1 CONTACT Kristyn Burton kristyn.burton@okstate.edu © 2025 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http:// creativecommons.org/licenses/by-nc/4.0/), permitting all noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Clinical Theriogenology 2025, 17, 12176, http://dx.doi.org/10.58292/CT.v17.12176 Case Report Pregnancy termination in a mare with hydroallantois* Kristyn Burton, Reed Holyoak, Adam Bassett, Dale Kelley College of Veterinary Medicine, Oklahoma State University, Stillwater, OK, USA Abstract We describe the successful management, resolution, and subsequent return to breeding of a mare with hydroallantois. A 14-year, American Quarter Horse mare in late pregnancy was referred for marked abdominal distention with a history of treatment for pla- centitis. Physical examination findings were consistent with hydroallantois. It was decided to terminate pregnancy. During fetal delivery, dystocia was diagnosed and corrected under field anesthesia; recovery was uneventful. Mare conceived the following sea- son and delivered a healthy filly. Keywords: Hydroallantois, mare, termination of pregnancy Background Hydroallantois is a rare condition characterized by a sudden increase in allantoic fluid volume leading to marked abdomi- nal distention, typically between 6-10 months of pregnancy. It occurs quickly over several days, with few to no notable abnor- malities preceding its onset.1 Initial signs typically include dra- matic increase in abdominal distention, reduced locomotion, difficulty feeding, dyspnea, and obvious discomfort.1,2 The con- dition is considered a serious threat to mare and foal;1 without intervention, it can seriously impact mare’s reproductive future. Diagnosis is made by transrectal palpation of the markedly distended fluid-filled uterus, combined with an inability to visualize the fetus on ultrasonography.3 In some cases, fluid excess of up to 100 liters has been reported.1 Although the fetus is often developmentally normal, survival to term and successful delivery depends on the mare’s ability to support the pregnancy to the point of viability.2-4 Therefore, probabil- ity of a live foal depends mostly on the interval from onset of symptoms to term. Pregnancy termination is often elected to protect the mare’s future reproductive health.1,4,5 Case presentation A 14-year, American Quarter Horse mare was presented on day 301 of pregnancy with abdominal distention (Figures 1 and 2) and a history of treatment for placentitis. Mare had a history of 5 full-term pregnancies; all were conceived via live cover and with no notable complications for the duration of pregnancy. The first 4 pregnancies resulted in live foals. The fifth was stillborn; it had an intact amniotic sac and visu- ally normal fetal membranes. All deliveries were unassisted. Mare was reportedly up to date on vaccinations (including Pneumabort) and had a Caslicks on presentation. Mare was initially examined by the referring veterinarian on day 284 of pregnancy, after owner noticed premature lacta- tion. Mare received twice daily oral trimethoprim-sulfame- thoxazole (30 mg/kg; Amneal Pharmaceutical Bridgewater, NJ), metronidazole (20 mg/kg; UniChem Pharmaceuticals Inc, New Brunswick, NJ), and once daily oral firocoxib (0.1 mg/kg; Equioxx®, Boehringer Ingelheim, St. Joseph, MO) at home the night before. Referring veterinarian felt a slightly thickened placenta with a combined uterus and placenta mea- surement of 15 mm. Medications were continued, and once daily oral altrenogest (0.044 mg/kg; Regu-Mate®, Merck Animal Health, Intervet Canada, Kirkland, QC, Canada) was added. Mare was reexamined by the referring veterinarian on day 300; size of mare’s abdomen had increased dramatically, with notable distention. Placenta was within normal limits, but due to the change in fluid volume a decision was made to refer. All medications were continued until arrival at the hospital. On physical examination, mare was bright and alert. Transrectal palpation and ultrasonography revealed the uterus to be above the pelvic brim with allantoic fluid at a greater volume than normal. Fetus could not be visualized with *Poster presented at the Society for Theriogenology 2023 conference; abstract (https://doi.org/10.58292/ct.v15.10001) updated to reflect additional medical records and case report represents the newest and most complete information. mailto:kristyn.burton@okstate.edu http://dx.doi.org/10.58292/CT.v17.12176 https://doi.org/10.58292/ct.v15.10001 2 Citation: Clinical