title: Reversibility of the effects of GnRH-vaccination used to suppress reproductive function in mares.
abstract: BACKGROUND
  Active immunisation against gonadotrophin-releasing hormone (GnRH) provides a reversible method for control of oestrous behaviour and fertility in mares. Previous reports failed to demonstrate the interval to resumption of cyclic ovarian activity after GnRH-vaccination.
  
  
  OBJECTIVE
  Administration of the GnRH-vaccine Improvac in a large group of mares of various ages will result in effective, reliably reversible suppression of ovarian activity within a 2 year period.
  
  
  METHODS
  The mares, subdivided into 3 age categories, were vaccinated twice (with a 35 day interval) using 400 µg Improvac and monitored via blood samples until Day 720 after initial vaccination for serum progesterone concentration determination by radioimmune assay and anti-GnRH antibody titre by enzyme immunoassay. Samples were collected until individuals resumed cyclic ovarian activity.
  
  
  RESULTS
  All mares showed suppression of cyclic ovarian activity by clinical examination and serum progesterone concentration (SPC) ≤ 1 nmol/l by Day 70 and 92.2% resumed cyclic activity by SPC at Day 720 with a mean interval = 417.8 days (s.d. ± 23.9; range 232-488 days, median 344 days). A significant age effect (P = 0.028) on the interval, but not on GnRH-antibody titre response, was observed between the youngest (≤ 4 years) and oldest (≥ 11 years) categories.
  
  
  CONCLUSIONS
  Immunising adult mares of all ages with Improvac resulted in a reversible suppression of cyclic ovarian activity in most mares. An age effect, with the youngest mares showing a longer interval to reversibility, was observed.
authors.affiliation: Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa. martin.schulman@up.ac.za
authors.forename: M L
authors.initials: ML
authors.lastname: Schulman
authors.forename: A E
authors.initials: AE
authors.lastname: Botha
authors.forename: S B
authors.initials: SB
authors.lastname: Muenscher
authors.forename: C H
authors.initials: CH
authors.lastname: Annandale
authors.forename: A J
authors.initials: AJ
authors.lastname: Guthrie
authors.forename: H J
authors.initials: HJ
authors.lastname: Bertschinger
mesh_terms: D000818:Animals; D000906:Antibodies / Q000097:blood; D004971:Estrus / Q000187:drug effects*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000276:immunology* / Q000494:pharmacology; D006736:Horses / Q000097:blood / Q000502:physiology; D007114:Immunization / Q000662:veterinary; D007115:Immunization Schedule; D012726:Sexual Behavior, Animal / Q000187:drug effects; D013997:Time Factors; D022642:Vaccines, Contraceptive / Q000276:immunology*
chemical_list: D000906:Antibodies; D022642:Vaccines, Contraceptive; D007987:Gonadotropin-Releasing Hormone