References
Understanding dystocia in the field: part 1
Abstract
Difficult births (dystocias) are not uncommon for horses and present significant risks to mare and foal, with potentially fatal consequences. A sound understanding of the normal physiological processes before and during parturition are essential to enable vets to quickly identify when these processes deviate from the norm. To rapidly diagnose the cause of the dystocia, the attending obstetrician should be assured in their approach; resolute in their decision making; and prompt in action if the risks of mare and foal morbidity and mortality are to be managed. Constructive communication with the mare's owners is essential to allow adequate planning to prioritise the outcome. The future fertility of the mare is imperative in many cases yet is often overlooked. The obstetrician must be dynamic and be prepared to alter their approach when necessary. This review outlines the causes of dystocia and the recommended approaches to resolution.
Dystocia in equines can be life-threatening to both dam and foetus, and represents a genuine emergency. Dystocia is reported to affect between 1–13.2% of foalings, with large Draft and pony breeds considered to be at higher risk than other breeds (Ginther and Williams, 1996; Lu et al 2006; Frazer, 2007). Dystocia was found to be less likely in maiden Thoroughbred mares (8.5%) than barren mares (12.7%) or mares that had previously foaled (14.2%) (Squires et al, 2013). Regardless of breed or parity, it is recommended that all equine births are attended so that if manual intervention is required, it can be provided in a timely fashion.
From approximately 2 months into gestation, the equine foetus is highly active (Allen and Bracher, 1992). The characteristically long umbilical cord facilitates free movement within the allantoic cavity between the uterine body and gravid horn, and alterations in foetal polarity between cranial and caudal presentation occur regularly. Between 5–7 months, both horns contract and the foetus is confined to the uterine body. By 7 months, the uterine horns form an acute angle over the dorsal aspect of the markedly enlarged uterine body. The foetus' hindfeet regain access to the gravid uterine horn, which is only possible while in cranial presentation and dorsal recumbency (Ginther, 1998). The hind-feet advance through the horn lumen with continued growth, to reach the horn tip by 10 months. Once the hindlimbs are enclosed within the gravid horn, the foetus is committed to a cranial longitudinal presentation. These uterofoetal interactions ensure that the vast majority (98.9%) of births occur in cranial longitudinal presentation; with only 1.0% in caudal longitudinal presentation and 0.1% in transverse presentation (Vandeplassche, 1987). Significant episodic foetal activity continues to term, which can be vigorous and sometimes clearly visible through the mare's flank. The soft eponychium extending from the pointed foetal hooves is thought to provide some cushioning to protect the uterus from potential injury.
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