If we hear the word ’diabetes,’ we usually think about blood sugar. However, diabetes really means the excessive production of urine. The word has both Greek and Latin derivations, meaning ”one who straddles” and ”siphon.”
Diabetes mellitus is sugar diabetes, and high blood sugar will make a horse urinate excessively. However, when the kidney doesn’t concentrate the urine normally, the horse could have diabetes insipidus, which occurs either because the brain isn’t producing normal amounts of a hormone called ADH - the antidiuretic hormone also known as vasopressin - or because the kidneys don’t respond to ADH as they should.
The major symptoms of diabetes insipidus are excessive thirst and urine production. The constant loss of large volumes of fluid causes the horse to drink a lot of water. Testing the urine will show the urine is diluted, not concentrated. There’s nothing wrong with a horse drinking lots of water, of course, but you should be aware it could also be a symptom.
Before diagnosing diabetes insipidus, kidney damage and idiopathic polydipsia must be ruled out. Idiopathic polydipsia is a term for horses that simply drink a great deal of water but don’t have anything wrong with them. Kidney damage can be detected by blood tests.
To determine if the urination is the due to liking water or caused by true diabetes insipidus, you need to see what happens if water is withheld. When heavy drinking is habit, withholding water will slow urination and the urine becomes concentrated. If the horse has diabetes insipidus, he’ll produce dilute urine anyway but also become rapidly dehydrated. Because of the risk of dehydration, it’s dangerous to limit the amount of water a horse with true diabetes insipidus drinks.
While your vet can inject the horse with a synthetic ADH to determine if it’s diabetes insipidus or a kidney problem, it's expensive and risky. In addition, the disease is actually rare in horses and the cause is unknown. The wisest course of action is to rule out a kidney problem and then allow the horse to drink water free choice.
Although you’ll have the annoyance of a really wet stall, these horses do fine as long as they have constant access to as much water as they need to make up for the urinary losses.
The main thing to bear in mind is that heavy or prolonged exercise, especially in hot weather, may not be advisable because of an increased risk of dehydration.
The horse may be at an increased risk of developing electrolyte abnormalities, especially if his water is interrupted. However, do not deliberately increase or decrease any electrolyte supplementation. You should simply provide free-choice salt like you normally would. If you do pick up on a sign that the electrolytes may be disturbed, such as a change in energy level, irritability or any muscular symptoms, contact your veterinarian to run a blood sample to check electrolyte levels.