Effects of two porcine zona pellucida immunocontraceptive vaccines on ovarian activity in horses

Abstract

Feral horse population growth rates as high as 25% are of concern to those responsible for managing range lands as well as conservation groups. Current methods to control these populations include adoption and long-term holding, which are both costly and controversial. Porcine zona pellucida (pZP) immunocontraception may have the greatest potential to control fertility because it has proven to be effective in other studies and vaccines are easy and safe to administer. One pZP vaccine formulation, SpayVac 1 (ImmunoVaccine Technologies, Inc., Halifax, NS, Canada), has demonstrated single-dose, multi-year contraceptive efficacy in other wildlife species, which would make it both practical and economical for field application. Over a 7-month period during the breeding season, we assessed the effect on ovarian activity of 2 formulations of SpayVac, 1 non-aqueous with modified Freund's adjuvant (MFA) and the other, an aqueous emulsion with MFA, compared to controls (n ¼ 7 per group). Comparative reproductive parameters included serum concentrations of progesterone (P 4 ) determined by enzyme-linked immunosorbent assay (ELISA), ovarian activity assessed by transrectal ultrasound and palpation, as well as gross and histological examination of ovaries upon necropsy (n ¼ 9 or 3 mares from each group) or after ovariectomy (n ¼ 12 or 4 mares from each group). We determined serum antibody titers using ELISA. Mean serum concentrations of P 4 were less in the non-aqueous MFA treatment group compared to control mares (P < 0.025). Ovaries collected from control mares weighed more (P ¼ 0.002) and had greater variation (P ¼ 0.003) than those from either vaccinated group. Both treatment groups also had smaller ovaries and fewer follicles compared to controls (P < 0.001). Three to 4 months after vaccination, 93% of SpayVac-injected mares ceased cycling; whereas all control mares continued to cycle throughout the study. Relatively constant antibody titers were reached by week 6 post-vaccination, although we found appreciable variation within treatment groups, especially 4-8 weeks post-vaccination. Based on our study, the SpayVac formulations impair ovarian function but do not affect other major organ systems, and could provide a safe and effective immunocontraceptive option for mares. Additional research to elucidate the vaccine's mechanism of action, actual contraceptive efficacy, and long-term effects are still needed. Ó 2013 The Wildlife Society.

Ovarian function and pregnancy outcome in pony mares following immunocontraception with native and recombinant porcine zona pellucida vaccines

“…The current study, however, demonstrated suppression of ovarian function in 6 of 7 pony mares following pZP treatment, characterised by small, Equine Veterinary Journal 49 (2017) 189-195 © 2015 EVJ Ltd inactive ovaries and basal ovarian hormone levels. Our findings confirm recently reported ovarian quiescence in mares treated with long-acting pZP vaccines [13], suggesting that suppressed ovarian function is not unique to long-acting formulations. Our findings confirm recently reported ovarian quiescence in mares treated with long-acting pZP vaccines [13], suggesting that suppressed ovarian function is not unique to long-acting formulations.…”

“…The discrepancy between our findings and that of an earlier report of unaltered oestrous cyclicity during short-term treatment of mares with conventional pZP vaccine [1] may be due to the higher pZP dose administered in our study (100 lg vs. 65 lg pZP), selected to reflect the current dose administered to feral horses [24][25][26]. Our findings confirm recently reported ovarian quiescence in mares treated with long-acting pZP vaccines [13], suggesting that suppressed ovarian function is not unique to long-acting formulations.…”

“…An unexpected finding was the prevalence of injection site reactions, supporting Bechert et al [13] who reported injection site reactions in 43% of treated mares. Our findings failed to support anecdotal reports of injection site reactions occurring less frequently when administered into the gluteal rather than the neck musculature [32,33].…”

“…A subsequent study in the same population reported no association between the incidence of ovulatory failure in mares and their duration of treatment [12]. Recently, investigators demonstrated ovarian inactivity in 13 of 14 mares within 4 months of treatment with single-dose pZP vaccine formulations [13].…”

“…In a previous study, 1 year of pZP treatment was found to have no effect on ovarian function [1]; however, longer periods of contraception, specifically ≥3 years, were associated with declining oestradiol levels and ovulation rates in feral horses of the USA [10,11]. Recently, investigators demonstrated ovarian inactivity in 13 of 14 mares within 4 months of treatment with single-dose pZP vaccine formulations [13]. Recently, investigators demonstrated ovarian inactivity in 13 of 14 mares within 4 months of treatment with single-dose pZP vaccine formulations [13].…”