EARLY FETAL LOSSES^*

MRLS was first observed in pregnant mares presented for fetal sexing between 60 and 70 days of gestation. Beginning on April 26, 2001, some of these mares had dead fetuses surrounded by allantoic and amniotic fluids containing hyperechoic material. In just a few short weeks the estimated fetal losses reached nearly 3000. These large numbers had never before been reported in central Kentucky or anywhere in the world. The only comparable experience in central Kentucky was in 1980 and 1981, when smaller numbers of early fetal losses, estimated to be 256 in 1980 and 162 in 1981, were seen. Breeding dates and gestational ages were similar in the 1980-1981 and 2001-2002 losses; however, ultrasonography was not available in the early 1980s, so detailed comparisons could not be made. Another similarity was that mares occasionally retained the fetus and placental membranes after abortion, an uncommon finding at this stage of gestation. Interestingly, in the four years in which losses occurred (1980-81, 2001-02), entomologists reported the presence of a greater than normal number of eastern tent caterpillars (ETC). No cause was ever determined for the losses in 1980 and 1981.

Mares experiencing early fetal loss from MRLS typically presented with no outward signs. Occasionally the mare had a serosanguineous or purulent vulvar discharge. Some mares were found with membranes protruding from their vulvas with the fetus located either in the vagina or uterus. A small percentage (estimated at less than 5%) exhibited mild signs of colic, abdominal straining, or low-grade fevers (101° to 101.5°F) 1 to 3 days before early fetal loss.

Often with MRLS losses the mare appeared to be pregnant when examined by rectal palpation. The uterus would have normal to slightly less fluid distention expected for the stage of gestation, and it was only on ultrasonographic examination that the mare was found to have a compromised or dead fetus. Before the appearance of MRLS in 2001, ultrasound examinations were conducted only through 30 to 35 days of gestation to confirm pregnancy in Thoroughbred mares in central Kentucky, unless fetal sexing was requested. Since the MRLS outbreak, mares are scanned through 60 to 90 days of gestation to confirm that the fetus is viable. The typical ultrasonographic appearance of a pregnancy afflicted by MRLS is a dead fetus surrounded by echogenic allantoic and amnionic fluid, with amnionic fluid being more hyperechoic than allantoic fluid.

The majority of losses in the author’s practice (Riddle) occurred between 40 and 80 days of gestation, with a range of 32 to 140 days (Table 56-1). In 2001 these mares were bred between February 10 and April 10, whereas in 2002 breedings ranged from February 15 to April 1. In 2001 losses were identified between April 26 and July 2, whereas losses in 2002 began on April 29 and ended on June 3.

Table 56-1 Gestational Ages Represented

Most losses in 2001 occurred over a 3-week period beginning April 26, whereas losses in 2002 were more evenly distributed over a 5-week period beginning April 29. In 2001 there were 56 losses on the seven farms managed by the practice (Riddle), whereas in 2002, 18 losses occurred on only two of the seven farms. These figures indicate a 68% decrease in MRLS losses from 2001 to 2002. The majority of equine practitioners in central Kentucky experienced a similar decrease.

Pregnancy rates between 15 and 35 days in 2001 and 2002 were not adversely affected by MRLS. In 2001 the status of the mares that aborted in the author’s practice (Riddle) was equally represented with 19 barren mares (34%), 20 maiden mares (36%), and 17 foaling mares (30%) aborting. In 2002, maiden mares represented 56% of the losses (5 barren mares, 28%; 3 foaling mares, 16%). This finding is likely related to a heavy load of ETC in fields on one farm that housed the maiden mares. Of the 56 mares that aborted from MRLS in 2001, 31 remained in the care of the author (Riddle) in 2002. Thirty of these 31 conceived, and 1 mare remained barren. Two of the 30 pregnant mares were again affected by MRLS on their subsequent pregnancy in 2002.

A number of ancillary diagnostic tests were performed on aborting mares in an attempt to identify the cause. Serum progesterone concentrations were measured in 16 mares when fetal death was noted. Concentrations were adequate to maintain pregnancies in these mares (>4 ng/ml). Complete blood counts and blood chemistry values were within normal limits. In 2001 and 2002 uterine cultures were taken on all aborting mares in the author’s practice. Bacteria isolated included alpha and beta streptococcus, Escherichia coli, and Enterobacter cloacae. Uterine cytologic findings within 7 days of abortion from 5 mares in 2001 showed moderate to severe inflammation. This inflammation was consistent with a recent abortion and was not unique to MRLS. Following abortion, all mares received uterine lavages and were treated for 5 days with the appropriate antibiotic based on culture and sensitivity. On subsequent heats uterine cytologic findings were within normal limits, and no bacteria were isolated on uterine culture in most mares. In 2001 uterine biopsies were performed on 10 mares 1 to 3 months after MRLS abortion. Histologic findings were mild, and those present could not be related to MRLS.

Hyperechoic allantoic and amniotic fluids were consistent findings in mares that aborted between 35 and 140 days from MRLS. Before 2001, echogenic fetal fluids were not reported during thousands of ultrasonographic examinations conducted to determine fetal sex between 58 and 75 days of gestation. In 2001 and 2002, hyperechoic fetal fluids were reported in all mares that retained a dead fetus and in many mares with a live fetus. In an attempt to determine the makeup of the hyperechoic fetal fluids, allantoic fluid aspirates were obtained from three mares that retained a dead fetus in utero in 2001. Culture of the fluid grew alpha Streptococcus in two cases and E. coli

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