[Suppression of reproductive cyclicity by active immunization against GnRH in the adult ewe]

The objective of the present study was to evaluate the effect of an anti-GnRH vaccine on cycling activity in the adult ewe during the breeding season. For the experiments 22 cycling White Alpine sheep, aged between 2 and 4 years, were randomly divided into a treatment and control group of 11 animals, each. Sheep of the treatment group were immunized twice at an interval of 4 weeks with 2 ml (400microg GnRH-protein conjugate) of Improvac (Pfizer Animal Health, Australia) subcutanously in the neck. Sheep of the control group received the same amount of saline solution. Blood progesterone concentrations were measured weekly from 3 weeks before to 32 weeks after first immunization and anti-GnRH titers were determined monthly. All vaccinated ewes ceased cycling within 2 - 8 weeks after first immunization. Plasma progesterone was suppressed for a minimum of 12 weeks (2 ewes) with individual variation of 14 (1 ewe), 25 (1 ewe) and more than 25 weeks (7 ewes). Four animals resumed cyclicity while 7 animals remained suppressed until the end of the study. Antibody titers peaked one month after the booster injection and thereafter continuously dropped until the end of the study (8 months after first immunization) to values between 10.9 and 40.8 % binding. From our results it can be concluded that two vaccinations with Improvac 4 weeks apart suppress cycling activity in adult ewes for at least 12 weeks. The inhibitory effect on ovarian activity, however, varies individually and may last more than 31 weeks.