IN-DEPTH: CONDITIONS AND MANAGEMENT OF THE PERIPARTUM MARE 2. Twin Pregnancies


Survival of twin equine fetuses to term is limited. Although early pregnancy detection with the ultra- sound scanning and manual twin “crush” by 16 days from ovulation has largely eliminated twin pregnan- cies, very occasionally, twins may be missed and present as a complication in mid-to-late preg- nancy. Several management tools for reducing twins to a singleton pregnancy exist from early pregnancy to approximately 150 days’ gestation, including transvaginal ultrasound–guided twin re- duction, craniocervical dislocation, and transabdom- inal ultrasound–guided cardiac puncture.5 For twin pregnancies that exceed these periods of gesta- tion, options are limited and termination of the preg- nancy is generally always recommended. If twins survive to late gestation, the likelihood of both or even one surviving and proving viable is relatively unusual, especially if an equine athlete is required. In a study of 124 sets of twins, only 35.5% of mares delivered live single or twin foals, of which only 18 foals lived more than 2 weeks.6 Severe growth re- striction and often prematurity are very likely to result in small, weak individuals that initially strug- gle to survive and may suffer significant complica- tions. Including failure of ossification of the small cuboidal bones in the knees and hocks especially, that will impact negatively on future athletic ability. The mare is also at much higher risk of dystocia and subsequent reduced fertility during the following breeding season. Historically there have been very few reports of successful maintenance of twin pregnancies with one surviving twin and one mummified fetus.7–9 Despite their small size at birth, surviving foals were delivered alive and apparently thrived. This scenario is a rare outcome that requires vigilant monitoring of pregnancy and parturition, and exten- sive client education regarding the risks and costs involved for both the mare and foal. In the event of two live twins that continue to grow into late gesta- tion, the risks of a catastrophic ending are substan- tial and pregnancy termination is recommended.


3. Umbilical and Placental Conditions That Affect Late Pregnancy


Umbilical cord torsion is most likely a sudden, rapid event, without any premonitory signs, and fre- quently results in abortion. There is some evidence that Doppler ultrasonography is of value for assess- ing umbilical torsion in human babies and therefore might be useful for screening mares.10,11 Few stud- ies, to date, confirm that this technique has suffi- cient sensitivity or specificity to be of value in detecting umbilical cord torsion in equine fetuses. There are currently ongoing epidemiological studies attempting to better understand the etiology of cord torsion and identify potential risk factors.12 Pres- ently, pregnant mares with suspected risk for um- bilical cord torsion undergo empirical therapeutic practices that are aimed at maintenance of preg-


150 2019  Vol. 65  AAEP PROCEEDINGS


nancy. Individual mares may be enrolled on a monitoring program if they have had more than one loss, but these are likely to concentrate on CTUP and other fetal biophysical parameters, but are not usually targeted specifically at reducing the risk of cord torsion. Research studies have shown that placental pro- gesterone production releases neuroactive steroids into the fetal circulation.13 These are responsible for maintaining a sleep-like behavioral state in the fetus during pregnancy and there has been some speculation that altrenogest treatment may have a similar effect and might lessen the risk of cord tor- sion by reducing fetal mobility. This premise has been suggested as a reason for differences in inci- dence of umbilical cord torsion between the United Kingdom and North America. Although the effect of severe cord twisting is obvi-


ous, the consequences of milder or intermittent cord twisting is unknown in horses, but is recognized in human pregnancies.14 Constricted blood flow may lead to production of thrombi within the umbilical vessels which can be a factor in the development of ischemic necrosis of the placenta. Aspirin (acetylsal- icylic acid) inhibits platelet aggregation via inhibition


of platelet thromboxane A2 synthesis. Low-dose aspirin therapy is used in pregnant women for the treatment of pre-eclampsia to improve uterine and placental perfusion and preventing microthrombi for- mation.15 Arecent study utilizing Doppler ultrasono- graphic measurement of blood flow in the uterine arteries of mares showed that uterine perfusion was improved by twice-daily administration of5gof oral aspirin.16 These findings suggest prophylactic aspi- rin therapy might reduce the risk of thrombi forma- tion, causing cord torsion and ischemic necrosis of the placenta.


Pentoxifylline also shows promise for improving uterine perfusion. A study by Ousey and co- workers17 demonstrated that long-term treatment with pentoxifylline (from 50 to 120 days of gestation) resulted in increased uterine artery blood flow in young pregnant mares. Improved blood flow was documented during treatment and through to the end of pregnancy. Similar effects were not identi- fied in aged mares with endometrosis, a population of mares that would benefit most from increased uterine blood flow. An interesting finding from this study was the increased placental vascular resis- tance after pentoxifylline treatment caused an un- wanted reduction in placental blood flow. Thus a better understanding of what effects pentoxifylline might have on uterine blood flow is needed to deter- mine whether the drug might have harmful effects on the fetus. Additional therapies for equine pregnancies af-


fected by disturbed placental blood flow often mimic those used for mares with bacterial placen- titis due to the similarity in clinical signs. Pla- centitis treatment protocols commonly include combinations of antimicrobials, altrenogest (double


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