A mare affected by hydrallantois or hydramnion will typically present after six months of gestation with a relatively rapid (on the order of days to weeks) onset of increased abdominal size. Normal fluid volume in the allantoic cavity at term is approximately 10-15 liters, while in cases of hydrops allantois this can increase to well over 100 liters. For amniotic fluid, the normal volume is approximately 3-5 liters at term, with volumes exceeding 50 liters reported in some cases of hydramnion.
Hydrallantois is more common in the mare than hydramnion, but diagnosis and management are largely similar between the two conditions. A diagnosis relies upon clinical history, physical examination, transrectal palpation, and transabdominal ultrasound. Palpation per rectum classically reveals a domed, fluid-filled uterus, with the fetus unable to be palpated. When the uterus is evaluated by transabdominal ultrasound, a large volume of fluid dominates the field of view (> 20 centimeters in depth), with the fetus suspended within this fluid. In some cases the fetus cannot be visualized due to the extreme volume of fluid.
There are isolated case reports of delivery of a viable foal, if the hydrops condition develops in late gestation. However, the most common outcome is induced abortion of a nonviable or deceased fetus in order to salvage the mare. Mares affected by hydrallantois or hydramnion are at increased risk for the development of prepubic tendon rupture, body wall herniation, uterine rupture, and hypovolemic shock during abortion/parturition. Due to these risks and the poor prognosis for fetal viability, management of hydrops conditions is generally accomplished through controlled drainage of fetal fluids and fetal extraction with concurrent supportive care of the mare. The client and clinician should anticipate that the mare will likely retain her placenta.
The underlying factors that cause hydrops conditions in the mare are not precisely known, but recent research has identified significant differences in expression of genes influencing vascular development in normal placentas versus placentas from mares affected by hydrops conditions. The fetuses from hydrops cases often display gross congenital abnormalities, which have been hypothesized to contribute to the development of the condition through altered fluid dynamics in the fetus (e.g., swallowing and umbilical blood flow). Zoonotic Leptospira bacteria have been isolated from the fetus and placenta of a small number of hydrops cases. Therefore, personal protective equipment is recommended to be donned when working with hydrops cases. Recurrence of hydrops conditions in subsequent pregnancies is not a recognized risk. In one retrospective analysis of 30 cases of hydrops at a referral hospital, 95% of affected mares produced a normal foal in their subsequent pregnancy. Despite the poor prognosis for fetal survivability, hydrallantois and hydramnion are associated with a good prognosis for a mare’s future broodmare career, if intervention is sought early. Potential heritability of hydrops is a subject of slight controversy, with differing reports found in the literature. A definitive study to answer this question would be ambitious to achieve, given the rarity of hydrops occurrence.
Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 31, Issue 2, funded by Equus Standardbred Station Inc. and M&J Insurance. It was written by Karen Von Dollen, DVM, DS, Dipl. ACT, of Hagyard Equine Medical Institute, in Lexington, Kentucky.