Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function in feral horses (Equus caballus).

The research article investigates the long-term effects of consistent porcine zonae pellucidae (PZP) immunocontraception on ovarian function in feral mares. The tests indicate that three consecutive years of PZP treatment significantly decrease pregnancy rates and may negatively affect normal reproductive functions.

Research Context

• The researchers performed tests on a group of feral mares that roam freely in Maryland, USA. They focused on understanding the impact of PZP, a form of immunocontraception, on the horses’ ovarian function if administered for three consecutive years.
• The animals received a PZP protein inoculation carried out before the breeding season, spanning from 1988 to 1990.
• The researchers tracked the ovarian function of selected mares in the peak of the breeding season after the third annual PZP injection. This monitoring was done by measuring urinary oestrone conjugates and unspecified progesterone metabolites.

Observations and Results

• The research noted that none of the ten mares that were inoculated with PZP during the experiment became pregnant in 1990.
• In contrast, 55% of the control group mares, which did not undergo PZP treatment, became pregnant. This percentage includes two mares which had their ovarian function monitored.
• In the untreated mares, three demonstrated apparent normal ovarian activity, with typical preovulatory oestrogen peaks, simultaneous progesterone lows at ovulation, breeding activity, and luteal-phase progesterone increases after ovulation.
• Two of the seven PZP-treated mares monitored showed ovulatory cycles, yet they did not conceive. One of these mares was already pregnant from conception in the previous year and exhibited an expected late-gestation endocrine profile.
• Four PZP-treated mares did not display any signs of ovulation, and their urinary oestrogen levels were significantly reduced.

Conclusions

• The experiments concluded that a third consecutive annual PZP booster shot is over 90% effective in preventing pregnancies in mares.
• The research also showed that three consecutive years of PZP treatment could interfere with the normal ovarian function. This interference is demonstrated by the noticeable decrease in oestrogen secretion.