A. Tast et al., Effects of active and passive gonadotrophin-releasing hormone immunizationon recognition and establishment of pregnancy in pigs, REPROD FERT, 12(5-6), 2000, pp. 277-282
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.