A Brief Overview of the Equine Oviduct

November 20, 2019

Figure 1 OviductWhen considering common causes of infertility in the mare, the oviduct is one of the last places an equine reproductive specialist looks. Uterine infections, poor conformation and stallion subfertility are much more likely causes for a mare’s failure to produce a pregnancy. That being said, there are rare cases where oviduct pathology is suspected, and in the past few years there have been advances for oviductal treatment that are cost effective, safe and have restored fertility in some patients. This summary will highlight some unique features of the mare oviduct, discuss when to suspect oviductal pathology or dysfunction, and review some of the current treatments available.

The anatomy of the mare’s oviduct is unusual compared to other species. At the level of the ovary, the oviduct has a funnel-shaped opening that drapes over the ovulation fossa (the site oocytes emerge after ovulation). Here the oviduct “catches” the oocytes and transports them to a narrower portion, the ampulla, where fertilization occurs. The oviductal environment is key to the production and development of the early embryo. It transports the sperm and oocyte to the place of fertilization, facilitates fertilization through the fluid environment, and then supports and transfers the early embryo to the uterus for ongoing development. Unique to the mare is the narrow, muscular sphincter present at the junction of the oviduct and the tip of the uterine horn. This connecting segment of the oviduct to the uterus allows embryos, but not unfertilized eggs to pass into the uterus. In other species, the unfertilized egg passes into the uterus and degenerates, and the oviductal opening into the uterus is usually much larger.

It is hypothesized that this unique narrow oviductal region in the mare causes cellular accumulations and degenerated eggs to build up, and these “plugs” can be seen with the naked eye and are a normal finding in most mares. In fact, in one study from 1986, when the oviducts of mares from slaughter houses were evaluated, these “plugs” were found in over 87% of mares.1While these oviductal accumulations seem common, their effect on fertility is not known, as they are even found in pregnant mares. Mares rarely have clinical signs of severe oviduct infection or distention with fluid, as can be seen in pigs or cows, but there is evidence that on a microscopic level inflammation exists with moderate frequency and its effect on fertility is not known. This information is relevant when we consider mares that have suspected oviduct dysfunction, as many of the treatments are aimed at removing the “plugs,” restoring patency of these delicate narrow tubes, and resolving inflammation.

The History of the Timed Insemination Protocol - UltrasoundOviduct pathology is suspected when a mare fails to produce an embryo or establish an early pregnancy despite appropriate breeding management, absence of other reproductive tract pathology, and a history of prior unsuccessful breeding to several stallions with proven fertility. Mares that have a history of producing a pregnancy and losing it, do not likely suffer from oviduct dysfunction, as the egg has been successfully fertilized, developed, and made it to the uterus to establish pregnancy. There are no practical tests available for testing oviduct function and reproductive history is probably most significant, for if the mare can establish or produce a pregnancy it is evidence that the oviducts are working. To test oviduct patency, dyes and microscopic beads have been applied to the oviducts and then recovered in the uterus or abdomen as evidence that they can pass through the oviduct, though these tests do not assess function.3 In most cases the reproductive history and exclusion of all other reproductive factors that cause infertility, leads us to suspect oviductal dysfunction. Mares are often treated without direct diagnostic tests of oviduct patency (beads, dye application or flushing) because the tests may cause issues and the treatment itself, if successful is a diagnosis in itself.

There are 3 treatments most commonly used to treat suspected oviduct dysfunction at this time: 1) application of Prostaglandin E2 (PGE2) to the surface of the oviduct, 2) flushing of the oviduct from the uterus (hysteroscopic hydrotubation), and 3) application of Prostaglandin E1 (PGE1) to the oviductal entrance in the uterus.

