The Effect of Extended Preoperative Fasting in Mares Undergoing Surgery of the Perineal Region

Horses with third-degree perineal lacerations require surgical treatment. To reduce the risk of postoperative dehiscence of the surgical site caused by tension, bacterial contamination, and mechanical abrasion, a preoperative and postoperative fasting up to 14 days was performed. A complete blood cell count, concentrations of electrolytes, urea, creatinine, glucose, lactate, cholesterol, triglycerides, total protein, and the activities of aspartate aminotransferase, glutamate dehydrogenase, γ-glutamyl transferase, creatine kinase, and alkaline phosphatase were determined. Fasting resulted in a marked decrease in intestinal motility and an increase in the concentrations of triglycerides and cholesterol. The activity of the glutamate dehydrogenase increased three-fold during the presurgical period. The other biochemical values showed few fluctuations. Based on these data, we concluded that presurgical and postsurgical fasting causes no severe imbalances in the metabolism in mares.     

A Single-Stage Technique for Repair of Rectovestibular Lacerations in the Mare

Delayed reconstructive surgery was performed as a one-stage procedure on 10 mares with rectovestibular lacerations, using monofilament nonabsorbable suture in a 3-layer closure, with all internal sutures buried. Primary healing was obtained in 9 cases. One case was complicated by severe postoperative straining, resulting in dehiscence of the rectum, which healed by second intention. A small rectovestibular fistula formed in this case and was later repaired. Recurrence of injury at subsequent foalings appears to be a significant problem.     

Use of Acupuncture to Induce Cyclicity in Anestrous Mares

Acupuncture is part of an Eastern system of medicine used for diagnosing, treating, and preventing disease. It has been used for the treatment of reproductive disorders in both human beings and domestic species. We hypothesized that acupuncture could be used to induce cyclicity in anestrous mares. In experiment 1, six anestrous mares were treated with acupuncture at Bai Hui, governing vessel (GV)-2, bladder (BL)-22, BL-23, BL-31, and BL-33 twice a week for 3 weeks, then once a week for an additional 8 weeks, whereas six anestrous mares served as controls. In experiment 2, six anestrous mares received acupuncture stimulation at Bai Hui, GV-1, BL-22, BL-23, and BL-28, with electroacupuncture (10 minutes at 50 Hz) at BL-22 and BL-28 bilaterally. In addition, vitamin B12 (3 mL, 1,000 mcg/mL) was injected in the vaginal fornix at the 3, 6, and 9 o’clock positions around the cervix. Six anestrous mares served as controls. Examinations and treatments were carried out twice a week until ovulation. In both experiments, ovarian activity was examined in all mares by transrectal ultrasonography and blood sample was obtained twice a week to determine progesterone concentrations. The time to first ovulation for experiment 1 was 14.0 ± 0.7 and 13.5 ± 1.0 weeks (mean ± SD) for the control and acupuncture groups, respectively (P = .60). In experiment 2, the time to ovulation was 9.0 ± 0.6 and 10 ± 0.3 weeks (mean ± SD) for the control and acupuncture groups, respectively (P = .19). No significant difference was found between the treated and control mares in either study.     

Comparison of Compounded Deslorelin and hCG for Induction of Ovulation in Mares

Ovulation-inducing agents are routinely used in broodmare practice. The objective of this study was to compare the efficacy of two compounded deslorelin products and human chorionic gonadotropin (hCG) in inducing ovulation in a clinical reproduction program. Breeding records of 203 mares administered an ovulation-inducing agent during the 2006 breeding season were reviewed. Estrous cycles were included for comparison if agents were administered when the largest follicle was 35 to 45 mm in diameter and endometrial edema was present. There was no significant difference (P > .05) in interval to ovulation for mares receiving deslorelin (1.9 ± 0.7 days) or hCG (2.0 ± 0.7 days). The percentage of mares that ovulated within 48 hours after treatment was also not significantly different between the agents (90.1% and 88.3%, respectively). In summary, clinical efficacy at inducing a timed ovulation in estrual mares with follicles 35 to 45 mm was similar between compounded deslorelin and hCG.     

