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

Abstract

This study investigated the effects of a reduction in gonadotrophins, by means of differently timed active and passive gonadotrophin-releasing hormone (GnRH) immunization at various stages, on the maintenance of early pregnancy 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 the first oestrus, and a booster immunization was administered 10 days (n = 7) or 20 days (n = 4) after mating. Plasma samples were collected every second day and assayed for GnRH antibodies and progesterone. Pregnancy testing was carried out by real time ultrasound. None of the sows receiving the booster immunization 10 days after mating were pregnant on Day 18 after mating. All sows receiving the booster on Day 20 after mating aborted, with a mean vaccination-to-abortion interval of 10.0 1.5 days. In the second experiment, crossbred gilts (n = 6) were passively immunized by infusing (i.v.) GnRH immune pig serum on Day 12 after mating. Luteinizing hormone profiles were determined on the day before immunization and one day afterwards. Daily 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-immunized controls. Booster immunization 10 days after mating resulted in failure of embryonic development and establishment of pregnancy before the corpora lutea (CL) regressed, according to progesterone profiles, whereas immunization 20 days after mating resulted in regression of CL followed by abortion. Passive immunization 12 days after mating had a similar effect to the active immunization 10 days after mating. These results demonstrate two different outcomes of active GnRH immunization depending on the timing of immunization, and indicate that loss of pregnancy between Days 12 and 18 may occur due a reduction in progesterone rather than complete failure of the CL, as occurs at later stages. The findings may provide an explanation for the reduced fertility of pigs in the summer–autumn period.