Chronic renal insufficiency - Causes and Risk Factors
As you know from reading the overview, there must be damage to the nephrons before we see signs of renal insufficiency. The most common cause of damage is a complex syndrome called chronic interstitial nephritis. (That's another $10 term which takes about a $120 textbook to explain.) Suffice it to say that there is inflammation in the nephrons that results in fibrosis or scarring of the kidneys, and the nephrons cannot function anymore. This inflammation may be from infection (a disease called pyelonephritis) but that is actually less common than plain old inflammation. The cause of the inflammation, when it's not infection, is often never known.
Chronic renal insufficiency has been a bit of a mystery disease, and there is active research into a definitive answer as to why so many older cats get it. The prevailing theory is that this is a multifactorial disease, with contributions from:
genetics, breed-related disease (Maine Coon, Abyssinian, Siamese, Russian Blue, Burmese, Balinese)
the trend toward urinary-acidifying cat foods in the last 15 years
gradual depletion of potassium with age
high blood pressure
The vaccine connection:
A very interesting study came out of Colorado State University in 2002. Researchers found that cats vaccinated with commercial fvrcp (feline panleukopenia, calici, and rhinovirus) had detectable levels of anti-renal antibodies. In other words, they had antibodies to their own kidney cells. To explain this, a little lesson in vaccine manufacture is needed.
In order to make vaccines, manufacturers inoculate tissue cultures in bottles with the viruses they need to grow. The virus invades a tissue cell lining the bottle, replicates, leaves the cell and a legion of new viruses invade the neighboring cells. Large numbers of viruses result, and end up in the nutrient fluid sitting on top of the cells. The fluid over the tissue cultures is removed, the viruses are purified and then processed (usually freeze-dried) to make vaccines.
Since the inception of feline vaccines the tissue line used in their manufacture is feline kidney cells. Inevitably in the manufacturing process little bits of the cell membrane and proteins from these kidney cells end up in the vaccine. There is just no way to separate small proteins from the virus. With every vaccination, goes the conjecture, we are injecting kidney cell proteins. The immune system does its job admirable, and gives us large levels of antibodies toward the virus, and also antibodies toward the kidneys. Over time, in theory, these anti-renal antibodies build up and start to damage the cat's own kidneys.
That's the theory, and although it looks plausible it has not been proven. It is, however, another reason why we want to minimize the number of vaccines that a cat receives over his or her lifetime.