Effect of oxytocin on suppression of oestrus in mares exhibiting normal oestrous cycles

Abstract

AIM: To compare the efficacy of oxytocin given once daily, either I/V or I/M, on Days 7–14 post-ovulation, on the expression of oestrus in mares through to 65 days post-ovulation.

METHODS: Eighteen mares of various breeds that were displaying normal oestrous cycles were randomly assigned to one of three treatment groups on the day of ovulation (Day 0), detected using transrectal ultrasonography. Mares in the control group (n = 6) were given 1 mL saline I/V; mares in the I/V and I/M groups (n = 6 per group) were injected with 10 IU oxytocin I/V and I/M, respectively. All treatments were given once daily on Days 7–14. Mares were teased by a stallion three times per week, up to 65 days post-ovulation, to detect oestrous or dioestrous behaviour. Ovarian follicular and luteal activity were monitored using transrectal ultrasonography three times weekly, and daily when a follicle >30 mm diameter was present until ovulation. Blood samples were collected weekly for analysis of concentrations of progesterone in serum. Prolonged dioestrus was defined as a period of >30 days of dioestrous behaviour after Day 0, confirmed by detection of corpora lutea and concentrations of progesterone in serum >4 nmol/L.

RESULTS: Overall, 8/18 (44%) mares showed prolonged dioestrus. These included 2/6 (33%) mares in the control group, compared with 5/6 (83%) and 1/6 (16%) mares in the I/V and I/M groups, respectively (p = 0.11). The median duration of the first dioestrus was longer for the I/V group (64 (min 16, max 66) days) compared with the control group (18 (min 12, max 64) days) (p = 0.05), but was not different between the control group and the I/M group (16 (min 13, max 65) days) (p = 0.57). For all mares there was strong agreement between teasing behaviours, ultrasonographic assessment of ovarian activity, and concentration of progesterone in serum.

CONCLUSIONS AND CLINICAL RELEVANCE: This study found that low doses of oxytocin did not increase the proportion of mares with prolonged dioestrus, compared with controls, although I/V oxytocin did increase the median duration of dioestrus. The results must be interpreted with some caution as group numbers were small, and a variety of breeds were used. Further investigation of oxytocin given I/V may be warranted as a potential method of oestrus suppression in mares exhibiting oestrous cycles that is low cost, safe and well-tolerated, and potentially reversible with prostaglandin.     

Complications associated with the presence of two intrauterine glass balls used for oestrus suppression in a mare

This report describes the identification and removal of 2 glass balls, both of which had fragmented, from the uterus of a mare. The glass balls had been placed into the uterine lumen to prolong function of the corpus luteum for suppression of oestrous behaviour. This report adds to a growing body of evidence that severe complications can result from the use of intrauterine glass balls for oestrus suppression in mares, particularly if they are retained for a prolonged time.     

A comparison of the uterine proteome of mares in oestrus and dioestrus

Proteomic analysis of mare uterine flush fluid provides a minimally invasive technique for studying protein changes associated with the oestrous cycle. The aim of this study was to identify differentially abundant proteins in the uterine flush fluid of mares in oestrus and dioestrus. In this study, uterine flush fluid samples were collected from eight reproductively healthy mares in either oestrus (n = 5) or dioestrus (n = 3). Proteomic analysis was performed using liquid chromatography‐tandem mass spectrometry. Of 172 proteins identified, six proteins (immunoglobulin lambda‐like polypeptide 1, haemoglobin subunit alpha, alpha‐1B‐glycoprotein, serotransferrin, apolipoprotein A‐1, and haemoglobin subunit beta) were significantly more abundant in oestrus. These proteins may contribute to the endometrial defence system through roles in inflammation, immunity or antimicrobial activity. In other species, some of these proteins have been described as immunoglobulins, negative acute phase proteins or defence agents against micro‐organisms. During dioestrus, immunoglobulin alpha‐1 chain C region‐related, complement factor I, CD 109 antigen and uterocalin, were significantly more abundant. Research in other species suggests that these four proteins contribute to the immune response through proposed immunoregulatory characteristics, complement system involvement or roles in B cell–T cell interactions. In conclusion, ten differentially abundant proteins were identified in the uterine flush fluid of mares in oestrus and dioestrus. Targeted studies on these proteins could elucidate their role in uterine defence mechanisms during the oestrous cycle in the mare.     

