This is a very rare condition in which there is failure of the kidney’s ability to concentrate urine caused by lack of a hormone (Anti-Diuretic Hormone – ADH, or vasopressin).
Loss of kidney concentrating ability happens either because of a lack of Anti-diuretic Hormone (ADH) or by the kidney’s insensitivity to ADH. ADH is normally produced in the hypothalamus and stored in the pituitary gland. If these areas of the brain are damaged, then it may not be produced. This is extremely rare in the horse.
ADH normally causes the concentrating parts of the kidney to take up water when the body’s hydration reaches a certain level. If ADH is not produced by the body, or the kidney cells cannot react to it normally for some reason, then excessive water is just lost in the urine. Horses with this condition must drink large quantities of water (polydipsia) to keep pace with the losses, and they urinate frequently (polyuria). This condition is not related to other types of diabetes. These conditions just share the name, which relates to the excessive urination seen in both conditions.
ADH cannot be measured readily in the horse. The diagnosis requires ruling out of other conditions that result in the inability to concentrate urine and retain water. This is done using a water deprivation test.
The treatment is injection of ADH (vasopressin). This is generally impractical in the horse and so the prognosis is poor.
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