Systemic Disease in the Mare
Colic and colic surgery are risk factors for fetal death or premature delivery. Foal survival rates after medical colic are superior to rates after colic surgery, which are reported as between 54% and 80%.70-73 Specific risk factors have varied among studies but have included the duration of anesthesia, intraoperative hypotension, intraoperative hypoxemia, perioperative toxemia, stage of gestation, and age of the mare.
A retrospective study reported that pregnant mares older than 15 years of age undergoing colic surgery were less likely to produce a live foal than mares aged 15 years or younger.72 In the same study mares that had colic surgery at 16 to 39 days of pregnancy were less likely to produce a live foal (49%) than mares that were older than 40 days in foal. The management of mares postoperatively typically includes supplementation with a progestin, although no clear benefit has been reported.71 Likewise, the tocolytic agent clenbuterol hydrochloride has been recommended, but again without significant evidence.In acute colic, the progestagen concentration declines hours to days before abortion. The concentration may fall to less than 2 ng/mL.21
Laminitis is often a chronic condition that is exacerbated in pregnant mares because of added body weight. The condition is associated systemic hypertension, and a recent study of pregnant laminitic mares reported hypertension, shorter gestational periods, and lower placental and foal birth weights than a control group of normal mares.74 The authors reported a number of vascular abnormalities in the placenta, including reduced vascular lumen and capillary area in microcotyledons, and thickened vascular walls.
There have been several retrospective studies of uterine torsion in mares. Survival rates of mares have varied between 60% and 90.5%.75 Survival of the mare and the foal is determined, in part, by gestational age at the time of the torsion, degree of rotation, and method used for correction. Foal survival is also related to the interval from torsion to correction. A multicenter retrospective study reported an overall foal survival rate of 82.3%; positive foal outcomes were highest when torsion occurred before 320 days' gestation (90.6%) and when corrected by standing flank laparotomy (88%).75 The authors suggested that improved outcomes at earlier stages of gestation were likely due to lighter foals exerting less pressure on uterine vessels by crossed broad ligaments. This is coupled with reduced oxygen demands in younger fetuses. Avoidance of general anesthesia and dorsal recumbency must also have comparatively less impact on placental blood flow.