Renal failure is a common finding in older ferrets due to the kidneys lack of significant regenerative capacity. As all animals age, including humans, as kidney tissue wears out, it is not replaced by more kidney tissue, but by fibrous connective tissue, which doesn't do anything (just takes up space, like a scar.) Over a lifetime, we continually lose renal tissue, and our kidneys lose ability to perform necessary tasks, such as to filter waste into the urine, and reclaim water as necessary to maintain proper hydration. As we get older, this is manifested by having to urinate more frequently, etc. This is normal.

Chronic renal failure occurs when the process progresses to the point where there is so little functioning renal tissue that the substances which normally are excreted by the kidney, are no longer excreted, and build up into the blood. Because renal tissue is replaced by scar tissue, there is no hope of recovery. Early in the process, remaining kidney tissue is able to undergo hypertrophy and function at a higher level, but eventually, even this hyperfunctional tissue is lost. When 90% of renal tissue is lost, fatal chronic renal failure occurs. (For those of you who remember this particular subject discussed in October, you may notice that the amount of tissue that can be lost has risen from 75% to 90%. This change is the result of an excellent lecture I recently attended at the Veterinary Pathologist's conference earlier this month, which actually now factors in the effect of nephron hypertrophy in chronic renal failure.)

How do you measure renal failure? In general, by measuring certain blood and urine parameters which only go awry once critical renal mass is lost. After around 66% of the kidneys are lost, you lose the ability to concentrate urine. Measurements of the specific gravity of the kidneys will be low and the animal's frequency of urination will increase. However, very few of us watch closely enough to pick up the early signs. After 90% of the kidney is lost, certain substances which are normally excreted by functional kidneys tend to back up and are present in increased levels in the blood - urea, creatinine, and phosphorus - which can be picked up by routine blood tests. As the level of blood urea nitrogen (BUN) increases, the animal will start to feel ill and may go off food. Intractable pain has not been described by human patients in renal failure, but some non-specific aches (like we get with the flu) may occur. This is probably the first obvious sign of renal failure that is seen by owners. Severely increased BUN may result in an ammonia smell to the breath and the presence of mouth ulcers (which are far more rare in ferrets than in dogs or cats with renal failure.)

There really is no cure for chronic renal failure, as that tissue will never be replaced, only palliative treatment designed to decrease levels of toxic substances in the blood and thus clinical signs. When the levels are high, large amounts of intravenous or subcutaneous fluids will help to flush out some of these substances and decrease the levels. They will of course, build up again over time, with animals with less functional renal tissue having faster increases.

Common wisdom in veterinary medicine suggests that lower protein diets are renal-sparing, and we generally reduce protein content markedly in the diets of animals with renal failure. As a precaution in cats and ferrets, two species who suffer inordinately high rates of renal failure as a species, it is probably a good idea to decrease feed protein at the times when renal changes are first seen microscopically - about 3.5 years in ferrets. At or around that age, I personally change my ferrets over to adult cat maintenance feeds and Totally Ferret geriatric food.