Page 534 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 509

VetBooks.ir Ultrasound examination to identify an ovulation at mare. It usually has no effect on the oestrous cycle,
but if the ovulation and subsequent luteinisation of
foal heat is useful and should be repeated until an
ovulation is detected. Mares with persistent CL will
release on day 15–16 of the cycle, the immature CL
respond to intramuscular prostaglandin and return this follicle occurs 1–4 days prior to prostaglandin
quickly to a normal oestrous cycle. If the mare is is unaffected and can be retained, leading to pro-
in true anoestrus, the treatment is not effective. longed dioestrus.
The mare’s environment and nutrition should be
improved and any stress factors removed if possible. FAILURE OF FOLLICLES TO OVULATE
Similar regimes of management and drug therapy
to that undertaken in the treatment of transitional Definition/overview
oestrus/anoestrus have been used with variable This is a separate condition from that seen in the
success in this condition. The use of exogenous pro- transitional period of mares in the spring and
gestagens for 10 days is a good first-line treatment. autumn months, as it occurs in the normal breeding
Experimentally, GnRH given via a continuous infu- season, often in certain mares as a recurrent prob-
sion pump delivering 2.5–5.0 µg/hour has led to lem. The cause is unknown. Mares with enlarging
cyclic activity within 10–14 days and treatment with follicles that fail to ovulate remain in oestrus for
the GnRH analogue buserelin (10 µg q12 h), which prolonged periods until regression or luteinisation
can be effective in some cases. Oral domperidone occurs. Repeated rectal palpation and reproductive
(a dopamine antagonist) given at 1 mg/kg twice ultrasonography will help determine the presence of
daily can also induce follicle activity and encour- an anovulatory follicle. Spontaneous resolution of
age lactation as a side-effect. Prevention using extra the problem will lead to normal cyclic behaviour in
light exposure (14.5 hours) for early- foaling mares most cases, but recurrence is quite common in some
from early December to February/March (northern specific mares.
hemisphere) has been effective.
Aetiology/pathophysiology
Prognosis The failure of follicles to ovulate can be part of
The prognosis is guarded in true anoestrus cases, the syndrome of the transitional period seen in the
particularly where conditions are causing stress to spring and autumn months (p. 407). The follicles
the mare and/or she is in poor body condition. Some enlarge and persist for up to 2 months. Eventually,
mares (particularly older animals) have difficulty in the spring, one of the follicles ovulates and the
maintaining body condition when lactating repeat- mare starts to cycle normally. In the autumn tran-
edly. They enter into lactational anoestrus when sitional period the declining gonadotropin levels
they have a foal at foot, and for this reason often only allow the enlarged follicles to persist throughout
produce a foal every other year. the period.
Occasionally, in the normal breeding season, a
DIOESTRAL OVULATION dominant enlarged follicle does not ovulate and in
some mares this can be a persistent problem over a
Definition/overview number of oestrous cycles. Various names have been
It is quite common for mares that are cycling nor- used to describe these follicles including HAF or
mally to ovulate in dioestrus when a CL is already PAF. It has been reported to be more common in
present and functioning. The mare shows no oes- pony mares (5% incidence). The cause is unknown,
trous behaviour and this ovulation is only detected but it is probably endocrine-related, either from the
if the mare is examined regularly by rectal palpa- pituitary gland or, more likely, the follicle itself.
tion and ultrasonography. This situation appears to
be a normal variation in the oestrous cycle of the Clinical presentation
mare and some clinicians feel it may be related to Mares with anovulatory follicles may remain in
the mid-dioestral peak of FSH that occurs in the oestrus for prolonged periods of up to 15 days.