In horses, like other mammals, urination is usually linked to a full bladder. An exception to this occurs during courtship and mating, when both sexes may deliver small squirts of urine.
However, a slow dribble of urine out of the vulva or prepuce is reason for concern, as it may indicate a serious medical condition or injury.
One of the common causes of urine leakage is neurological damage: it may result from a disease of, or injury to, the central nervous system, the spinal cord, or even the base of the tail.
Equine herpes virus (EHV)-1 infection, for example, can affect the brain, causing the horse to lose balance or even become partially paralysed. A horse like this may be able to urinate normally, but urine will also dribble out as it moves.
Mycotoxins in feed can also damage the central nervous system.
Accidents are a common cause of damage to the spinal cord. A horse may rear up and fall over backwards, breaking its tail or pelvis, or fracturing a vertebra, or it may slip on tar or concrete and twist its spine.
Injuries to local nerves, such as the hypogastric branch and pudendal nerve, can have an effect on the bladder or urethral sphincter. If nerves are damaged, the horse will have no bladder control and be unable to urinate normally. Instead, the bladder will overfill and urine will leak out constantly.
In stallions or geldings, nerve damage may lead to penile paralysis, with the horse being unable to retract its penis into its sheath after urination.
All of the foregoing can lead to secondary conditions such as cystitis, due to the multiplication of bacteria or yeast build-up in the urine. Over months, the retention of urine can result in a cheese-coloured sediment inside the bladder.
Bacterial cystitis can also be due to a primary infection without any nerve involvement. This can follow infections carried over by a stallion during mating or chronic inflammation of the vagina due to poor conformation of the vulva.
Damage to the vagina and bladder may also occur during foaling. Bladder and kidney stones are relatively rare in horses, but have been described.
Diagnosis & treatment
You will notice urine stains below the vulva and on the back legs and tail of a mare. In a gelding or stallion, dribbling urine may cause burn marks and swelling of the sheath.
The urine may be clear, milky, bloody or purulent, and might have a strong or unpleasant smell.
If you think your horse has urinary incontinence, it’s essential to consult a vet, preferably an equine specialist, as soon as possible. He or she will perform a neurological evaluation, as well as a rectal examination of the pelvis and bladder. Sonar or endoscopy may also be used.
Urine is usually sent to a laboratory for analysis and swabs may be taken to test for bacterial or yeast infections. A full diagnostic workup is required and blood samples will be taken.
Treatment depends on the cause and is more likely to be successful if action is taken immediately. Longstanding cases may have to be humanely euthanased.
Dr Mac is an academic, a practising equine veterinarian and a stud owner.