Early Embryonic Loss
Basic Information 
Definition
The failure to maintain pregnancy between fertilization and day 40 of gestation. This stage corresponds to the time of transition from the embryonic to the fetal stage of conceptus development.
Epidemiology
Species, Age, Sex
• EEL is usually seen in very young yearling mares or older mares in a range of 2.5% to 24%, with the highest incidence seen in mares older than 18 years of age.
• Younger mares usually experience EEL between days 15 and 35 of gestation.
• Older mares experience much higher embryonic losses during both oviductal (days 0–6) and the early uterine period (days 6–16).
• The effect of age on fertility usually originates in the oocyte, and embryos from the oviducts of especially older mares have shown to have significantly fewer cells and poor morphology.
Clinical Presentation
Physical Exam Findings
• Transrectal palpation results in failure to detect a chorionic vesicle at the base of either horn at or after 28 days of gestation. There may be a loss or decrease in uterine tone.
• Transrectal ultrasonography may reveal a sudden disappearance of the embryonic vesicle between examinations without evidence of premonitory signs. Other findings include signs of impending embryo loss such as an irregularly shaped embryonic vesicle, prolonged embryonic mobility (beyond day 16), signs of estrus such as excessive endometrial edema despite presence of the embryo, underdeveloped or small for their age embryos, absence of embryonic heartbeat, dislodgement of the vesicle with loss of fluid, increased echogenicity of fluid within the conceptus, or abnormal development of embryonic membranes.
Etiology and Pathophysiology
Contagious equine metritis caused by Taylorella equigenitalis may cause embryonic loss after initial infection (see “Contagious Equine Metritis” in this section).
Bacterial endometritis may be caused by several species of bac teria, the most important being Streptococcus equi subsp. zooepidemicus. Others include Escherichia coli, Klebsiella pneumoniae (capsule 1, 2, and 5), Pseudomonas aeruginosa, and Bacteroides fragilis (especially in postpartum and foal heat mares).
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