Relation Among Neutrophil Enzyme Activity,Lipid Peroxidation,and Acute-Phase Response in Foal Heat in Mares

Apart from functional abnormalities, genetic structural disorders and management problems endometritis is one of the major causes of infertility or subfertility in mares. However, the causes of postbreeding endometritis in foal heat have not been clearly resolved to date. The aim of this study was to search for the relationship between neutrophil activity, acute-phase proteins, and oxidative status to indicate the parameters, which can influence fertility in cold-blooded mares in foal heat. The blood for the experiment was collected from 16 cold-blooded mares at five time points: 6–8 days before parturition, 24 hours after parturition, at the first postpartum breeding on the ninth day, 24 hours after breeding, and 48 hours after ovulation. The obtained samples were assigned for hematological tests, assays of neutrophil activity, plasma malondialdehyde (MDA), and fibrinogen concentrations. We estimated that in susceptible mares during persistent postbreeding endometritis, neutrophil activity increased together with MDA and fibrinogen plasma level. Elastase release in resistant mares before parturition was 48.91 ± 1.75%, whereas in susceptible animals, the value reached 45.57 ± 1.9% of the maximal release. Myeloperoxidase release in resistant mares before parturition reached 13.95 ± 2.1%, then increased at three consecutive measurements, and returned to a value from before parturition at the last measurement. Myeloperoxidase level in susceptible mares was slightly lower than in resistant ones, then these values augmented at all measurements, reaching the maximum at the fourth one. The obtained results may help to indicate the predisposition to persistent postbreeding endometritis in cold-blooded mares bred at foal heat.     

A Field Study of Serum,Colostrum, Milk Iodine,and Thyroid Hormone Concentrations in Postpartum Draft Mares and Foals

Iodine, thyroxine (T4) and triiodothyronine (T3) are required for normal fetal growth, maturation, and neonatal survival. There is a lack of robust information on iodine levels found in colostrum, milk, and serum of mares and foals after a healthy pregnancy. Our objective was to characterize colostrum, milk, and serum iodine levels in healthy postpartum mares and foals (n = 10) and explore relationships with thyroid hormone concentrations. Colostrum, milk, and jugular blood samples from draft breed mares and foals with an estimated average iodine daily intake of 39 mg per mare during pregnancy were obtained at Day 0 (foaling date) and/or 10 days later. Parameters studied were (1) mare basal concentrations of serum: TT3, TT4, and iodine; (2) iodine in colostrum at Day 0 and milk iodine (Day 10); and (3) foal basal: TT3, TT4, and serum iodine (Days 0 and 10). Median ± median error colostrum iodine levels (165 ± 15.1 μg/L) were higher than milk (48 ± 5.6 μg/L; P = .007) levels. Median ± median error foal serum iodine (268.5 ± 7.6 μg/L), TT4 (1,225 ± 47.8 nmol/L), and TT3 (14.2 ± 1.1 nmol/L) at foaling date were higher than at 10 days (serum iodine: 70 ± 3.6 μg/L; TT4: 69.6. ± 20.4 nmol/L; and TT3: 5.4 ± 0.3 nmol/L). In conclusion, equine mammary tissue concentrates iodine beyond plasma levels, making colostrum and milk a significant source of iodine. Foal serum iodine levels are high in the neonatal period and are positively correlated with TT4, which is important for neonatal adaptation.     

Effect of Chronic Administration of Oxytocin on Corpus Luteum Function in Cycling Mares

The objective of this study was to determine if intramuscular administration of 60 units of oxytocin once daily for 29 days, regardless of when treatment was initiated during the estrous cycle (i.e., without monitoring estrous behavior and/or detecting ovulation), would induce prolonged corpus luteum (CL) function in cycling mares. Mares were randomly assigned to two groups: (1) saline-treated control (n = 7) and (2) oxytocin-treated (n = 9) subjects. Control mares received 3 cc of saline, and oxytocin-treated mares received 60 units (3 cc) of oxytocin intramuscularly for 29 consecutive days. Treatment was initiated in all mares on the same day (day 1), independent of the day of the cycle. Jugular blood samples for determination of progesterone concentration were collected three times weekly (M, W, and F) for 21 days before treatment was initiated to confirm that all mares had a luteal phase of normal duration immediately before treatment. Beginning on the first day of treatment, blood samples were collected daily for eight days and then three times weekly through day 80. Mares were considered to have prolonged CL function if serum progesterone remained >1.0 ng/mL continuously for at least 25 days after the end of the treatment period. The proportion of mares with prolonged CL function was higher in the oxytocin-treated group than in the saline-treated group (7/9 vs. 1/7, respectively; P < .05). Three of the seven oxytocin-treated mares that developed prolonged CL function initially underwent luteolysis within 4–7 days of the start of oxytocin treatment and then developed prolonged CL function after the subsequent ovulation during the treatment period. In the other four oxytocin-treated mares that developed prolonged CL function, progesterone remained >1.0 ng/mL throughout the treatment period and into the post-treatment period. All mares with prolonged CL function maintained elevated progesterone concentrations through at least day 55 of the study. In conclusion, intramuscular administration of 60 units of oxytocin for 29 consecutive days effectively prolonged CL function in mares, regardless of when treatment was initiated during the estrous cycle. Importantly, this represents a protocol for using oxytocin treatment to prolong CL function that does not require detection of estrous behavior or day of ovulation.     

