46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
73
The suppression of reproductive
function and immunocontraception
in horses
Schulman, ML1* and May, CE1
1
Section of Reproduction, Faculty of Veterinary Science, University of Pretoria
Background
Suppression or prevention of reproductive function and behaviour is possible in both
stallions and mares and may be achieved by reversible or irreversible surgical,
hormonal or immunological methods. Currently, these methods are rarely employed
as a contracep tive strategy in equine practice, but are primarily directed towards
prevention of aggressive and other undesired behavioural traits to facilitate
management and athletic performance.
Immunocontraception describes the strategy of vaccine administration to induce an
adaptive immune response with reversible prevention of fertilisation. This method is
employed for population control in various species including the horse where it has
been extensively applied in the management of feral populations particularly in North
America.
In mares, the control of reproductive function is a vital management aspect from
both a fertility and behavioural perspective. Anecdotally, oestrous behaviour is
reported to negatively influence the performance ability of mares engaged in work
and competition events. To date, fertility control and the suppression of sexual
behaviour in mares have been successfully managed by administration of progestins
in oral or injectable formulation to simulate dioestrus. The high costs and the
frequent administration associated with the short duration of action often make their
use impractical and potentially-negative effects on future reproduction after repeated
administration of progestins in prepubertal fillies have also been described. The
major alternative to pharmacological manipulation for long-term control of
reproductive cyclicity and behaviour is the surgical removal of both ovaries
(ovariectomy). This invasive method resulting in irreversible loss of breeding
potential has been associated with: trauma, various post-operative complications and
some mares continuing to show signs associated with oestrous behaviour.
Ovariectomy is generally considered undesirable unless circumstances are
exceptional.
The commonest, universally applied method to prevent reproductive function and
behaviour employed (as old as equine practice itself), is the surgical castration of
stallions. This irreversibly eliminates the source of testicular steroids and sperm
resulting in loss of reproductive potential. There are many reported complications of
castration and these are the most common cause of malpractice claims against
equine practitioners in North America. Various minimally invasive methods have
been described to temporarily or permanently castrate males of various species.
These include the administration of various hormones, gonadotropin-releasing
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