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

Reasons for performing study: Few studies have investigated ovarian function in the mare undergoing porcine zona pellucida (pZP) immunocontraception despite reported ovarian dysfunction in other species. Objectives: This study aimed to describe ovarian function and oestrous cyclicity in pony mares following treatment with either the conventional pZP vaccine or a novel recombinant form of the vaccine derived from porcine ZP3 and ZP4 (reZP). In addition, the contraceptive efficacy of pZP versus reZP was assessed. Study Design: Mares (n=21) were randomized into three groups of seven: Group I received the conventional pZP vaccine, with a booster five weeks later; Group II received the reZP vaccine, with a booster five weeks later; and Group III (controls) received two treatments, five weeks apart, of saline and adjuvant alone. Data were collected over a 24 week period coinciding with the physiological breeding season, with treatments commencing in week four. Methods: Mares underwent weekly monitoring via trans-rectal palpation and ultrasound examination of the reproductive tract, with daily monitoring during oestrus. Serum samples were obtained for antibody titres and ovarian steroid level analyses at seven day intervals. Cycling mares were bred via fresh semen artificial inseminations, over a maximum of two consecutive oestrous cycles that commenced five weeks post booster vaccination. Results: Control mares cycled throughout the trial. Post-treatment, six of seven pZP mares (86%) and one reZP mare (14%) had extended anoestrus that correlated with basal serum oestradiol and progesterone levels. All mares resumed cyclicity by ten months post treatment. Pregnancies were diagnosed in all controls, four reZP- (57%) and none of the pZP- immunized mares. Conclusions: The current study demonstrates the reversible suppression of ovarian function in pony mares following treatment with pZP. The effect of the reZP vaccine on pregnancy outcome requires further investigation.