Why Are Mares with Pneumovagina Susceptible to Bacterial Endometritis? A Personal Opinion

Pneumovagina is the abnormal presence of air in the vagina, which may lead to pneumometra and endometritis. Predisposing factors are abnormal perineal conformation and weakness of the vestibulovaginal sphincter. This report examines why ponies and some other breeds are less susceptible to pneumovagina than Thoroughbreds and consequently less susceptible to endometritis. Observations on vaginal ballooning during vaginoscopic examination led to the conclusion that there is a fundamental difference in intra-abdominal pressures between horses and ponies, and between mares with high or low body condition scores. This difference in the extreme cases, for example, a small fat pony versus a large thin Thoroughbred, means that the pressure gradient in the former is from the uterus through the vagina to the exterior, whereas the case is reversed in the latter. Although the uterus is at a negative pressure relative to the exterior, evacuation of fluids is seriously disadvantaged, leading to an inability to resolve uterine infections.