Randomized Comparative Trial of Acupuncture and Exercise Versus Uterine Ecbolics in the Treatment of Persistent Postbreeding Endometritis in Mares

Although exercise and acupuncture are frequently used therapies to treat persistent postbreeding endometritis, their efficacy to date is unproven. The objective of this study was to determine if exercise and acupuncture are effective methods to reduce intrauterine fluid and compare the effectiveness of these treatments to the use of uterine ecbolics. Twelve mares susceptible to postbreeding endometritis were enrolled in the study with a randomized cross-over design using both positive and negative controls. During each estrous cycle, mares were randomized into one of six treatment groups, including stall rest (SR), oxytocin, cloprostenol, exercise, electroacupuncture, and oxytocin and exercise. Each mare was challenged with an insemination dose of 500 × 10⁶ dead sperm at time 0 hours. Intrauterine fluid measurements were taken at 0, 4, 24, 48, 72, and 96 hours postbreeding. Associations between treatment efficacy and fluid clearance were investigated using a random-effects logistic regression model that controlled for positive uterine culture. Compared with the SR negative control, exercise was the most effective treatment and had 29.7 times increased odds of fluid clearance. The second most effective treatment was oxytocin alone, with 16.9 times increased odds of fluid clearance. This was followed by cloprostenol that had 10.6 greater odds of fluid clearance, and finally, the treatment that combined exercise with oxytocin had 8.4 times greater odds of fluid clearance. Results from this study confirm that exercise and exercise combined with oxytocin are effective methods to clear intrauterine fluid.     

Feasibility and Safety of Endometrial Injection of Autologous Bone Marrow Mesenchymal Stem Cells in Mares

​Chronic degenerative endometritis (CDE) is an important cause of fertility problems in older mares. It is estimated that 30% of mares breeding are over 18 years old and the high value of their progeny encourages the use of these animals in assisted reproduction procedures. Currently, cell-based therapies are broadly used in human and veterinary regenerative medicine and have been showed a good effect on the treatment of liver fibrosis. Thus, the present study aimed to evaluate the feasibility and safety of endometrial injection of autologous bone marrow MSCs in mares. Mares were examined by transrectal ultrasound and the endometrial stem cells injections performed during diestrous, using injection needles coupled to a teflon catheter through the biopsy channel of a flexible endoscope. After treatment clinical evaluations (heart reat, respiratory rate, staining of the mucosa, capillary refill time, body temperature and lameness score) were performed daily during seven days. The intrauterine fluid and endometrial edema was evaluated before, 24 and 48 hours after the procedure by ultrasonographic exams and the inflammatory infiltrate (polymorphonuclear cells and mononuclear cells) and degree of fibrosis by histological evaluations before (D0) and 15 (D15), 30 (D30) and 60 (D60) days after endometrial injections. The results were evaluated by Kolmogorov-Smirnov and Kruskal-Wallis test followed by Dunn test and for fertility rates was used Chi-square, considering a 5% significance level. Neither clinical alteration was observed in mares after treatment, as well as, intrauterine fluid and endometrial edema were not detected in any mare before and after cell therapy. After cell therapy, significantly more PMNs were found in D15 biopsies, however, these cells were not observed in D30 and D60 biopsies. No worsening on the histological architecture after treatment was observed in any mare. In conclusion, the results of this study showed that endometrial hysteroscopic injections of stem cells in mares is a safe procedure.     

