The first article in this series focused on physiology, safety concerns, applications of prostaglandins in oestrous cycle manipulation, prevention of anovulatory follicles and other uses related to ovarian function (Kelly, 2024). In this second part, the focus will turn to other prostaglandin applications related to equine reproduction.
Termination of pregnancy
Indications for termination of pregnancy include mismating, twins, abnormal fetus and health risks to the mare (Douglas et al, 1974). If pregnancy is terminated in the first trimester there are few (if any) complications. However, when termination is performed in late pregnancy, serious complications can occur and include retained fetal membranes, cervical trauma, dystocia and uterine infection (Dascanio, 2014).
Prostaglandins induce abortion by interfering with progesterone production, causing uterine smooth muscle contractions and possibly cervical relaxation (Douglas et al, 1974; Daels et al. 1995). The protocol used depends on the stage of pregnancy, as PGF2α becomes less effective as the pregnancy advances (Table 1). This is because maintenance of pregnancy by luteal progesterone is gradually replaced by placentally produced progestogens as the pregnancy progresses (Holtan et al, 1979; Madej et al, 1987) (Figure 1). Up to day 45, maintenance of pregnancy is dependent on progesterone produced by the original corpus luteum. Progesterone is additionally produced by accessory corpora lutea formed typically between days 45–60 (McCue, 2019). These accessory corpora lutea are formed under the influence of equine chorionic gonadotrophin, which is produced by the endometrial cups (which begin to form at approximately 35 days). Placentally-produced progestogens are then responsible for pregnancy maintenance from day 90 to term (McCue, 2019).
| Stage of gestation | Type of prostaglandin | Dose regimen | Published studies | Comments |
|---|---|---|---|---|
| Up to 35 days (before endometrial cup formation) | PGF2α | A single standard luteolytic dose, (cloprostenol 250 μg or dinoprost tromethamine 2.5–10 mg) | Dascanio, 2014 | The mare should be re-examined 4–7 days after administration to ensure pregnancy is terminated (Dascanio, 2014) |
| 35–120 days (endometrial cup formed with equine chorionic gonadotrophin production) | PGF2α | Cloprostenol 250 μg or dinoprost tromethamine 2.5–10 mg) be administered once a day for 3–5 days | Dascanio, 2014 | Endometrial cup formation complicates return to a fertile oestrus |
| Transcervical administration of 500 µg cloprostenol at approximately 65 days | Cuervo-Arango et al, 2015 | Reported to be very effective at 65 days. Re-treatment may be needed if pregnancy not terminated in 2 days. Also 37.5% of mares had a normal ovulation within 15 days of abortion despite endometrial cups and elevated equine chorionic gonadotrophin levels | ||
| 120 days to term (endometrial cups regress 120–150 days) | PGF2α | Twice daily intramuscularly injections of 250 µg Cloprostenol or 10 mg Dinoprost trometamol for 2–4 days | van Leewen et al, 1983; Rathwell et al, 1987; Daels et al, 1995 | Paccamonti (1991) reported that, because of multiple injections being required, prostaglandins are not the preferred method at this stage of pregnancy and that other methods, such as large volume saline infusions, are recommended instead |
| 25 mg cloprostenol twice daily intramuscularly for 2–4 days | Douglas and Ginther, 1975 | No major side effects were seen at this high dose rate |
Termination of pregnancy up to day 35
A single standard luteolytic dose of PGF2α (eg 250 μg cloprostenol intramuscularly) will cause lysis of a mature corpus luteum (over 5 days old), and a drop in progesterone leading to failure of pregnancy maintenance (Douglas et al, 1974). The mare should be checked 4–7 days later to ensure that the pregnancy has been eliminated (Dascanio, 2014).