1.) PGE2 is secreted by the equine embryo that allows it, and not unfertilized eggs to pass from the oviduct into the uterus. By laparoscopically applying this in a gel over the surface of the oviduct it’s thought to cause contractions in the oviduct that restore function and patency. In one study based on a small group of mares that had been barren for 2 years with no other cause of infertility, 93.3% (14/15) of the mares became pregnant the same or the next year after treatment with PGE2.4 The disadvantage of this procedure is it is costly (around $1,500), and requires hospitalization and surgery (a flank incision for laparoscopic approach) to apply the gel. The advantage is it’s relatively harmless to the oviduct, and does not require any contamination or exposure of the uterus to potential inflammation.

2.) Hysteroscopic hydrotubation is a delicate technique that was perfected by a Japanese veterinarian Yuji Inoue, and introduced in 2013. It involves placing an endoscope in the uterus, identifying the oviductal opening, catheterizing it and then flushing from the uterus to the ovary with sterile saline. This technique was used clinically in mares with infertility, and had similar results to those mares treated with PGE2.5 The cost associated with this procedure was $800-1,000 but requires experience with the technique for good results. Due to the reduced cost and the fact the mare does not need surgery, this technique was considered more practical compared to laparoscopic PGE 2 application. The disadvantage of the technique is it requires insufflation of the uterus to pass the oviductal catheter, which can cause severe inflammation in some mares. Also, there is always the risk of peritonitis or salpingitis as you are flushing from the uterus, a potential source of contamination into the oviduct and the abdomen, though this complication has not been reported.

3.) The last, and definitely most practical technique is the deep-horn application of PGE1to the oviducts. This procedure was developed by Marco Alvarenga, in Brazil and was presented in 2018.6 In this study PGE1 (Misoprostol) instead of PGE2 was used to hypothetically induce oviductal contractions and sphincter relaxation. Specifically, one Cytotec ® tablet (200mcg of Misoprostol, Pfizer, USA) was diluted in 3mL of sterile water and is applied to the tip of the uterine horn through a deep-horn insemination pipette. Within 2 cycles after the procedure 68% (15/22) produced a pregnancy. In some mares a mild to moderate inflammation occurs after infusion, and so performing the procedure during estrus and following with uterine lavage 12-24 hours later is beneficial. This treatment provides a new and easy-to use approach with minimal downsides for the practitioner in the field and does not risk peritonitis, trauma and is very cost-effective (less than $200).

While oviduct pathology is rare in the mare, it can be a cause of subfertility and in some cases infertility with traditional breeding programs. The reproductive history and full examination of the mare should be performed before jumping to the oviduct as the cause for infertility. When appropriate case selection is made, the current treatments available are effective and considered safe. Hopefully future work comparing the efficacy of these treatments using matched controls would be helpful to determine the real efficacy of some of these treatments. The recent addition of the deep-horn PGEapplication allows more people to treat the oviduct in the field in a cost-effective and safe manner.

References

1) Saltiel A, Paramo R, Murcia C, et al. Pathologic findings in the oviducts of mares. Am J Vet Res 1986;47:594–597.

2) Fiala-Rechsteiner M, Amaral MG, Augusto Cruz L, Rodrigues RF, Pimentel AM, Mattos RC. Inflammatory lesions in the oviducts and its relationship with endometrial inflammation and ovarian activity in Criollo mares. J Eq Vet Sci2015 ,35(9); 731-734.

3) Arnold CE, Love CC. Laparoscopic evaluation of oviductal patency in the standing mare. Theriogenology 2013;79:6, pages 905-910.

4) Allen WE, Kessy BM, Noakes DE: Laparoscopic application of PGE2 to re-establish oviductal patency and fertility in infertile mares: a preliminary study. Equine Vet J 2006;38:454-459.

5) Walbornn RS, Schnobrich MR, Bradecamp EA, Scoggin CF, Woodie JB. Pregnancy rates after laparoscopic application of PGE2 or hysteroscopic hydrotubation of the uterine tubes. AAEP Proc 2018;64:245.

6) Alvarenga M, Segabinazzi G. Application of Misoprostol as a Treatment of Unexplained Infertility in Mares. J Eq Vet Sci, 71, p46-51, 2018.