One of the veterinary hats that I wear is as an in-home euthanasia provider. It may sound a little bit morbid, but helping pets pass peacefully at home, surrounded by their loved ones, is actually very rewarding (funny that I still feel the need to justify this choice of work, though).
Anyway, I had a notable week awhile back. I saw an unusually high number of cats, and every single one of them was in kidney failure. Statistically, this is probably not too surprising. Kidney disease is the number one killer of older cats, after all, but it still got me thinking, "Why are all these cats dying of kidney disease?"
First a little background. Kidney failure can be divided into two categories: acute and chronic. Acute kidney failure develops quickly, usually as a result of an identifiable problem, such as getting into antifreeze, a kidney infection, low blood pressure during anesthesia, etc. Chronic kidney failure develops more slowly, usually in older cats, and is the result of the gradual loss of nephrons, the functional unit of the kidney (healthy cat kidneys have hundreds of thousands of them).
Nephrons can’t regenerate themselves. Once one is damaged and no longer working, it is gone forever and can’t be replaced. Many things cause the loss of nephrons: a bout of acute kidney failure can knock out a whole bunch at one time, but everyday wear and tear builds up too. Some cats may also be born with fewer nephrons than is normal. So you can see how over time a cat could essentially "run out" of nephrons.
When faced with an owner’s questions about chronic kidney failure, I’ve heard some veterinarians quip that "cat kidneys were designed by committee," but that’s not really the case. They were designed by natural selection, which usually does a great job, on a population level at least, of promoting health. So what’s the deal?
In my opinion, the epidemic of kidney failure in domestic cats is our fault, but not in the way that you might think. I don’t blame inappropriate diets, lifestyle choices, litter box issues, or over-vaccination as some do, I blame excellent husbandry and veterinary care for giving our cats the opportunity to live much longer than they were ever designed to.
Look at the stats. Outdoor cats typically don’t live for more than five to seven years, and truly feral kitties (those that receive no supplemental nutrition, veterinary care, etc.) often only survive to the age of two or so. But even in such a short life, intact males and females can produce many litters, ensuring that their genes are passed on to the next generation… the goal of natural selection.
If all this can be accomplished in just a couple of years, and a cat was likely to die of an infection, be eaten by a predator, or otherwise not make it past the age of five, who needs kidneys that last for 20 years? It’s all about allocation of resources. The energy used to maintain an over-designed kidney has to be taken from somewhere, maybe resulting in weaker muscles, a poorer hunting ability, and fewer offspring.
These days, I think some cats are essentially out-living their kidneys because of their owner’s excellent care. After all, we all have to die of something.
Dr. Jennifer Coates
Image: Julia Shepeleva / Shutterstock