By Anne Kahler, Imelda M. McGonnell, Harriette Smart, Alycia A. Kowalski, Ken C. Smith, D. Claire Wathes, Amanda M. de Mestre

Early pregnancy loss (EPL) occurs in approximately 8% of equine pregnancies, although the aetiology is mostly unknown and embryonic/fetal morphological abnormalities associated with EPL are not defined.

The objective of this study is to compare the morphology of EPL to clinically normal embryos/fetuses and previously described embryonic/fetal developmental milestones. To identify morphological abnormalities associated with equine EPL.

Abstract/background

• Study design: The study design included an observational case-control study
• Methods: Embryos/fetuses were obtained from clinically normal Thoroughbred and pony pregnancies (n = 11) and following EPL from Thoroughbred mares (n = 27). The crown-rump length (CRL) of embryos/fetuses was measured and macroscopic morphology and developmental age were determined independently by three blinded examiners. Sagittal sections of EPL (n = 13) and control (n = 6) embryos/fetuses were assessed microscopically. Fisher’s exact test was used to determine significance (P < .05) and correlations were expressed by Pearson coefficient.
• Results :Age and CRL were strongly positively correlated in clinically normal Thoroughbred and reference (n = 15, R = .9 (95% CI: 0.8-1.0), R2 = .9, P < .0001) but not EPL embryos/fetuses (n = 19, R = .1 (95% CI: −0.4 to 0.5), R2 = .01, P = .75). Relative to controls, the CRL of EPL embryos/fetuses was smaller, with evidence of intrauterine growth retardation (IUGR) in 3/8 fetuses assessed. In 9/13 EPL embryos/fetuses, nonspecific neural tissue alterations were identified including disruption of developing pros-, mes- and rhombencephalon and the presence of haemosiderin, indicating premortem haemorrhage. Failed neural tube closure was identified in 1/13 EPL embryos/fetuses. Subcutaneous haemorrhage was present in 14/27 EPL embryos/fetuses. • Main limitations: Autolysis significantly affected 15/27 EPL embryos/fetuses, excluding them from complete assessment. The IUGR reference cut-off values were based on a small number of controls. • Conclusions: Morphological features associated with equine EPL were a mismatch between embryonic/fetal size and age, and alterations of the developing neural tissue and localised subcutaneous haemorrhage. Failed neural tube closure was confirmed as a rare specific abnormality.

1. Introduction

Pregnancy loss is of significant importance in equine reproductive medicine and is associated with animal welfare(1) and economic implications.(2) In mares, the majority of pregnancy failure occurs between the initial diagnosis and day 65 of gestation and is commonly referred to as ‘early pregnancy loss’ (EPL). In the United Kingdom, EPL was recently described to have an incidence of 6.4% between days 14 and 42 and of 1.6% between days 43 and 65 of pregnancy,(3) comparable to observations in distinct broodmare populations worldwide over the last 20 years.(4-8) Multivariable and epidemiological studies have identified several risk factors for EPL,(4-6,9) although the aetiology remains unknown in >80% of mares suffering this condition.(10,11).. To read the complete article you need to be a subscriber

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