The hazards associated with the use of intrauterine glass balls to suppress oestrus in mares

Placing a sterile glass ball or marble into the uterine lumen is a popular method to suppress unwanted oestrous behaviour in mares. This is in spite of the original report of Nie et al. (2003), which detailed the lack of efficacy of the technique. Recent reports in the literature have described a number of untoward problems associated with the technique. This paper describes two further cases, one involving fragmentation of marbles in the uterine lumen and a second describing the laparoscopic removal of a marble after it had tracked through the endometrium and became pedunculated between the myometrium and serosa. The inefficiency of suppressing oestrus with glass balls, combined with the potential problems their placement may cause, argue strongly that this practice should be considered unethical.     

Cystic calculus in a mare due to a misplaced uterine glass marble

A 9-year-old Quarter Horse mare was referred for evaluation of continuous oestrus, polyuria and suspected pyometra after a glass marble was placed in the uterus by the owner. Palpation per rectum and transrectal ultrasonography confirmed the presence of a spherical object, but it was not possible to discern whether it was located within the lumen of the uterus or urinary bladder. Thus, an endoscopic examination of both uterus and bladder was carried out and a spherical, spiculated surface urolith was found on the floor of the bladder. The cystic urolith with suspected marble core was removed using a 12-mm specimen retrieval bag with endoscopic guidance. Follow-up with the owner one month after discharge revealed that the mare had not shown any signs of discomfort while urinating nor signs of oestrus.     

Chronic intermittent colic in a mare attributed to uterine marbles

A 13‐year‐old Quarter Horse mare presented for evaluation of chronic intermittent colic. Following extensive diagnostics, abdominal radiographs revealed two round, radiopaque objects in the caudal abdomen. Palpation per rectum and transrectal ultrasonography of the reproductive tract confirmed that the round objects were uterine marbles. Dinoprost tromethamine (Lutalyse, 5 mg i.m. q. 24 h for 2 days) was administered to bring the mare into oestrus, and both uterine marbles were manually removed from the uterus following digital dilation of the relaxed cervix. Follow‐up with the owner 12 months after discharge revealed that the mare had shown no further signs of abdominal discomfort since having the uterine marbles removed. To the authors' knowledge, this is the first published report of chronic intermittent colic attributed to uterine marbles in a mare.     

Recognition and quantification of pain in horses: A tutorial review

Pain management is dependent on the quality of the pain evaluation. Ideally, pain evaluation is objective, pain‐specific and easily incorporated into a busy equine clinic. This paper reviews the existing knowledge base regarding the identification and quantification of pain in horses. Behavioural indicators of pain in horses in the context of normal equine behaviour, as well as various physiological parameters potentially useful for pain evaluation, are discussed. Areas where knowledge is sparse are identified and a new equine pain scale based on results from all reviewed papers is proposed. Finally, the most important considerations in relation to the implementation of a pain scale in a hospital setting are discussed.     

Higher doses of Melatonin affect ovarian and middle uterine arteries vascular blood flow and induce oestrus earlier in acyclic ewes

The present study aimed to evaluate the effect of a single intramuscular injection of melatonin on response of oestrus, rate of conception, ovarian and uterine blood supply during the non-breeding season. Melatonin powder was dissolved in dimethyl sulfoxide (DMSO) and olive oil and injected in two dose of 25 mg (MEL-1; n = 10) and 50 mg (MEL-2; n = 10) in May. Ten ewes were served as the control (CON). Results showed that oestrous response was higher in the MEL-2 group than in other groups (p < .01). The time to onset of oestrus in both the MEL-1 and MEL-2 groups was significantly shorter, compared with the CON group (p < .01). Pregnancy rates reached 80% and 90% at 45 days of gestation. Doppler indices of the right and left ovarian artery were lower in the melatonin groups than in normal groups (p < .01). Both Doppler indices in uterine arteries were not different among two treatment groups (p > .05), but there was a significant difference when compared with the CON group (p < .01), no difference found between two treatment group in the Doppler velocities of middle uterine arteries (p > .05). In conclusion, the definite changes in the studied parameters showed two novel findings; (a) the increase in melatonin dose results in increasing the blood flow expressed by increased Doppler velocities in fatty tailed ewes, (b) as well as, melatonin can initiate the breeding season regardless of the season earlier by 1–2 months in ewes.     

