The kidneys have a myriad of functions. The two most important are

1. water/electrolyte balance
2. removal of water soluble toxins and chemicals

The kidneys perform these functions by filtering the blood through tiny tubules. There is considerable reserve in kidney function and a horse can lose up to 2/3rds of their functional kidney mass before the horse shows signs of illness. Once illness occurs, we call this renal failure. Chronic renal failure (CRF) represents a substantial and permanent decrease in the amount of blood filtered by the kidneys. With chronic renal failure usually two abnormalities develop first:

1. an inability to concentrate water
2. a build up of toxins.

More slowly electrolyte imbalance occurs. This article deals with causes, clinical signs, diagnosis, and appropriate management of the horse with CRF. Prognosis is also discussed.

Not a common disease in the horse, the reported incidence of CRF is around one in 400 horses and rises to one in 200 in older stallions and primarily a disease of horses over 15 years old. Since CRF is commonly a consequence of other severe or long term disease like colic, surgery, sepsis, hemolysis, rhabdomyolysis, or drug administration is in the history. Note: the insult may have occurred years before the renal failure is detected.

The first abnormality to become clinically apparent is a decreased ability to concentrate urine. With this decreased ability more urine is formed (polyuria) and to replace it the horse drinks more water (polydipsia). Onset may be slow, and therefore hard to notice. Owners may notice a wetter stall.

As more kidney function is lost, a build up of toxins, primarily urea (BUN) and creatinine, occurs. This is called azotemia. This causes many of the clinical signs first seen: mild depression, decrease in feeding (anorexia), weight loss. Prior to this there may be ventral edema. As the disease advances the poor doing worsens and the horse becomes weaker and depression worsens.

CRF generally represents a chronic inflammatory disease. The inflammation may first be set up as a result of severely decreased blood or oxygen supply to the kidney, exposure to toxic substances, and developmental abnormalities. Blood and oxygen deprivation may occur secondary to shock, hemolysis, and tying up (rhabdomyolysis). The most common toxic substance would be NSAID's (bute). There appear to be immune-mediated forms often associated or initiated by infections like EIA or Strangles. Less common, ascending infectious or bilateral kidney stones may have formed.

Developmental or congenital problems are less well defined and may represent under development, abnormal development, or a particular form where cysts form in the body of the kidneys. CRF in horses less than five years should be suspected for congenital kidney disease. Paints seem predisposed to the polycystic form.