Low Concentration of Anti-Müllerian Hormone in Mares with Delayed Uterine Clearance

Luteal dysfunction has been observed in mares with defective uterine clearance. Association of low ovarian reserve with luteal dysfunction and abnormal endometrial thickness has been reported in bovine. Anti-Müllerian hormone (AMH) has been indicated as a marker for ovarian reserve in bovine and originates primarily from the ovary in equine. The present study evaluated serum AMH concentration in mares with delayed uterine clearance versus that in mares without delayed uterine clearance. Of 49 mares assigned to the study, 12 and 37 mares were diagnosed with and without delayed uterine clearance, respectively. Delayed uterine clearance was determined based on history and the observation of intrauterine fluid in ultrasonographic examination 24 hours after natural breeding. Serum AMH concentration was measured during estrus. Concentration of AMH was lower in mares with delayed uterine clearance (0.4 ± 0.1 ng/mL) than in those without delayed uterine clearance (1.1 ± 0.1 ng/mL). In conclusion, the present study indicated possible associations between ovary-lined mechanisms and uterine clearance failure in mares.     

The Effect of Cloprostenol on the Incidence of Multiple Ovulation and Anovulatory Hemorrhagic Follicles in Two Mares: A Case Report

Reproductive records of the entire lives of two mares with abnormally high incidence of multiple ovulation and hemorrhagic anovulatory follicles were analyzed retrospectively. Chi-square analysis was used to test statistically the effect of cloprostenol, a prostaglandin F2 analog, on the incidence of multiple ovulations, hemorrhagic anovulatory follicles, and its ultrasonographic appearance. A total of 319 estrous cycles during a 17-year period were analyzed. Cycles induced with cloprostenol were more likely (P < .000) to develop hemorrhagic anovulatory follicles than spontaneous cycles. The incidence of multiple ovulation was higher in induced cycles than in spontaneous cycles in one of the two mares.     

The Level of Prolactin,Serum Amyloid A,and Selected Biochemical Markers in Mares Before and After Parturition and Foal Heat

The aim of the study was to determine the level of prolactin (PRL), serum amyloid A (SAA), and selected biochemical markers (T-Chol, AST, TP, Mg²⁺, P⁺, and Ca²⁺) in the blood of mares during the perinatal period. The study involved 14 mares of the Polish Coldblood Horse breed, which were in the third trimester of pregnancy. Blood was collected for testing 2 weeks before parturition and then 24 hours after delivery and in the foal heat (9 days) and 9 days after ovulation and breeding. The research revealed significant differences in the level of PRL and SAA before and after delivery. The highest PRL level was found 24 hours after delivery, lowest in foal heat and 9 days after ovulation. Serum amyloid A concentration was within the accepted norms; however, on day 9 after foaling, a significant increase of this protein was observed. All biochemical markers were within physiological limits. However, significant increases in T-Chol, AST, and TP levels was observed 24 hours after the delivery, whereas in foal heat and after ovulation levels of T-Chol and TP significantly decreased and the AST level remained at a similar level. There were no significant changes in electrolyte levels such as Mg²⁺, P⁺, and Ca²⁺. The pregnancy rate in the foal heat was at 43%. Collectively, the results of this study in conjunction with clinical observations demonstrated that when the perinatal period was normal, no disturbances in health related to pregnancy, parturition, lactation, and reproductive status during the postpartum period were found.     

Effect of a Nonsurgical Embryo Transfer Procedure and/or Altrenogest Therapy on Endogenous Progesterone Concentration in Mares

Endogenous progesterone levels may decline after transcervical embryo transfer in some mares. Progestogen therapy is commonly used to support endogenous progesterone levels in embryo transfer recipient mares or those carrying their own pregnancy. The goal of this study was to determine the effects of the transcervical transfer procedure and/or altrenogest therapy on luteal function in mares. Mares were assigned to one of six treatment groups: group 1 (untreated control; n = 7 cycles), group 2 (sham transfer, no altrenogest; n = 8 cycles), group 3 (sham transfer plus altrenogest; n = 8 cycles), group 4 (pregnant, no altrenogest; n = 9 mares), group 5 (pregnant plus altrenogest; n = 9 mares), and group 6 (nonpregnant plus altrenogest; n = 10 cycles). Mares in groups 4-6 were bred and allowed an opportunity to carry their own pregnancy. Blood samples were collected for 22 days beginning on the day of ovulation. Sham embryo transfer (groups 2 and 3, combined) did not result in a decline in endogenous progesterone levels compared with control mares (group 6). However, sham embryo transfer did result in luteolysis and an abrupt decline in endogenous progesterone levels in one of the 16 (6.2%) sham-transferred mares. Altrenogest therapy in sham-transferred mares (group 3) was associated with lower endogenous progesterone levels on days 10, 12, and 13 postovulation when compared with sham-transferred mares that did not receive altrenogest (group 2). Administration of altrenogest to pregnant mares (group 5) was associated with lower concentrations of endogenous progesterone from days 14 to 18 and on day 21 compared with endogenous progesterone levels in pregnant mares not administered altrenogest (group 4). In conclusion, a transcervical embryo transfer procedure can cause luteolysis in a low percentage of mares. Altrenogest therapy may be associated with a reduction in endogenous progesterone secretion, presumably mediated by a reduction in pituitary luteinizing hormone (LH) release and a decrease in luteotropic support.     

