Acute endometritis in a mare
Keywords: endometritis, endometrium, mare, equine, biopsy
An image of the stratum spongiousum in an endometrial biopsy from a mare with acute endometritis. The image shows the presence of neutrophils (examples ringed in red), round cells i.e. lymphocytes or monocytes (examples ringed in yellow) and normal stromal cells (examples ringed in green). Some of the darker and more compact "round cells" could be plasmacytes. The presence of the neutrophils marks this as a case of acute, suppurative endometritis. In some fields, it is possible to see neutrophils migrating out of blood vessels but this is not visible here. Although bacteria are common in endometrial cytology, it is rare to see them in endometrial biopsies; probably because of staining techniques.
A large number of bacteria and fungi can cause endometritis but the most ubiquitous pathogen in mares is Streptococcus equi var (ssp) zooepidemicus often referred to only as Streptococcus zooepidemicus
Even for experienced pathologists, it is often difficult to distinguish between different types of round cells in endometrial biopsies but it is important to note that there is always a "janitorial population" (a normal protective population level) of round cells in the lamina propria, especially in the stratum compactum. There may also be low numbers of plasmacytes in normal mares. The author has arrived at this conclusion after examining large numbers of endometrial biopsies from donation mares with no histories of reproductive pathology. If all these cell types are present in large numbers in the absence of neutrophils, there may indeed be chronic-to-subacute endometritis. However, unless one is familiar with normal endometrial biopsies, caution should be exercised before interpreting low populations of round cells and plasmacytes as being pathological.
