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GnRF Immunization in Mares: Ovarian Function, Return to Cycling, and Fertility
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Gonadotrophin-releasing factor (GnRF) immunization provides effective temporary control of estrous cycling and fertility in mares.
1. Introduction
Gonadotrophin-releasing factor (GnRF) immunization suppresses luteinizing hormone and follicle stimulation hormone, causing immune-mediated anestrus in mares. The objective of the study was to characterize responses to the GnRF vaccinea on estrous cycling and fertility in multiparous mares.
2. Materials and Methods
A randomized, blinded, placebo-controlled field study was performed. By July 2004, 82 foaling mares, mean age of 6.0 ± 2.7 (standard deviation) yrs, were determined to be cycling, randomized to two groups (n = 61, GnRF vaccine [a] ; n = 21, placebo), and treated twice 4 wks apart. All mares received ultrasound examinations every 2 wks until December 2004. In 2005, all mares underwent repeated ultrasound examinations before and after pasture breeding. Mares barren in 2005 were similarly examined and pasture bred in 2006.
3. Results
In 2004, 4 wks after the second dose, 98% of immunized mares were anestrus or transitional, compared with 15% of placebo mares. Vaccinates ceased cycling for the remainder of breeding season, but placebo mares cycled normally, with seasonal anestrus reaching 90% by early December. After the 2004 GnRF vaccination, 100% of placebo and 88% of vaccinated mares were detected cycling in the 2005 breeding season, and 98% of the vaccinated group were cycling by 2006. In 2005, 100% of the placebo group and 75% of the vaccinated mares were pregnant after pasture exposure, and by 2006, 90% of the vaccinated group had confirmed pregnancies. Live foal rates in bred mares were 90% in the placebo group and 95% in the vaccinated group.
4. Discussion
Immunization with the GnRF vaccine [a] caused immune anestrus by rapidly curtailing estrus cycles in 60/61 (98%) of mares for the remainder of the 2004 breeding season. Immune anestrus was temporary, with 88% of the vaccinated group cycling in 2005 and 98% by 2006. Pregnancy rates (PRs) in pasture-exposed mares were high (placebo mares, 100% PR in 2005; vaccinated mares, 75% PR in 2005 and 90% PR by 2006). Live foal rates in immunized and placebo mares were similar.
This study was financed by Pfizer Animal Health.
Footnote
[a] EQUITY, Pfizer Animal Health, Parkville, Victoria 3052, Australia.
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