Reversibility of the effects of GnRH-vaccination used to suppress reproductive function in mares

Authors

  • M. L. SCHULMAN,

    Corresponding author
    1. Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
      Dr Schulman's present address is: Section of Reproduction, Department of Production Animal Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag XO4, Onderstepoort, 0110, South Africa. Email: martin.schulman@up.ac.za
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  • A. E. BOTHA,

    1. Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
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  • S. B. MUENSCHER,

    1. Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
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  • C. H. ANNANDALE,

    1. Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
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  • A. J. GUTHRIE,

    1. Equine Research Centre, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
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  • H. J. BERTSCHINGER

    1. Section of Reproduction, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, South Africa
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Dr Schulman's present address is: Section of Reproduction, Department of Production Animal Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag XO4, Onderstepoort, 0110, South Africa. Email: martin.schulman@up.ac.za

Summary

Reasons for performing study: 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.

Hypothesis: 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.

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