Effects of active and passive gonadotrophin-releasing hormone immunizationon recognition and establishment of pregnancy in pigs

This study investigated the effects of a reduction in gonadotrophins, by me ans of differently timed active and passive gonadotrophin-releasing hormone (GnRH) immunization at various stages, on the maintenance of early pregnan cy in pigs. In the first experiment crossbred sows (n = 11) were immunized against GnRH using a commercial vaccine on the day of farrowing, mated at t he first oestrus, and a booster immunization was administered 10 days (n = 7) or 20 days (n = 4) after mating. Plasma samples were collected every sec ond day and assayed for GnRH antibodies and progesterone. Pregnancy testing was carried out by real time ultrasound. None of the sows receiving the bo oster immunization 10 days after mating were pregnant on Day 18 after matin g. All sows receiving the booster on Day 20 after mating aborted, with a me an vaccination-to-abortion interval of 10.0 +/- 1.5 days. In the second exp eriment, crossbred gilts (n = 6) were passively immunized by infusing (i.v. ) GnRH immune pig serum on Day 12 after mating. Luteinizing hormone profile s were determined on the day before immunization and one day afterwards. Da ily plasma samples were assayed for GnRH antibodies and progesterone. None of the gilts were pregnant 18 days after mating, compared with 5 of 6 non-i mmunized controls. Booster immunization 10 days after mating resulted in fa ilure of embryonic development and establishment of pregnancy before the co rpora lutea (CL) regressed, according to progesterone profiles, whereas imm unization 20 days after mating resulted in regression of CL followed by abo rtion. Passive immunization 12 days after mating had a similar effect to th e active immunization 10 days after mating. These results demonstrate two d ifferent outcomes of active GnRH immunization depending on the timing of im munization, and indicate that loss of pregnancy between Days 12 and 18 may occur due a reduction in progesterone rather than complete failure of the C L, as occurs at later stages. The findings may provide an explanation for t he reduced fertility of pigs in the summer-autumn period.