When a mare develops PBIE, several outcomes can occur. The mare may resolve the endometritis on her own, using her innate immunological and physical defences to clear the uterine environment. If unable to clear the endometritis, the inflammation becomes well established and the mare develops chronic endometritis, and this can cause further damage leading to permanent changes in the lining of the uterus. The uterus can also become contaminated with bacteria during these episodes of endometritis and develop an acute infectious endometritis, which if not treated correctly, becomes a chronic infectious endometritis. Mares with chronic infectious endometritis become bad breeders because they are persistently plagued with fluid in their uterus. These mares require intensive treatment and management to become pregnant and carry a pregnancy to term.
INNATE DEFENCE MECHANISMS
When a mare is initially covered, the sperm cells and associated products are recognised as foreign. The body then mounts an immunological response to deal with this foreign intrusion and eliminate any excess sperm, seminal fluid, debris, or bacteria from the uterus. Pregnancy, however, relies on the ability of the uterus to eliminate the sperm and debris from the environment, and then return to a state that is suitable for an embryo to move around until it implants in the uterine wall. The body must recognise the embryo as friendly and not reject it, because half the DNA in the embryo comes from the same sperm that initially induced the immunological response and triggered the process to rid the uterus of foreign material.
The natural immunological and physical mechanisms invoked include uterine contractions that help evacuate fluids and debris from the uterus into the vagina and the mobilisation of immune cell types that defend the uterus. Uterine cells can react and release cells capable of eating up (phagocytosis) sperm and bacteria. These phagocytes, full of unwanted bacteria and sperm pieces, are then removed from the uterus via the lymph circulation. Oestrogens amplify the immune response when the uterus is challenged, and offer better protection to the mare during oestrus when the cervix is open so breeding can occur (oestrogens are higher during oestrus).
A natural microbiome exists in a mare’s uterus and this is thought to add to the protective mechanisms, but the full role it plays in maintaining a healthy uterine environment is still being investigated. This biome changes according to what stage of the oestrus cycle the mare is in and is also influenced by the levels of circulating oestrogens.
Factors that negatively affect the mare’s ability to deal with endometritis include damage to the normal vulval seal, disruption of the vaginal defence structures, incompetent cervix, pendulous uterus and damage or degradation to the lining of the uterus. These are commonly encountered in aged mares and mares that have had multiple foals. Mares that have torn the vulval lips or stretched the vaginal walls are susceptible to infectious endometritis as they are incapable of forming a protective seal that is refractory to bacteria and faeces, allowing these contaminants access to the uterus.
Overly large foals and dystocias (difficult foalings), especially where tools have been used to assist in the deliveries, can damage the cervix and prevent it from closing properly. The cervix is the opening to the uterus and when functioning effectively, is open during oestrus, and closed when the mare is not in season or pregnant to stop any bacteria from ascending from the vagina into the uterus.
Multifarious mares (those that have had many foals) and aged mares can progressively stretch the ligaments that suspend the uterus from the body wall, causing the uterus to sag down below the level of the pelvic brim and vulva, making it difficult for any fluids that have accumulated in the uterus to be expelled. Normal uterine contractions are unable to force the fluid up a gradient as this means they must work against gravity.