THERIOGENOLOGY


and colleagues42 reported foal-heat pregnancy rates of 50% and 20% for Thoroughbred mares that passed their placentas in less than 3 hours and greater than 3 hours, respectively. Consequently, it is recommended that early prompt treatment be initiated to remove retained fetal membranes both for the health of the mare and to optimize her repro- ductive potential.43,44


Excessive Lochia


The term, lochia, refers to placental fluid, inflamma- tory cells, and debris remaining in the uterus after foaling. Mares normally pass a small amount of turbid red-brown non-malodorous lochia fluid for several days after foaling. A small amount of fluid may be observed in the uterine lumen during ultra- sound examination for the first 3 to 6 days after foaling, after which the fluid volume should reduce significantly.9,38 The presence of a small volume uterine fluid during the first post-partum estrus was not associated with a reduction in pregnancy rate in two studies.45,46 There is no clinical need to treat a mare for the presence of a normal amount of lochia discharge early in the post-partum period. However, a pro- longed duration of lochial discharge, an abnormal odor or character to the discharge, or an increased volume or echogenicity of lochia within the uterus visible on ultrasound examination beyond 5 to 7 days after foaling are all abnormal and may de- crease fertility or increase embryonic loss in the early post-partum period.6,27,47–49 In these circum- stances, a therapeutic uterine lavage along with ad- ministration of oxytocin should be considered.46 Exercise may also be helpful for uterine involution and evacuation of uterine fluid in post-partum mares.33,50–52


Uterine Involution


The term, “involution”, has been used to describe the restoration of the uterus to a pre-gravid state, and includes expulsion of uterine fluid and debris, a re- duction in uterine size, and regeneration of the en- dometrial lining of the uterus.51,52 In a study evaluating uterine involution, biopsy of the uterus the day after foaling revealed distinct microcar- uncles, distended endometrial glands and marked edema.53 Re-evaluation 7 days after foaling showed that microcaruncles were no longer present, the luminal epithelium was intact, and the endome- trial glands were no longer distended, indicating that uterine involution was complete. The previously gravid horn remains larger than


the contralateral horn for approximately 3 weeks after foaling.9,47 Histologically, the endometrium returns to normal by 14 days postfoaling.8,53 The endometrium of the mare sustains only limited dam- age as a consequence of pregnancy, foaling, and pas- sage of the placenta. This has been attributed to the non-invasive diffuse epitheliochorial placenta- tion of the horse.54


140 2020  Vol. 66  AAEP PROCEEDINGS Culture and Cytology


Culture of the uterus of post-partum mares com- monly results in growth of Streptococcus equi subsp. zooepidemicus and/or Escherichia coli.5,6,53,55–57 In some reports, a decrease in foal-heat pregnancy rate was noted in mares with a positive uterine culture,56,58 whereas other studies did not note a difference in pregnancy rates based on the presence or absence of bacterial growth.5,6 Mares with a positive culture on their foal heat typically have a negative culture on the subsequent heat.55,57 Ulti- mately, there is no clear relationship between re- sults of culture and pregnancy rates in mares bred on the foal heat. Post-partum mares also exhibit considerable vari-


ability in the amount of inflammation present in the uterus as determined by the number of inflamma- tory cells in cytologic samples.5,6,24,59 The number of neutrophils in uterine cytology samples was re- ported to increase from day 2 postpartum to day 5, but pregnancy rates after foal-heat breeding were not different between mares with neutrophils and mares without neutrophils on uterine swabs.5 A second study noted that the number of neutrophils and amount of cellular necrosis decreased from par- turition to the onset of the foal heat.24 A third study revealed that 58% of post-partum mares had a significant degree of inflammation present at day 7 and yet 73% of mares with significant inflammation became pregnant after a foal-heat breeding.6 In summary, there is not a clear relationship between the presence or absence of inflammatory cells in a sample collected during the foal heat and the poten- tial for generating a pregnancy at a foal-heat breeding. The post-partum period is the only period in the


life of a mare in which the presence of bacteria (i.e., Streptococcus sp. and E. coli) on culture and white blood cells on cytology may have limited clinical relevance. If a stallion owner requires a “clean cul- ture” prior to a live cover, it is anticipated that many foal-heat mares will be disqualified. If present, the bacteria and inflammation will usually be elimi- nated during and/or after the foal heat and a subse- quent culture should be negative and cytology clean on the 30-day heat. The author’s current recommendation is that de-


cisions regarding foal-heat breeding should not be based solely on endometrial cytology or culture sam- ples collected early in the post-partum period.


Cervical Function


Speculum examination of the vagina early in the post-partum period may reveal a reddened or hyper- emic cervix, and a cloudy discharge. The cervix of a post-partum mare remains relaxed and open until after the first ovulation, whether that is on the foal heat or a subsequent heat.9 Closure of the cervix is due to an elevation in serum progesterone levels that occurs after the first post-partum ovulation.52 Lacerations or bruising of the vulva, vestibule,


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