Microscopic examination of endometrial biopsies of retired sports mares: An explanation for the clinically observed subfertility?

• Subfertility of retired performance mares is a clinical problem.
• During the breeding season 50% of these mares show endometrial maldifferentiation.
• Glandular disorders are inactivity (8%) and irregular differentiation (42%).
• Non-performance mares have a much lower incidence of these glandular disorders.
• Glandular dysfunction is a likely cause for subfertility in retired sports mares.

After their retirement from sports, performance mares often show a poor breeding success. The objective of this study was the microscopic evaluation of endometrial biopsies of retired sports mares (n = 189) to search for alterations that may explain subfertility. Mares of this study aged 3–23 years showed endometritis (30%) and endometrosis (77%); mild forms predominated. In regard to those mares biopsied during the breeding season (n = 99), 50% had glandular differentiation disorders, i.e. glandular inactivity (8%) or irregular glandular differentiation (42%). Compared to literature data retrieved from mainly non-performance mares, the sports mares of this study showed a similar prevalence of endometrosis and endometritis, but a much higher prevalence of glandular differentiation disorders. The most common cause of the latter is an ovarian dysfunction. Results of this study indicate an association between glandular maldifferentiation of the endometrium and the clinically observed reduced fertility of retired sports mares.