Termination between days 35–120
The formation of endometrial cups occurs at 28–35 days gestation (Ginther, 1998). The chorionic girdle, specialised tissue on the early conceptus, invades the endometrium around 35 days of gestation to form the endometrial cups (Ginther, 1998). The endometrial cups produce large quantities of equine chorionic gonadotrophin, which can be detected in maternal blood from around 37–40 days post ovulation. Levels peak around day 55–70, and thereafter decline until equine chorionic gonadotrophin is no longer detectable peripherally at days 100–50. Equine chorionic gonadotrophin induces the formation of accessory/secondary corpora lutea within the ovaries (Ginther, 1992). Because of these secondary corpora lutea it is recommended that multiple daily doses of PGF2α are administered intramuscularly for 3–5 days to lyse those that are present or in development (Squires et al, 1980; Rathwell et al, 1987). However, the endometrial cups will remain functional until 60–80 days after their formation (120–150 days of gestation) and while these cups are functional and producing equine chorionic gonadotrophin, it has been reported that mares will not return to normal oestrous cycles after pregnancy termination (Allen, 1982).
Transcervical administration of PGF2α has also been used to terminate 65-day old pregnancies in polo ponies and successfully caused abortion in 27/32 mares (84.3%) within 48 hours (Cuervo-Arango et al, 2015). A dose of 500 μg of cloprostenol was diluted in 8 ml of saline and deposited transcervically using a standard artificial insemination pipette.
Notably, with this protocol 12/32 (37.5%) mares returned to normal cyclicity with a normal ovulation occurring within 15 days of abortion despite elevated levels of equine chorionic gonadotrophin. The mean time from abortion to normal ovulation was 28 days (range: 5–65 days). If this period was consistent, then termination of pregnancy at 65 days does not mean that the entire season is wasted, and the mare could potentially cycle and ovulate normally, achieving a successful pregnancy in the same reproductive season. Cuervo-Arango et al (2015) highlighted that equine chorionic gonadotrophin levels at the time of abortion do not correlate well with the luteal structures present or time until next ovulation.
Termination at 120 days to term
The mechanism by which PGF2α induces abortion at this stage of pregnancy (when not dependent on luteal progesterone) is thought to be because of its ecbolic effect. Sustained myometrial contractions lead to placental separation, a drop in circulating oestrogen levels and expulsion of the fetus (Daels et al, 1995). Although prostaglandins can be used successfully to terminate pregnancies after the first trimester of pregnancy, multiple injections are required (Madej et al, 1987). Paccamonti (1991) reported that because multiple injections are required, prostaglandins are not the preferred method at this stage of pregnancy and that other methods, such as large volume saline infusions, are recommended instead.
When using PGF2α to terminate 35–300 day pregnancies, twice daily injections are usually administered for 2–4 days (van Leeuwen et al, 1983; Rathwell et al, 1987; Daels et al, 1995). Douglas and Ginther (1975) reported that injecting 2.5 mg of cloprostenol twice daily resulted in abortion in 100% of the study population of 13 mares. On average, mares aborted after 3.7 injections (range 1–7 injections), and no anorexia, diarrhoea, colic or sweating was observed at this high dose.
PGE1 and PGE2, although not used for inducing abortion itself, have been recommended as part of some protocols to cause cervical dilation/relaxation. PGE1 (misoprostol, 1–2 mg) or PGE2 (dinoprostone, 0.5 mg) is applied directly to the cervical os and lumen, with cervical relaxation occurring 2–4 hours later (Dascanio, 2014).
Treatment of oviductal blockage
The use of PGE1/2 has increased since several studies found that applying these prostaglandins to the oviduct or the uterotubal junction led to pregnancies in mares that for unknown reasons could not previously conceive. Oviductal blockage, which is usually a diagnosis of exclusion, was thought to be the underlying cause in these mares (Allen et al, 2006) (Table 2). PGE1 and 2 cause dilation of the oviductal lumen via their effects on the oviductal smooth muscle (Weber et al, 1995; Troedsson et al, 2005). Pregnancies resulting after their application therefore support oviductal blockage as the cause of infertility in these mares.
| Indication | Prostaglandin | Dose regimen | Published studies | Comments |
|---|---|---|---|---|
| Treatment of oviductal blockage (unexplained infertility presumed to be caused by oviductal blockage) | PGE1(misoprostol) | 200 µg PGE1 dissolved in 3 ml of sterile water deposited onto each uterotubal junction via deep uterine insemination catheter. Can be administered pre or post ovulation | Alvarenga and Segabinazzi, 2018 | Lower success rate (68.2%) |
| PGE2 (dinoprostone) | 0.2 mg PGE2 in gel applied along length of each oviduct via laparoscopy in a standing sedated mare | Allen et al, 2006 | Higher success rate (93%) |
Physiologically, PGE2 plays a key role in oviductal embryo transport by relaxing the circular muscles (Weber et al, 1995) while increasing the contractility of the longitudinal muscles of the oviduct (Troedsson et al, 2005). It is produced by day 5 equine embryos (Weber et al, 1991) and these effects on the oviduct smooth musculature may explain the clinical effects of shortened embryo oviductal transport time (Robinson et al, 2000) and increased fertility in selected mares with presumed oviductal blockage.
