Abstract
Endometritis is the commonest reproductive disease in the mare resulting in decreased
fertility and economic losses to the equine industry. Endometritis is manifested by
accumulation of intrauterine fluid and cellular debris and its therapy is mainly aimed
towards enhancing uterine clearance. Impaired uterine contractile activity (UCA) in
susceptible mares has been shown to contribute to defective uterine clearance.
In this thesis the importance of UCA in uterine clearance was demonstrated by
converting the uterus of a genitally normal mare, using clenbuterol, a (L
sympathomimetic, into a susceptible uterus, after bacterial infusion. All clenbuteroltreated mares had intrauterine fluid collections 48 h after the infusion. Uterine
contractile activity is mainly controlled by the action of the ecbolic hormones, oxytocin
(OT) and prostaglandin F2a (PGF2a). The profiles of these two hormones were
investigated in resistant and susceptible mares around artificial insemination (AI) and
after OT injection. Other stimuli applied only to resistant mares included oestrous and
dioestrous teasing, natural service, intrauterine saline infusion and manual manipulation
of the genital tract. All stimuli caused OT release and there were no differences in OT
profiles between resistant and susceptible mares. However, significantly fewer
susceptible mares released PGF2a in response to endogenous OT release or exogenous
OT administration.
Qualitative measurement of UCA has been reported in mares using ultrasonography and
its effect on ecbolic hormone release and subsequently UCA was investigated. There
was no evidence to show any effect of ultrasonography on UCA. Utilizing the same
technique,'differences in UCA before and after OT administration in oestrous resistant
and susceptible mares were investigated. Oestrous resistant mares had higher baseline
UCA and OT administration caused uterine spasm in all oestrous mares. However the
repeated use of OT at short time intervals caused uterine refractoriness. Daily OT
administration in the early postovulatory period significantly affected UCA up to day 3
postovulation.
It was finally concluded that UCA is responsible for the clearance of uterine fluid from
the uterus. Mares susceptible to endometritis have lower baseline uterine motility and
UCA is restored slower after OT administration. There are no differences in the OT
profiles between resistant and susceptible mares and different mechanical and
psychogenic stimuli associated with reproductive events, seem to trigger OT release.
However, most susceptible mares do not release PGF2a in response to endogenous OT
release or exogenous OT. Oestrous resistant mares respond to OT administration with
uterine spasm and PGF2a release. However in dioestrus, OT stimulates UCA only up to
day 3 postovulation
