selection of mares (e.g. retire mares with poor endometrium quality or a badly
damaged cervix), minimizing stress and minimizing the risk of introducing or
spreading infectious disease among broodmares.
There are few pharmacological means of preventing EPL, and not all of the
underlying causes are amenable to treatment. Progestagen supplementation can
avert EPL when a mare shows clear signs of returning to estrus despite the presence
of an apparently normal conceptus in her uterus, has a history of repeated EPL
associated with loss of the primary CL, is endotoxemic or has undergone severe
acute stress likely to compromise CL maintenance. In these cases, the logical
approach is to supplement with a suitable progestagen (e.g. altrenogest:
Regumate™) from as soon as CL failure is suspected or systemic disease is
identified or, in the case of repeated EPL, from before day 6 after ovulation. When
the incident takes place in early gestation, supplementation should continue until
adequate maternal progesterone production is certain, i.e. from sometime after day
75 of gestation, by which time the placenta has assumed the role as major provider
of the progestagens required to maintain pregnancy. Alternatively, progestagen
supplementation can be discontinued from around day 45 if ultrasonographic
examination demonstrates formation of eCG-induced accessory CLs or endogenous
plasma progesterone concentrations exceed 4 ng/ml. While progesterone
supplementation clearly has a role in protecting pregnancies threatened by maternal
progesterone deficiency or endotoxin or inflammation induced PGF2a release, it is
important to subsequently monitor the pregnancy for viability since many will fail
despite the progestagen therapy (suggesting an alternative underlying cause) and
the progestagens will then prevent the mare returning to estrus.
An additional treatment that has been shown to improve pregnancy rates,
presumably by preventing EPL, is the administration of a single injection of 20-40 mg
buserelin (Receptal™) between days 8 and 12 after ovulation. While this treatment
leads to a fairly consistent 5-10% increase in pregnancy rates,12 it is not known how
it exerts its effects, although it does not appear to be by boosting circulating
progesterone concentrations or preventing luteolysis.13 The utility of systemic
antibiotics to counter suspected endometritis in early pregnant mares is less clear;
the use of a 5 day course of broad-spectrum antibiotics to treat embryo transfer
recipients that receive an embryo from an obviously infected donor (i.e. very cloudy
flush) seems a sensible precautionary measure. On the other hand, if an early
pregnant mare shows ultrasonographic indications of endometritis (uterine free fluid
or marked oedema), antibiotics are generally ineffective at saving the conceptus and
more likely to result in false ho pe and wasted time.
Conclusions
In summary, although EPL is a significant cause of loss to the horse breeding
industry, it is difficult to predict, may occur without any premonitory signs and is, in
many cases, not treatable or preventable. While some methods of reducing specific
types of pregnancy loss are now commonplace (e.g. early detection and resolution of
twin pregnancy, maintaining a closed broodmare band, progesterone therapy in
cases of suspected failure of pregnancy recognition / endotoxemia / severe stress),
development of strategies to combat EPL has been slowed by inadequate
understanding of the underlying causes.
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
9