Risk Factors for the Development of Haemorrhagic Anovulatory Follicles in the Mare

J Cuervo‐Arango,Jr Newcombe

doi:10.1111/j.1439-0531.2008.01260.x

Abstract

Haemorrhage into the dominant follicle during the reproductive season is a subtle but definitive cause of infertility in the mare population. This condition however can be of high relevance for an individual in which its incidence is abnormally high. Little is known about the nature and factors affecting the incidence of haemorrhagic anovulatory follicles (HAFs) in the mare. The objectives of the study were to define and characterize the ultrasonographic development and incidence of HAFs and to investigate possible risk factors influencing its occurrence. Detailed reproductive and ultrasound records of seven mares studied during their entire reproductive lives (>10 years and 612 oestrous cycles) were analysed retrospectively and computed into a statistical mixed model. Of all animal studied, two mares were found to have an unusually high incidence of HAFs of approximately 25%. Time of season and use of induction treatments (Cloprostenol) were found to influence its incidence. It appears that early-enhanced stimulatory effect of LH on an ovary with the presence of small and immature follicles might increase the risk of ovulatory failure of those follicles later in the cycle. Mares during the months of highest follicular activity (May to August) and after treatment with hormones to induce oestrus and ovulation are at greater risk to develop HAFs. The potential relevance of this study is two folds: clinical relevance for the practitioner to better understand this condition and so improve reproductive management of mares with abnormally high incidence; and to provide useful insights for researchers willing to further investigate the nature of this phenomenon.

Highlights

Failure to ovulate the dominant follicle and subsequent persistence of the anovulatory unruptured structure has been reported to occur in several domestic animal species and in women
A distinct cause of ovulatory failure observed in the mare is haemorrhage of the dominant follicle(s) with subsequent organization of follicular contents and, in most occasions, luteinization of follicular wall without previous follicular collapse
It has been proposed that the incidence is higher during the autumn months and in mares aged > 20 years (Ginther et al 2007). To better understand this phenomenon in the equine species, this study focuses on risk factors affecting the development of haemorrhagic anovulatory follicles

Summary

Introduction

Failure to ovulate the dominant follicle and subsequent persistence of the anovulatory unruptured structure has been reported to occur in several domestic animal species and in women. Research studies on methods of contraception and ovulatory process have shown experimentally-induced unruptured follicles in rabbits (Salhab et al 2003; Grinwich et al 1972), rats (Armstrong and Grinwich 1972) and women (Killick and Elsein 1987). It seems that this phenomenon might occur naturally in all domestic species, only in species like the cow and the mare in which follicular dynamics are and routinely followed ultrasonographically, allow sufficient number of observations to characterize this syndrome. This condition has been referred to in different ways: first reference to what seemed to be the same sort of follicle was back in the 40’s (Burkhardt 1948) where it was described as occurrence of large persistent follicles during the months of October to November

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