Luteoma in two mares treated by ovariectomy

A 15-year-old Paint mare (Case 1) and a 15-year-old Arabian mare (Case 2) were presented for aggressive and undesired behaviour. Reproductive evaluation revealed, initially, an enlarged ovary with a smaller/normal sized contralateral ovary in each case. Granulosa cell tumour (GCT) panel testing revealed elevated inhibin B in both cases and elevated anti-Mullerian hormone (AMH) levels only in Case 1 determined from currently established reference ranges. Based on a presumptive diagnosis of GCT, bilateral standing ovariectomy was elected for both mares. In each case neoplastic tissue consistent with luteoma was detected only on histological examination in the smaller/normal sized left ovary; the right ovary appearing normal. Long-term follow-up was performed 1 year and 5 years post-operatively by telephone. The owners were satisfied with the outcome and each patient demonstrated resolution of aggressive stallion-like behaviour. Each case returned to the owners' intended use. In conclusion, luteoma should be considered as a differential diagnosis for ovarian neoplasms in horses related to behavioural abnormalities, even in normal-sized ovaries.     

Post-partum uterine rupture: Standing repair in three mares using a laparoscopic technique

Three mares underwent diagnostic laparoscopy because of suspicion of post-partum uterine ruptures. All three horses showed clinical signs of a uterine rupture between 1 and 3 days after parturition and underwent diagnostic laparoscopy. In all cases a full thickness uterine rupture could be detected and was sutured laparoscopically. Availability of suture material and surgeon experience were responsible for the surgical methods chosen for repair. In the first case, a hand-assisted laparoscopic approach was chosen for suturing the ruptured uterus, whereas in the other cases the approach was entirely laparoscopic. In the second case, extracorporeal knots were used for the repair and in the last case described a barbed loop suture was available for closure of the uterus. Two of three mares were alive for at least 12 months after surgery without any abdominal problems. One of these mares delivered a healthy foal 2 years after surgery. The remaining mare died 3 months after surgery but no necropsy was done. Laparoscopy should be considered for post-partum mares with signs of peritonitis to access the uterus and repair a rupture if it is accessible. A laparoscopic approach using intracorporeal knots or barbed sutures for the repair of the uterine rupture as well as a hand-assisted laparoscopic approach are feasible. The use of the barbed suture for intracorporeal closure makes the minimal invasive laparoscopic technique easier to perform.     

Growth rate of ovulatory follicles during the first ovulatory oestrus (after seasonal anoestrus) and subsequent oestrous period in Irish Draught mares

It is believed that during the spring transition, the developing follicle tends to grow more slowly, persist longer and grow to a larger diameter prior to ovulation than at subsequent oestrus periods. A general suspicion, that the first ovulation of the year is less fertile than subsequent ovulations could be explained by a slower growth rate of the ovulatory follicle during transition with the consequent production of a subfertile oocyte. By detailed serial examination of the same group of Irish Draught mares over three winter/spring periods, no significant difference was found in either growth rate or pre-ovulatory diameter when compared with subsequent ovulations. Mean growth rates over the ten days prior to ovulation were 2.20 mm/day (range 1.18 to 3.64) and 2.19 mm/day (range 1.25 to 3.41) for first and subsequent ovulations respectively. Mean maximum pre-ovulatory diameters were 44.7 mm (range 35 to 59) and 43.5 mm (range 31 to 57.5) for first and subsequent ovulations respectively. The impression gained by practitioners that the first follicle develops more slowly during the transition to the first ovulation of the season may be due to less frequent examinations and consequently a failure to observe and record that follicles may grow and then regress during this period. The largest follicle observed a few days previously is not necessarily the same large follicle found at a later examination.     

