By Jos Mottershead
Oxytocin acts on smooth muscle in the body, causing it to contract. One such area of tissue, which is of tremendous importance relative to breeding, is the mare’s uterus.
It has been recognized for some time that fluid build up in the uterus can be detrimental to establishing or maintaining pregnancy, and with a view to removal of such fluid there has been research into the use of oxytocin.
Uterine fluid is most commonly seen by ultrasound post-breeding, and is usually present as a result of a natural response to a foreign object in the uterus – semen. In most mares, this fluid poses no problem to the establishment or maintenance of pregnancy, and is cleared within 24 - 48 hours by the mare herself. In some mares however, especially multiparous, older, or mares with poor reproductive conformation, a condition known as "delayed uterine clearance" may occur. This results in the fluid not being cleared, and will possibly result in the conceptus encountering an inhospitable environment when it enters the uterus 4.5 to 6 days post-ovulation.
Uterine fluid encountered in estrus mares during an ultrasonic evaluation prior to breeding (often during an examination of the follicular status of the ovaries) has recently been established to be detrimental to establishing or maintaining pregnancy1. Generally amounts in excess of one-half inch depth are considered excessive, and suitable for treatment with oxytocin. Amounts in excess of one inch are considered to be more likely to respond favourably to lavage, and the resulting exudate should also be evaluated for inflammatory cells and bacteria, as a bacterial uterine infection may be the cause.
The post-breeding treatment that I have seen to be most effective has been intramuscular oxytocin, combined with an intra-uterine infusion of a broad-spectrum antibiotic. Experimental work by others has also found this to be more effective2. I have also noted that in the case of excessive fluid build-up post breeding, multiple treatments with oxytocin may work better. The first intramuscular injection of oxytocin (see below for dosage information) should be at least 4 hours after breeding, which allows adequate time for all sufficiently progressively sperm to make their way to the oviducts (which are not affected by the oxytocin treatment). This is then followed by an intra-uterine infusion of antibiotic 2 hours later, and then three more intramuscular injections at six hourly intervals (the first of which takes place 6 hours after the uterine infusion). It should be noted that oxytocin has a lesser effect post-ovulation.
Pre-breeding, it is considered to be more important to solely remove the fluid, rather than infusing antibiotic, as long as no uterine bacteria have been found to be present.
Initially the suitable dosage level of oxytocin was most commonly considered to be 20 international units, but recent research3 has suggested a lower dose - 10 i.u. - to be more suitable. The rationale behind this thought being that the higher dosage results in a more concerted constriction of the smooth muscle, whereas the lower dosage results in a methodical constriction starting at the distal portion of the horns, and working caudally, which would result in a more complete and systematic evacuation of the fluid.
Some users of this treatment protocol are favouring the use of the synthetic prostaglandin analogue Cloprostenol, rather than oxytocin, as it is felt to have a longer active period than does the oxytocin. Additionally, intramuscular use rather than intravenous has been favoured as allowing a slower, longer passage4.
If the cervix is found to be excessively tight, treating the mare with Estradiol Cypionate may also be indicated. This may be done either pre- or post-breeding, depending upon the need. It should be noted that higher dosage rates of estradiol prior to ovulation should be carried out with caution, as this may delay ovulation.
© 2000 Jos Mottershead. Used with permission.
1: Pycock, J.F. and Newcombe, J.R. (1996) The relationship between intraluminal uterine fluid, endometritis and pregnancy rate in the mare.Equine Pract. 18, 19-22.
2: Pycock, J.F. (1994) Assessment of oxytocin and intrauterine antibiotics on intrauterine fluid and pregnancy rates in the mare. Proc. 40th Ann. Conv. Am. Ass. Equine Pract.
3: Cadario, M.E., Merritt, A.M., Archbald, L.F., Thatcher, W.W., LeBlanc, M.M. (1998) Changes in intrauterine pressure after oxytocin administration in reproductively normal mares and in those with a delay in uterine clearance, Theriogenology (51)5 (1999) pp.
4: Burns, P.J. and Emond, R. (1999) Personal communication.
For more horse related information, visit http://www.equine-reproduction.com.
Back to Articles of Interest