74
46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
hormone (GnRH) agonists, GnRH antagonists, and immunisation against GnRH as
well as the use of GnRH toxin conjugates.
In stallions, hormone treatments include progestagens (e.g. altrenogest) to suppress
secretion of GnRH and LH with resultant decreased testicular testosterone release
to reduce libido and aggressive behaviour have shown variable efficacy and
necessitate frequent administration at high daily dose rates. Furthermore: potential
side effects are undefined, progestagens cannot be used in horses intended for
human consumption, and are considered as a doping-offence in many equine-sporting
events. The application of high doses of GnRH antagonists in stallions (for pitiutary
down-regulation) to reduce LH and testosterone secretion has a limited or no effect
on libido in mature stallions despite a dramatic fall in testosterone concentrations
and occasionally even enhances gonadotropin secretion.
A practical alternative may be active immunisation against GnRH, which is reversible
allowing recovery of normal testis function and libido when immunisations are
discontinued. This has been shown to be highly effective as an alternative method
for: surgical castration of cryptorchids, controlling undesirable aggressive and sexual
behaviour, facilitating handling and enhancing performance and preserving potential
genetic material in future breeding stallions.
A. GnRH vaccine studies
Introduction
The formulation of a GnRH vaccine involves the conjugation of GnRH (or an
analogue) to a large, foreign protein such as ovalbumin. In order to further stimulate
the immune response, an adjuvant is added to the antigen. The administration of
these compounds induces the production of anti-GnRH antibodies. In the target
animal the antibodies bind endogenous GnRH in the hypothalamic-pituitary portal
vasculature to prevent binding to pituitary gonadotropic receptors. The ensuing
failure of gonadotrophin secretion in males results in a decline in gonadal steroid
secretion with associated suppression of testicular activity and sexual behaviour
(Janet et al. 2009). The few studies to date report variations in individual responses
(in both stallions and mares) as monitored by antibody titre, circulating steroid levels
(plasma progesterone and testosterone) and various measurements obtained from
gonads and semen evaluation parameters. In stallions, the inhibitory effect on
testicular function and sexual behaviour is highly variable lasting from 24 weeks to >
46 weeks. These variations in effect have been ascribed to a differential effect on
pituitary LH and FSH secretions, as FSH secretions are less affected with sufficient
circulating FSH and low testosterone concentrations are able to maintain
spermatogenesis. Sexual behaviour is not only testosterone-dependent but is also
influenced by factors such as age and previous sexual experience. The recovery
from vaccination in stallions is also reported to be complete, but no effects of longterm suppression have as yet been reported. Reported applications included the
effective treatment of equine viral arteritis in carrier stallions.
In mares, the few studies of GnRH vaccination to date report a successful if variable
effect on suppression of ovarian activity and corresponding prevention of oestrous
behaviour in adult mares. These studies have in most instances been limited by
relatively small numbers of mares and additionally an effect of mare age on treatment
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