Theriogenology 2025, 17, 12176, http://dx.doi.org/10.58292/CT.v17.12176 ultrasonography. Due to the length of pregnancy remaining and concern over mare’s future fertility, it was decided to ter- minate the pregnancy. Bloodwork on admission had creatinine kinase and calcium values within reference range, indicating the immediate lower risk of prepubic tendon rupture. Because the mare was pre- sented on a Friday afternoon, mare was kept for 2 days prior to induction to have full staff availability during the procedure. Mare was monitored closely during this time; condition remained stable for the duration. Treatment An intravenous catheter was placed; 1 liter of lactated Ringer’s solution (LRS, Dechra Veterinary Products, Overland Park, KS) was given, and heart rate was monitored throughout the proce- dure. Caslicks was removed and a sterile 21-inch CH18 Foley catheter was inserted through the cervix into allantois; over a period of 2½ hours a total volume of 45.4 liters of allantoic fluid was drained (Figure 3). After drainage, parturition was induced with 60 units of intravenous oxytocin (Bimeda-MTC Animal Health, Cambridge, ON, Canada) in 1 liter of 0.9% NaCl solu- tion (Dechra) given over 60 minutes. At 45 minutes fetal front limbs appeared in the birth canal; 10 minutes later hind limbs were felt near the right shoulder, indicating malposture. Palpation revealed fetus in anterior presentation, with the left hind limb extended and the right hind limb crossing over the left hindlimb; both were extended beneath the fetus (‘dogsitting’ posture). Intravenous detomidine (0.02 mg/kg; Dormosedan®, Zoetis, Parsipanny, NJ) and ketamine (1.5 mg/kg; Ketaset®, Zoetis) were given to anesthetize the mare, along with 240 mg buscopan (0.35 mg/kg; Boehringer Ingelheim) to facilitate repositioning and delivery of the fetus; foal was stillborn (4-6 weeks prema- ture). Mare was given an additional liter of intravenous LRS during recovery, followed by intravenous flunixin (1.1 mg/kg; Merck) and oral sulfathiamethoxine (2 mg/kg) ~ 2 hours after procedure. Mare did not experience hypovolemic shock and expelled fetal membranes 3 hours after recovery. Outcome Mare recovered uneventfully; rest for at least 2 estrous cycles and a breeding soundness examination prior to breeding were rec- ommended. Referring veterinarian observed (transrectal ultraso- nography) residual intrauterine fluid 2 weeks after discharge and antibiotic treatment was extended. Mare’s uterus was normal at the next appointment. Owner made a decision not to breed the mare until the next season due to an unrelated tendon injury. The following year, mare was bred via artificial insemination on a natural cycle, and the following spring delivered a healthy, Figure 1. Marked distention of the abdomen (lateral view) at presentation Figure 2. Marked distention of the abdomen (frontal view) at presentation Figure 3. Draining allantoic fluid http://dx.doi.org/10.58292/CT.v17.12176 Citation: Clinical Theriogenology 2025, 17, 12176, http://dx.doi.org/10.58292/CT.v16.12176 3 live foal at full-term (Figure 4) after a medically normal and uncomplicated pregnancy. Discussion The full pathophysiology of hydroallantois remains poorly understood, although it is suspected to occur in cases of com- promised placental function.2,4,5 However, placentitis is not predictive of hydrops, as there are more common etiologies.6 Therefore, although placentitis often precedes a diagnosis of hydroallantois, identification of placentitis is not commonly the point at which hydroallantois is selected as a primary dif- ferential. Contributing genetic and epigenetic factors have also been suggested.4 Although findings of concurrent placen- titis are not consistent,2 suspected placentitis and subsequent treatment were present in this mare’s recent medical history. At a rate of 10:1,1 hydroallantois is the more common form of hydrops in the mare compared to hydramnios. Diagnostically, it may be differentiated by a more rapid rate of abdominal distention (typically in days), substantially higher volume of fluid (sometimes in excess of 100 liters), and increased inabil- ity to visualize the fetus on transabdominal and transrectal ultrasonography (although possible in both conditions). Gross fetal deformities of the face and head are characteristic of hydramnios, and although fluid volumes of up to 50 liters have been reported, lesser amounts are more common and tend to accumulate over a longer interval.3 It is understood to be a disorder of fluid dynamics