Equine kidney disease is not very common in horses. The kidnies are the organs of the body which filter out toxins from the blood. When they stop working well more water is needed to clean the blood and a horse will need to drink more and urinate more in order to stay healthy.
Most kidney disease in horses is caused by specific infections that move up from the bladder to the kidney.
Kidneys may also be damaged when they have damaged muscles as in Azoturia (Ti-Up). The toxins that are released from damaged muscles may affect the kidneys badly when the are being excreted.