Effect of eFSH on Ovarian Cyclicity and Embryo Production of Mares in Spring Transitional Phase

The use of equine FSH (eFSH) for inducing follicular development and ovulation in transitional mares was evaluated. Twenty-seven mares, from 3 to 15 years of age, were examined during the months of August and September 2004, in Brazil. Ultrasound evaluations were performed during 2 weeks before the start of the experiment to confirm transitional characteristics (no follicles larger than 25 mm and no corpus luteum [CL] present). After this period, as the mares obtained a follicle of at least 25 mm, they were assigned to one of two groups: (1) control group, untreated; (2) treated with 12.5 mg eFSH, 2 times per day, until at least half of all follicles larger than 30 mm had reached 35 mm. Follicular activity of all mares was monitored. When most of the follicles from treated mares and a single follicle from control mares acquired a preovulatory size (≥35 mm), 2,500 IU human chorionic gonadotropin (hCG) was administered IV to induce ovulation. After hCG administration, the mares were inseminated with fresh semen every other day until ovulation. Ultrasound examinations continued until detection of the last ovulation, and embryo recovery was performed 7 to 8 days after ovulation. The mares of the treated group reached the first preovulatory follicle (4.1 ± 1.0 vs 14.9 ± 10.8 days) and ovulated before untreated mares (6.6 ± 1.2 vs 18.0 ± 11.1 days; P < .05). All mares were treated with prostaglandin F_2α (PGF_2α), on the day of embryo flushing. Three superovulated mares did not cycle immediately after PGF_2α treatment, and consequently had a longer interovulatory interval (22.4 vs 10.9 days, P < 0.05). The mean period of treatment was 4.79 ± 1.07 days and 85.71% of mares had multiple ovulations. The number of ovulations (5.6 vs 1.0) and embryos (2.0 vs 0.7) per mare were higher (P < 0.05) for treated mares than control mares. In conclusion, treatment with eFSH was effective in hastening the onset of the breeding season, inducing multiple ovulations, and increasing embryo production in transitional mares. This is the first report showing the use of FSH treatment to recover embryos from the first cycle of the year.     

Frequency of Rhodococcus equi in Feces of Mares in Central Kentucky

Rhodococcus equi (R equi) pneumonia is an important cause of disease and death in foals. Feces from mares can contain R equi, including virulent R equi, and thus may act as a source of the bacteria at horse breeding farms. A previous report documented that every mare at a farm in central Kentucky shed virulent R equi in at least one of four fecal samples collected serially during the periparturient period. The objective of this study was to assess the extent to which this high prevalence of fecal shedding could be replicated at other horse breeding farms. The frequency of detection of R equi and virulent R equi in fecal samples was studied among 131 mares from 24 farms in central Kentucky. The proportions of fecal samples from mares containing R equi and virulent R equi were 95% and 76%, respectively. These findings indicate that R equi and virulent R equi may be isolated with high frequency from feces of mares at breeding farms in central Kentucky, and that mares are a source of virulent R equi for the environment of their foals.     

Morphological Characteristics of the Placenta and Umbilical Cord of Arabian Mares Foaling in the United Arab Emirates

