Remember, under FEI rules a pregnant mare can compete until 4 month of her
pregnancy. Allen et al. (44) reported that manual trans rectal rupture of the
conceptus between Days 16 and 22 of gestation in 11 mares resulted in continued
CL function for at least 60 days in all of the mares, during which time none displayed
oestrous behaviour. Although there is a 100% success with this method, terminating
a normal, healthy pregnancy may be untenable to many horse owners.
• Down regulation with gonadotropin-releasing hormone (GnRH) analogs
‘Ovuplant’, a SC implant manufactured by Fort Dodge Animal Health, containing 2.1
mg deslorelin (as deslorelin acetate, a GnRH analog acetate) showed evidence to
cause prolonged anovulatory periods in some mares. (47- 49)
Ovuplant was initially developed to induce timed ovulation when mares are artificially
inseminated. (45, 46) It is inserted in the vulvar mucosa prior to breeding and
removed 48h later when the mare has ovulated. Research at Colorado State
University in 2001, showed that a deslorelin implant, left in place for longer periods,
causes reduced circulating concentrations of Follicle Stimulating Hormone (FSH)
with absence of a mid-cycle FSH peak leading to prolonged interovulatory interval.
(50) This down regulating effect of a deslorelin implant on the pituitary function has
been used clinically to suppress ovarian activity (51) Other data showed that treating
mares with a potent GnRH analog suppresses oestrus in mares for extended periods
of time. However anovulatory mares can show oestrous behaviour while suppressing
follicular activity that does not assure the absence of oestrous behaviour.
• Immunological approaches
Immunizing a mare against GnRH eliminates gonadotropin release from the pituitary
gland resulting in suspension of her ovarian activity. Tshewang et al. (53) reported
successfully suppressing oestrus for 25–30 weeks in mares treated with a GnRH
vaccine. All of the vaccinated mares in his study regained normal cyclicity and
oestrous behaviour and each mare conceived and produced a normal foal the next
year. Other studies (54–56) also agreed with the efficacy of the GnRH vaccine, but
noted that some of these vaccinated mares continue to show oestrous behaviour
despite their anovulatory state. Although most mares appear to regain their ovarian
activity within a reasonable period after a GnRH vaccination, some mares may not
return to their cyclicity, which could be particularly problematic if a return as
broodmare is desired. Although an equine GnRH vaccine is not available in North
America, commercial preparations are available in Europe and Australia. In France a
GnRH vaccine called ‘Improvac®’ (manufactured by CSL Limited, Australia) is used
in horses. Given IM it can show some serious adverse effects such as swelling and
pain at the injection site, stiffness of the neck, pyrexia and apathy. These reactions
are probably due to the adjuvant or the carrier protein as this vaccine was originally
designed to control boar taint in pigs. A GnRH vaccine called ‘Equity ®’ was then
developed and is more specifically designed for use in the horse. It is composed of a
protein conjugate of Gonadotropin releasing factor (GnRF) attached to an injectable
carrier for IM use (1ml) in the horses; Improvac is composed of 400µg GnRH- protein
conjugate given IM (2ml).
Fully vaccinated mares with a GnRH vaccine are in an anovulatory state, therefore
can show signs of oestrous behaviour a phenomenon also seen in ovariectomized
mares.
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
22