Practical approaches to hormone therapy in
broodmares
Stout T.A.E
Department of Equine Sciences, Utrecht University, Netherlands
Introduction
Exogenous hormones are commonly used to improve the efficiency of equine
reproductive management. However, approaches differ between countries
and sectors of the industry, and where the efficacy of, or rationale for, a
treatment is questionable the veterinarian must decide whether to use a drug
that “might help, and shouldn’t harm” or not to use it because “although it
might not harm, there is no evidence that it helps”. This presentation will
address the rational use of hormones in mare reproduction.
GnRH Agonists
Potent agonists of GnRH are used to induce a predictably timed ovulation in
cycling mares, e.g. when administered to oestrous mares with a dominant
follicle >35 mm in diameter, a subcutaneous implant containing deslorelin
acetate (OvuplantTM) will induce ovulation in 85-95% of mares within 48 h
(most betw een 36 and 42 h: Mckinnon & McCue, 2012). Removing the
implant after ovulation avoids the risk of subsequent suppression of follicle
development. Injectable deslorelin is now also available in some countries; 1.5
mg i.m. is recommended for inducing ovulation. Because deslorelin is a small
molecule (9 amino acids), treated mares should not produce antibodies, and
there are no reports of reduced efficacy after repeated use. In countries where
Deslorelin is not available other injectable GnRH agonists can be used to
induce ovulation, these include buserelin (ReceptalTM), although depending on
formulation efficacy appears to depend on multiple injections (e.g. 20mg at 12
hour intervals; Barrier-Batut et al 2001) or can be achieved with a single
administration (Suprefact; Levy and Duchamp, 2007). Administration of a
GnRH agonist (20-40 mg buserelin; ReceptalTM) on approximately day 10
after ovulation has also been described to improve pregnancy rates in mares
although it is not entirely clear how this effect is mediated (Newcombe et al,
2001).
Reproductive activity and behaviour in horses can be suppressed by
immunization against GnRH or using GnRH antagonists (e.g. cetrorelix,
antarelix). GnRH antagonists are, however, expensive. And whereas the
vaccine is affordable, safe, effective and reversible, the duration of
suppression is not very predictable and there is a risk of long-term or
permanent suppression of reproductive activity (Schulman et al, 2014). An
anti-GnRH vaccine (EquityTM, Pfizer, Inc.) has been registered in Australia for
use in mares.
Human Chorionic Gonadotropin (hCG)
hCG is a glycoprotein hormone used to induce ovulation in mares because of
its LH activity. hCG is usually administered when a mare is in oestrus, with a
follicle >35 mm. Ovulation is expected within 48 h, and usually occurs at
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
83