By Sonja Egan, PhD
The colloquial term ‘dirty mare’ is often used to describe mares suffering from endometritis – a condition that is frequently characterised by the absence of pregnancy. Endometritis is regarded as one of the most common causes of equine infertility1,2. Bacterial uterine infections effect 25-60% of barren mares, causing considerable losses to the breeding industry3, and a substantial headache for breeders.
Hurtgen described in his 2006 review of endometritis treatment and pathogens, that characterisation of uterine infection and inflammation falls under numerous categories; “acute, chronic, active, subclinical, post-partum, bacterial, fungal, viral, mating induced, persist and in many other forms”.(4) The author states that while there may be simplicity in the term ‘infection’, there is clear complexity in determining the origin of this condition, highlighting the difficulty in provision of successful treatment, and management plans.
The normal response to insemination is an acute increase in white blood cells to attack the existing bacteria and residual sperm cells remaining in the uterus. This process occurs approximately six to 12 hours after breeding, where the expulsion of uterine fluid is usually completed within 48 hours; a normal uterus contains little to no fluid.1 Mares who display rapid, efficient clearing are termed ‘resistant’.(3) However, the abnormal response found in breeding-induced endometritis is clinically characterised as the retention of uterine fluid after mating.
Impaired uterine clearance facilitates the multiplication of uterine bacteria, secondary inflammation, and the prevention of pregnancy progression. Mares who have difficulty clearing uterine fluid after covering or foaling are termed ‘susceptible’.(3) The absence of the physical uterine contractile response to clear the fluid is another strong indicator of a susceptible mare.
A 2016 review of mare infertility and endometritis1 described the impaired muscular response in the middle layer of the uterus, responsible for the induction of uterine contractions. The authors stated that in resistant mares there is normal neuromuscular signalling activity which is responsible for stimulating these contractions. However, in susceptible mares this response is delayed by two hours, followed by substantially reduced levels of signalling activity 12 hours after breeding; in some cases this activity was lower than that of pre-breeding levels... To read the complete article you need to be a subscriber
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