lameness, gait abnormalities, or apparent pain, and the vaccine proved safe for pregnant animals and neonates. For practitioners managing feral or free-ranging herds where reproductive control is required, this research indicates that a single vaccination offers limited temporary benefit, but a planned reimmunization strategy at four-year intervals could achieve meaningful population growth reduction without the welfare or handling complications associated with alternative contraceptive methods.
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Practical Takeaways
- •GonaCon-Equine provides only temporary fertility suppression as a single dose (2-3 seasons), so reimmunization is necessary for sustained population control in managed herds
- •Injection site swelling is common (62%) but does not cause lameness or locomotor problems, so this is a manageable cosmetic reaction rather than a clinical concern
- •This vaccine offers a reversible, non-lethal management option for mares where population control is needed, with safety confirmed in pregnant animals and offspring
Key Findings
- •Single GonaCon-Equine vaccination reduced foaling rates in treated mares for 2-3 seasons post-treatment but returned to control levels by season 4, demonstrating reversibility
- •Reimmunization (booster at 4 years) significantly increased contraceptive effectiveness with foaling proportions of 0.0-0.16 versus controls for 3 consecutive years (P<0.001)
- •Approximately 62% of vaccinated mares developed intramuscular swelling at injection sites, but no lameness, gait alterations, or range-of-motion abnormalities were observed over 8 years
- •Vaccine was safe in pregnant females and neonates with no overt behavioral side effects during breeding season