Reproductive and endocrine changes in ewes actively immunized against luteinizing hormone

The objective of this study was to examine reproductive and endocrine changes in ewes actively immunized against a conjugation of luteinizing hormone (LH) and ovalbumin (LH immunized n = 10) or ovalbumin alone (control; n = 10). Ewes were immunized at weeks 0, 3 and 5 of the 93-week study and exposed to a fertile ram during weeks 7-25 (first breeding season) and weeks 60-67 (second breeding season). Immunization against LH abolished estrus and prevented pregnancy in all but one LH immunized ewe during the two breeding seasons. Uterine weights of LH immunized ewes were lighter than those observed in control ewes while ovarian weights did not differ between the treatment groups. No differences were observed between basal circulating concentrations of LH measured by an in vitro bioassay for the two treatment groups. However, serum concentrations of immunological follicle stimulating hormone, and the number of medium sized ovarian follicles (4-6 mm) were increased in LH immunized ewes. It was concluded from this study that immunization against LH prevents ovulation presumably by blocking the preovulatory surge of LH and not altering basal concentration of LH.