Sustained-release deslorelin acetate implants disrupt oestrous cyclicity in the mare

There is a need for a safe, effective and practical method of oestrus suppression in the mare. The aim of this study was to monitor ovarian activity in mares exposed to either 9.4 or 28.2 mg deslorelin acetate, a GnRH agonist, in the form of a sustained-release implant. Following oestrus synchronisation, mares were randomly assigned to one of three groups (n = 4 per group) and administered either one (Des1 group; 9.4 mg) or three (Des3 group; 28.2 mg) implants of deslorelin acetate (Suprelorin-12, Virbac Australia) or one blank implant (Control group; Virbac Australia). Mares underwent weekly blood sampling for 12 weeks following implant placement (Day 0–Day 84), with transrectal palpation and ultrasonography of the reproductive tract at all sampling timepoints except Days 56, 70 and 77. All mares showed baseline serum progesterone concentrations (SPC; ≤1.3 nmol/L or 0.4 ng/ml) on Day 0. Cycling Control mares showed typical oestrous cyclicity characterised by peaks and troughs in SPC over time. Four of eight treated mares demonstrated a sustained elevation in SPC after the initial ovulation after implant placement; SPC declined to baseline levels (Des1 group; 2 mares) or remained elevated (Des3 group; 2 mares) at the final sampling timepoint on Day 84. Oestrous cyclicity was erratic in three of the remaining four treated mares. In total, 87.5% (7 of 8) of treated mares showed atypical oestrous cyclicity after implant placement. These results suggest that deslorelin acetate disrupts oestrous cyclicity in the mare, which warrants further research.     

Evaluation of poor performance in competition horses: A musculoskeletal perspective. Part 1: Clinical assessment

Lack of willingness to go forward freely, lack of power, shortened steps, stiffness of the cervical or thoracolumbosacral regions are common nonspecific signs of musculoskeletal causes of poor performance in sports horses. Understanding musculoskeletal causes of poor performance requires knowledge of how normal horses move, the requirements of specific work disciplines, the nomenclature used by riders to describe how a horse is performing and the interactions between horses and riders. Determining the underlying causes needs an in‐depth history and clinical assessment, including in hand, on the lunge and ridden. Ridden exercise should include all aspects with which the rider is experiencing problems. Change of the rider can sometimes help to differentiate between horse and rider problems, but most normal horses are compliant and just because a horse goes better for a more skilled rider does not preclude an underlying pain‐related condition. Lungeing and ridden exercise should include not only trot but also transitions and canter which may highlight gait abnormalities not seen at trot. An accurate history combined with thorough clinical examination of the whole horse should permit the establishment of a list of problems requiring further investigation.     

Pathology of lethal peripartum broad ligament haematoma in 31 Thoroughbred mares

Reasons for performing study: Broad ligament haemorrhage in peripartum mares is a life‐threatening disease and there are few reports on the aetiology and pathogenesis of broad ligament haematoma. Objectives: To obtain information regarding the sites for the early diagnosis and pathogenesis of broad ligament haematoma of mares. Methods: Thirty‐one mares that died of broad ligament haematoma peripartum were examined pathologically for bleeding sites. The arterial distribution of 5 young mares with several parities served as negative controls. Results: Age and/or multiparity were the predisposing factors for the disease. Arterial injuries were most commonly observed in the uterine artery (24 of 31 mares). Among these, the proximal uterine artery that lies within 15 cm of the bifurcation of the iliac artery was the most frequent site for rupture (18 mares). The lesions occurred preferentially at the bifurcations, lateral part of curvatures and abrupt flexures of the artery. The morphology of the injuries was classified into 3 types: ruptures with and without longitudinal fissures, and transections. Histologically, the arterial wall adjacent to the rupture showed atrophy of smooth muscle cells with fibrosis of the tunica media and disruption and/or calcification of the internal elastic lamina. Conclusions: Arterial injuries that led to broad ligament haematoma in peripartum mares occurred most frequently in the proximal uterine artery, and atrophy of smooth muscle cells with fibrosis of the arterial wall was as one of the predisposing factors in aged and multiparous mares. Potential relevance: Monitoring small aneurysms, mural tearing, medial fibrosis at the proximal uterine artery by transrectal echography could provide useful information for the early diagnosis and possible prevention of broad ligament haematoma of peripartum mares.