A total of 127 normal placentas from Arabian mares resident in the United Arab Emirates were examined. The mean linear dimensions of the placenta were, on average, 84% of those previously recorded for the placentas of the Thoroughbred. Significant differences in the size of the allantochorion between primigravid and multiparous mares were seen only in the linear dimensions of the body portion. The pregnant horn was more commonly on the right than left side of the uterus (P = .01; 74/127; 58%). Cord attachment was primarily at the base of the two placental horns (112/127; 88%), with the remainder showing anomalies from this position. The mean (±SEM) total length of the umbilical cords was 62.2 ± 1.2 cm, being composed of the allantoic portion (29.7 ± 0.9 cm) and amniotic portion (32.5 ± 0.6 cm), which averaged 53 ± 0.01% of the total length. The amniotic portion was usually, but not always, longer than the allantoic portion (79/127; 62%). Longer cords were associated with a greater proportion of allantoic length. An enlarged yolk sac remnant (YSR) was present in 16/127 (13%) placentas. Cords with YSRs displayed a significantly longer allantoic portion than those without (P = .02). The total cord length was not correlated with the weight or area of the allantochorion or amnion, any linear measurement of the allantochorion, gestation length, the month of foaling, parity of the dam, or birthweight or sex of the foal. The purpose for which the mare was bred (i.e., racing or showing) influenced the cord length, those of show mares being significantly longer.     

Biochemical Profile of Amniotic and Allantoic Fluid During Different Gestational Phases in Mares

Fetal fluids have different vital functions that sustain both pregnancy and normal parturition. The biochemical composition of amniotic fluid during gestation is not well established; thus the purpose of the present study was to determine the biochemical profile of both amniotic and allantoic fluids from mares during initial, mid, and latter third phases of pregnancy. Samples were collected after slaughter, using allantocentesis and amniocentesis. Sixty samples of fetal fluids were analyzed. Alkaline phosphatase (AP), glucose, total protein (TP), urea, creatinine, Ca, chloride (Cl), Na, and K concentrations were measured using commercially available kits. The AP concentration in amniotic fluid was higher than that in allantoic fluid during the three gestational phases (P < .05). There were no differences between glucose mean values of allantoic and those of amniotic fluids (P < .05). However, glucose values were higher in the allantoic fluid in the last trimester of pregnancy. TP was higher in the amniotic fluid than in allantoic fluid (P < .05). Urea values varied among the phases; however, there were no differences between the amniotic and allantoic fluid values (P > .05). Creatinine values were higher in allantoic fluid (P < .05). Na and Cl concentrations were higher in amniotic fluid (P < .05). However, Ca and K concentrations were higher in the allantoic fluid.     

Comparison of the Cytology Brush and Cotton Swab in the Cytological Evaluation of the Endometrium in Mares with Regard to Fertility

The aim of this study was to compare the cytology brush (CB) and cotton swab (CS) in the cytological evaluation of the endometrium in mares with regard to fertility. The study was conducted on 26 mares displaying spontaneous estrus. Samples for cytological evaluation were taken from each mare by using commercially available CS and CB. After sample collection, all mares were mated in the same estrus, and pregnancy diagnosis was performed 14-18 days after last mating. No vaginal cells were found in smears, and the CB technique yielded significantly more cells/high-power field (HPF) than the CS technique. Additionally, more cases of endometritis were diagnosed using the CB than the CS. It was also shown that the degree of inflammation is more important in diagnosis of infertility in the mare than the mere presence or absence of inflammation. In conclusion, further studies are needed to establish more precise criteria for the interpretation of inflammation, especially if samples are collected using the CB.     

Referred Vaginal Pain: Cause of Hypersensitivity and Performance Problems in Mares? A Clinical Case Study

In mares, pneumovagina develops because of perineal malconformation. This condition may aggravate to cause vaginitis, cervicitis, and endometritis. In animal models, visceral pain has been associated with genital infections causing hyperalgesia and allodynia in the affected organs. The pain from genital infections may be referred to dermal areas of the hindquarter.The aim of the current clinical case study was to determine whether performance problems attributable to hypersensitivity in dermal areas of the hindquarter, a classic symptom of nymphomania in the mare, may be caused by visceral pain resulting from irritative processes in the reproductive organs. Vulvoplastic surgery (Caslick operation) is a well-known treatment of mares in which poor perineal conformation is the underlying factor of genital infections. The current study is based on clinical examinations and performance of mares with hypersensitivity in dermal areas of the hindquarters before and after Caslick operation.Fourteen mares exhibiting performance problems such as kicking against the rider's legs, bolting, or refusing to move forward were selected for this study. Performance problems or stereotypic behavior were observed either permanently or intermittently during estrus. In 12 of 14 mares (86%), the performance problems were totally eliminated or diminished within 6 months after Caslick operation.The clinically observed performance improvement in the treated mares supports the hypothesis that problems attributable to hypersensitivity in dermal areas of the hindquarters may be caused by irritative processes in the labia vulvae, vestibulum, and vagina of the mare.