There are two main approaches for prostaglandin application – either laparoscopically onto the oviducts or via a deep uterine insemination technique to the uterotubal junction. PGE2 (0.2 mg dinoprostone) gel applied laparoscopically resulted in 14/15 mares (93%) becoming pregnant at the same or subsequent cycle (Allen et al, 2006). Despite this procedure's apparent high success rate, it is invasive and relatively costly. More recently, the use of PGE1 (misoprostol) applied via deep uterine insemination has been demonstrated to increase pregnancy rates in mares with assumed oviductal blockage (Alvarenga and Segabinazzi, 2018). Despite pregnancy rates being inferior to the laparoscopically-applied PGE2 (68.2% vs 93%), it may be preferable to many clinicians because it is less invasive (hence lower complication rates) with lower costs and no specialised equipment (Figure 2).
Suspected side effects of deep uterine insemination with misoprostol were thought to be increased uterine inflammation and anecdotal reports of systemic/anaphylactic reactions. One study reported no significant effect of misoprostol infusions on uterine inflammation in mares in dioestrus or oestrous and sham controls (Amorim et al, 2022). In all mares, the treatment produced a transient inflammatory reaction that resolved within 72 hours of treatment (Amorim et al, 2022).
There is one published report of a severe anaphylactic reaction (Kiviniemi-Moore, 2021); however, it should be noted that the mare in this case received 1600 μg/uterine horn compared to the standard dose of 200 μg/uterine horn, which might have contributed to the severity of the reaction (Amorim et al, 2022). It is still unclear as to whether mares experiencing systemic reactions do so because of the misoprostol itself or because of underlying conditions that are exacerbated by the drug. In any case, as this technique becomes more common, more mares who are potentially sensitive to misoprostol may be exposed. As a precaution, it is recommended to observe treated mares for 30–60 minutes post treatment for systemic reactions (Amorim et al, 2022).
The effect of misoprostol applied via deep uterine insemination on pregnancy rates in reproductively normal mares has been investigated. Donatsch et al (2022) found that misoprostol had no effect on pregnancy rates in this group, which highlights the importance of patient selection for this treatment, as it offers no benefit to ‘normal’ mares. Ideally, only mares with a history of unexplained infertility should undergo this procedure.
Foal fostering and treating foal rejection
The use of PGF2α to aid fostering orphan foals onto mares (Figure 3) has been widely used for many years (Daels et al, 2002), and its use in the treatment of mares who reject their own foals has also been successful (Barker et al, 2019) (Table 3). One report has demonstrated lactation induction and foal adoption could be successfully performed in mares with an early pregnancy (30 days gestation). This involved the use of PGF2α but with the progestogen altrenogest to prevent pregnancy loss (Podico et al, 2022).
| Indication | Type of prostaglandin | Dose regimen | Published studies | Comments |
|---|---|---|---|---|
| Foal adoption or treatment of foal rejection | PGF2α | 750–1000 µg intramuscularly cloprostenol or 25 mg Dinoprost tromethamine intramuscularly 15–20 minutes before introducing foal to mare |
Barker et al, 2019
|
Can be attempted multiple times Success rate 95.2% |
Therefore, the mare used for adoption could be:
The technique involves administering a supraluteolytic dose of cloprostenol (750–1000 μg) or dinoprost tromethamine intramuscularly approximately 15–20 minutes before introducing the foal (Barker et al, 2019). Once strong side effects such as sweating, colic signs and muscle tremors are seen, the foal is introduced.
