The umbilical cord contains two arteries, and a vein, that are responsible for transferring nutrients and oxygen and deoxygenated blood via the bloodstream between the mare to the foal. The cord also contains the urachus, a thin-walled tube connecting the foetal bladder to the sac around the foal, known as the allantois. The urachus allows foetal waste, in the form of urine, to be expelled from the foetus into the fluid of the allantois, which is the outer of the two sacks of fluid that the foal is suspended in.
During normal parturition, the umbilical cord spontaneously ruptures shortly after delivery, sealing this opening between the foal and the outside environment. Once the umbilicus closes, it quickly becomes shrivelled, remaining as a dried-out structure on the lower abdomen for several weeks before dropping off. When this process doesn’t occur correctly, or there are complicating factors that interfere with it, the foal is at risk of developing serious infections, some that can be severely debilitating and some that can be fatal.
Complications include the incomplete closure of the urachus, allowing urine to continue to leak from the cord; excessive bleeding from one or more of the blood vessels in the cord; and systemic infections and local trauma that compromise the health of the umbilicus. Frequently, the result of any impediment to the normal rupture and subsequent closure of the umbilical cord is infection.
A “patent urachus” is the medical term given to the incomplete closure of the urachus, allowing urine to escape from the bladder and leak out through the umbilical stump. The urachus connects directly to the apex of the bladder, and failure of this structure to close and seal results in either the continuous dripping/dribbling of urine from the umbilical stump, or the foal posturing and passing a stream of urine from the umbilicus. The owners are alerted to a problem when they see a wet area on the abdomen, or they see fluid dripping or being urinated from the umbilicus.
Patent urachus can occur as a primary condition (sometimes referred to as a persistent urachus), or it can be acquired in the days to weeks after foaling following excessive straining, prolonged periods of recumbency, or occasionally following increased abdominal pressure applied in the vicinity of the bladder. Excessive straining can occur in foals with retained meconium or those that have been administered multiple enemas, causing an irritation to the rectum that induces straining. Prolonged lying on the ground is thought to increase the internal abdominal pressures, forcing urine out through the urachus, but it could be seen more frequently in foals that are recumbent for long periods, as these are often the foals that have systemic infections or compromised immune systems and so are predisposed. Increasing intra-abdominal pressures can occur when foals are lifted from the ground by people holding them tightly around the abdomen or by using slings.