due to fetal inability to cycle amniotic fluid, compared to hydroallantois which is a primary dysfunction of the placenta and movement of allantoic fluid.1 Although there are reports of hydrops pregnancies resulting in live foals,2,4 most cases resulted in foals that were either still- born or euthanized soon after parturition.4,5 Furthermore, dys- maturity and severe fetal abnormalities were identified;7 foals born alive received substantial medical support and interven- tion.4 In contrast, prognosis for the mare is generally good with prompt and appropriate intervention. Unattended hydroallantois can quickly lead to herniation of the ventral abdominal musculature, prepubic tendon rupture,8 or even- tual uterine rupture.1 Given poor prognosis to mare and fetus, termination of pregnancy is commonly elected. The single most important factor in fetal viability appears to be the interval remaining to term when signs develop.1 Overall prognosis for live delivery and survival of the foal is guarded to poor,2 and generally limited to mares that present after 330 days of pregnancy. These cases typically represent relatively low fluid volumes and often involve controlled partial drain- age of the allantoic fluid.1,2 Regardless of desired outcome or stage of pregnancy, a controlled delivery is important, as hypo- volemic shock is well documented in mares after expulsion of fetal membranes.1 Given the early stage of pregnancy and large volume of fluid in the present case, the likelihood of maintaining pregnancy to term was determined to be neither probable nor rewarding. In this case, fetal malposture complicated delivery of the fetus, necessitating anesthesia of the mare and manual reposition- ing of the fetus; thereafter, delivery and recovery were unevent- ful. Fluids were given and an intravenous port was maintained for the procedure duration. Similar to this case, a majority of mares conceived and had nor- mal pregnancy after hydroallantois.1-5,7 Although the mare was deemed sound for breeding later in the same season, breeding was delayed for treatment of an incidental tendon sheath injury. Learning points • Hydroallantois is a rare but serious complication of equine pregnancy. Although full pathogenesis is unknown, it is closely correlated to placental dysfunction, and often iden- tified in cases that have been treated for placentitis. • Quick and effective intervention carries a favorable progno- sis for the mare and full return to breeding soundness is expected. • Prognosis for fetal viability is guarded to poor, heavily dependent on case presentation, and attempts to manage a hydrops pregnancy carry significant risks for the mare. Acknowledgement Authors thank Trisha Martin for her enthusiastic cooperation and willingness to provide information regarding the long- term outcome of this case. Conflict of interest The authors report no conflict of interest. References 1. Waelchli RO: Hydrops. In: McKinnon AO, Squires EL, Vaala WE, et al: editors. Equine Reproduction. 2nd edition, Ames; Wiley- Blackwell: 2011. p. 2368-2372. 2. Mitchell ARM, Delvescovo B, Tse M, et al: Successful management of hydrallantois in a Standardbred mare at term resulting in the birth of a live foal. Can Vet J 2019;60:495-501. PMC646 3767 3. Wolfsdorf K: Placental hydrops. Hagyard Equine Medical Institute. Available from: http://www.hagyard.com/placental-hydrops 2018 [cited 14 July 2023]. 4. Bocci C, Sgorbini M, Panzani D, et al: Impact of managing presumptive fetal membrane hydrops in a mare on fetal livability. Figure 4. Filly delivered after uneventful pregnancy the following year http://dx.doi.org/10.58292/CT.v16.12176 http://www.hagyard.com/placental-hydrops 4 Citation: Clinical Theriogenology 2025, 17, 12176, http://dx.doi.org/10.58292/CT.v17.12176 Am J Anim Vet Sci 2023;18:131-138. doi: 10.3844/ajavsp.2023. 131.138 5. Govaere JLJ, De Schauwer C, Hoogewijs MK, et al: Hydrallantois in the mare – a report of five cases. Reprod Domest Anim 2014;48:e1-e6. doi: 10.1111/j.1439-0531.2012.02013.x 6. Johnson AK: Equine placentitis: diagnosis and treatment options. Proc Auburn University CVM Annual Conference. 2023. 7. Arroyo E, Whitelock LM, Stanton ME, et al: Hydroallantois in a mare associated with schistosomus and unilateral ovarian agene- sis in the fetus. J Equine Vet Sci 2025;144:105228. doi: 10.1016/j. jevs.2024.105228 8. Morrison MJW, Back B, McClure JT: Hydroallantois and prepubic tendon rupture in a Standardbred mare. 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