The foal is slowly introduced to the mare, allowing her to smell and lick the foal first before allowing the foal to progress slowly towards the mammary gland area (ie the mare's head to flank to mammary gland). It is reported that the process can successfully take place in less than 15 minutes, after which the mare and foal can be allowed in the stable together. The undesirable side effects of exogenously administered PGF2α (eg colic, sweating – which are usually an aspect to be avoided) are assumed to be important in promoting maternal behaviour. The use of d-cloprostenol induces luteolysis without side effects (Kuhl et al, 2016) and has not been reported as an aid to fostering or foal rejection. Its minimal side effects would make it a poor choice for fostering if they are indeed important in this process.
Ecbolic
Uterine fluid accumulation is commonly encountered post mating/insemination (Figure 4) and is not necessarily as a result of infection or endometritis (Canisso et al, 2020). Other possible causes include exaggerated uterine inflammatory response to semen and impairment of uterine drainage, eg cervical incompetence or pendulous uterus (Canisso et al, 2016). Failure to remove this fluid before the embryo enters the uterus approximately 6 days post ovulation may lead to embryonic death and reduce fertility (Adams et al, 1997; Robertson et al, 2018). Oxytocin is probably the most frequently used ecbolic in mares, but PGF2α is also effective (Combs et al, 1996; Niikura et al, 2021) (Table 4). However, the use of PGF2α post-ovulation, its effect on the developing corpus luteum (potentially causing luteolysis and/or reduced progesterone levels) and subsequent pregnancy rates (possibly reduced), are a concern.
| Indication | Type of Prostaglandin | Dose regimen | Published studies | Comments |
|---|---|---|---|---|
| Uterine ecbolic | PGF2α | Cloprostenol 250 µg intramuscularly once daily (4 hours post breeding up to 2 days post-ovulation) |
Nie et al, 2003
|
Causes more prolonged but weaker uterine contraction than oxytocin. Conflicting reports on pregnancy rates post-treatment (Troedsson et al, 2001; Brendemuehl, 2002). Wise to use single daily injections pre-ovulation and up to 2 days post ovulation only |
Oxytocin and PGF2α are key mediators in uterine contractions (Goddard and Allen, 1985; Cross and Ginther, 1987); oxytocin is more effective than prostaglandins at clearing radiocolloid from the uterus (Combs et al, 1996). PGF2α causes more prolonged (4–5 hours vs 30–50 minutes) but weaker contractions than oxytocin (Troedsson et al, 1995). PGF2α administered for 2 days post-ovulation causes a temporary decrease in progesterone, with a subsequent increase to normal levels (Troedsson et al, 2001; Brendemuehl, 2002).
Reports are conflicting on pregnancy rates after using PGF2α as an ecbolic in the periovulatory period. Two studies reported decreased pregnancy rates using cloprostenol up to 2 days after ovulation (Troedsson et al, 2001; Brendemuehl, 2002). However, a single dose of dinoprost on the day of ovulation or a single dose of cloprostenol for 2 days post ovulation was also shown by other studies to have no effect on pregnancy rates (Nie et al, 2003; Niikura et al, 2021). If multiple doses over multiple days are administered, then it is much more likely to cause complete luteolysis and/or depress progesterone to levels which are incompatible with pregnancy maintenance.
In view of this risk, it is wise to avoid multiple doses and/or to only administer daily single doses in the pre-ovulatory or early post-ovulation period.
Treatment of uterine infection
The luteolytic and ecbolic effects of PGF2α can aid treatment of uterine infections. Increased levels of progesterone produced by an active corpus luteum lead to cervical tightening and inhibit myometrial contractions, which are both important in uterine clearance (Irvine, 1993). Therefore, the uterus is less resistant to infection during dioestrus while progesterone levels are raised.
Some uterine infections themselves lead to the release of endogenous PGF2α release and luteolysis, causing the mare to return to oestrus earlier than expected (Daels et al, 1989). If uterine fluid or infection is detected while the mare is in dioestrus, exogenous PGF2α should be administered to return the mare to oestrus to allow appropriate diagnostics and management.
Ovulation (and subsequent raised progesterone levels) occurring during a course of uterine therapy could interrupt treatment or potentially make it less effective. Uterine treatments administered in dioestrus have also been associated with development of fungal infections and resistant bacteria (Hinrichs et al, 1988; McDonnell and Watson, 1992). Serial PGF2α injections to create an aluteal cycle could avoid this interruption, allowing a more prolonged and potentially more effective course of therapy (Coffman and Pinto, 2016). However, no studies could be found investigating the efficacy of such a regimen in the treatment chronic endometritis.
