intrauterine fluid (≤ 10 × 20 mm) were identified during the luteal phase, but no
neutrophils or bacteria were recovered on uterine swabs when examined during the
subsequent oestrus. No changes for inflammation and glandular dilatation were
observed in the uterine biopsy.
• Oxytocin and other methods
Stout and Allen first described the use of oxytocin to disrupt normal luteolysis.
Subsequent work by Vanderwall et al.demonstrated that twice daily injections of 60
IU oxytocin during the mid-luteal phase disrupted luteolysis by prolonging luteal
function in 100% of the mares. Comparing once daily and twice daily injections of
oxytocin found no significant difference between the groups with prolonged luteal
function in 63% and 71% of animals, respectively. In each of these studies the
progesterone levels were measured as a means of determining the length of luteal
maintenance. (10,11)
Although administration of exogenous oxytocin to mares around the time of luteolysis
(day 11 to 15 post-ovulation) stimulates an acute onset of PGF2α secretion (34–36),
when oxytocin is administered in the mid-luteal phase prior to day 10 post-ovulation,
it does not induce PGF2α secretion and often disrupts luteolysis causing prolonged
CL function. (35) Carbetocin, a long acting oxytocin, when administered between 7
and 14 days post-ovulation, does not prolong the CL function, on the contrary it
effectively short cycled the mares and decreased the interovulatory interval. The
cause is unknown.
• Inducing late-diestrus ovulation
Hedberg and co workers induced a prolonged luteal phase in a limited number of
mares by inducing mid-diestrus ovulation after administration of human chorionic
gonadotropin (hCG). Overall three out of the four (75%) experimental mares treated
with hCG on a > 30mm mid-estrous follicle ovulated within 72 h and resulted in luteal
phases lasting from 58 to 82 days. The exact mechanism is unknown. Despite the
fact that this study looks promising for prolonging the CL function, it is important to
note that, some mares (in this study five out of nine) require multiple oestrous cycles
to develop a mid-diestrus follicle ≥ 30 mm. Some mares never developed a large
enough mid-diestrus follicle to warrant chg. treatment to induce an ovulation
therefore it was considered not to be a reliable ‘on-demand’ method of blocking
oestrous behaviour in mares.
In Allen’s laboratory they demonstrated a prolongation of the luteal phase for at least
30 days after intra uterine administration of 1 ml of various plant oils. (3)
C. Suppressing of the ovarian follicular activity
• Pregnancy
Pregnancy is another way of suspending cyclicity by allowing the natural ability of the
conceptus to block luteolysis and maintain CL function. This method brings some
disadvantages such as: the time and expenses necessary to establish a pregnancy
followed by the need to terminate pregnancy (unless an offspring is ultimately
desired). This is in general not very appreciated by many horse owners/breeders.
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
21