In some cases of pyometra, where the endometrium has been damaged or eroded, endogenous PGF2α is not released and the corpus luteum persists (Hughes et al, 1979; Threlfall, 1986). Administration of PGF2α to these cases will aid treatment but only if there are no other causes of impairment to uterine drainage, such as cervical fibrosis or transluminal adhesions (Van Camp, 1986). PGF2α and uterine therapy alone are unlikely to cause permanent resolution in chronic cases of pyometra and recurrence is likely. In these cases, bilateral ovariectomy, ovariohysterectomy and cervical stents have been advocated (Rötting et al, 2004; Jones et al, 2019; Krohn et al, 2019).
Cervical relaxation
The use of PGE2 to cause cervical relaxation before parturition has been investigated in both humans and cows (Trofatter et al, 1985; Duchens et al, 1993) (Table 5). Rigby et al (1998) administered 2–2.5 mg of PGE2 intracervically to mares 6 hours before inducing parturition. This was found to beneficial, facilitating fetal delivery, shortening delivery times and increasing foal vigour. This suggests that as part of a parturition induction protocol PGE2 is beneficial and should be considered.
| Indication | Prostaglandin | Dose regimen | Published studies | Comments |
|---|---|---|---|---|
| As part of parturition induction | PGE2 dinoprostone gel | 2–2.5 mg applied to cervix 6 hours prior to induction of parturition | Rigby et al, 1998 | Reported to facilitate foal delivery, shorten delivery time and increase foal vigour |
| Tight cervix causing uterine fluid accumulation | PGE1 misoprostol | 2 mg in 3 g of cream applied intracervically 2–3 hours pre-breeding | Le Blanc, 2006 | Reported to cause dilation for up to 8 hours after treatment |
| Misoprostol 1 mg as cream applied topically to cervix | McNaughten et al, 2014 | No measurable degree of cervical relaxation compared to controls |
Failure of cervical relaxation in non-pregnant mares can result in intrauterine fluid accumulation and infertility (Adams et al, 1987; Robertson et al, 2018; Nie and Barnes, 2003; Le Blanc, 2006). The benefits of prostaglandins to aid cervical relaxation in non-pregnant mares is less clear with conflicting reports on their use. One study that used PGE1 (misoprostol) intracervically reported cervical relaxation for up to 8 hours after treatment (Le Blanc, 2006). The treatment was administered 4–6 hours before mating or artificial insemination. However, another study found no evidence of cervical relaxation in treated mares vs untreated mares (McNaughten et al, 2014). The underlying cause of cervical failure to relax is likely to be important in the success (if any) of prostaglandins to treat this condition. If present, significant fibrosis of the cervix would mean that even if cervical smooth muscle relaxation occurred, cervical relaxation or lumen dilation would not.
Conclusions
The multiple applications of prostaglandin analogues in equine reproduction mean they have become an indispensable tool for clinicians. Prostaglandins are effective for termination of early pregnancy but become less effective as pregnancy advances requiring multiple doses. The protocol used will depend on the stage of pregnancy. Successful treatment of presumed oviductal blockage can be achieved using prostaglandins but is highly dependent on correct case selection. Clinicians should be aware that the pregnancy rates are not improved in reproductively normal mares. Adoption of orphan foals and treatment of foal rejection has been highly successful using PGF2α. Different types of mares can be used for adoption purposes using PGF2α, with altrenogest or exogenous progesterone for pregnant mares to prevent unintentional termination or pregnancy. The luteolytic and ecbolic effects of PGF2α can aid in the treatment of uterine infections and fluid accumulation. They can be used successfully up to 2 days post-ovulation. However, caution should be used to prevent irreversible damage to the early corpus luteum which is detrimental to pregnancy maintenance. Prostaglandin use to cause cervical relaxation is controversial, but it appears to be more beneficial as an adjunctive aid in induction of parturition. The full extent of the effects of prostaglandins on the reproductive cycle and equine fertility are still not fully understood. Serious side effects to patients, clinicians and surrounding personnel exist. For this reason, they should be used judiciously and only in cases where they are most likely to be efficacious.