Kidney’s don’t regenerate like the liver and by the time kidney disease is diagnosed it is normally far down the pike and the best anyone can hope for is not very encouraging, very costly and takes great owner dedication... many of these dogs and cats being fed the worst of the worst foods in the first place don’t stand much chance of owners being that dedicated to formulating a diet that will keep them alive.
They don’t physically regenerate, but they often recover a lot of function after an acute poisoning episode. Many cats who have been critically ill with acute kidney failure from the contaminated foods, are being gradually weaned off fluids as their kidney function returns. Some will end up not needing ongoing fluid therapy at all; others will require much lower daily quantities of fluids than they needed while the toxin was still in their system — depends on how much permanent physical damage the kidneys sustained.
As for me, my only renal failure kitty is a 21 year old whose renal failure is due to being ancient, not due to any poisoning. He’s gotten fluids every single night for almost 2 years now, and will continue to need them for the rest of his life.
I hope people whose pets have sustained permanent damage requiring lifelong fluid administration won’t rush to put them to sleep out of the belief that the pet will regard the daily needle+fluids as some sort of torture. My kitty LOVES getting his fluids. He lies on my bed and purrs up a storm, because he gets combed during fluid admin and thinks that’s just the most excellent treat. If I’m late with his fluids, he gets up on the bed and parks himself in The Spot, glaring at me if I’m within glaring range. Other people train their kitties to look forward to a special food treat, like pure tuna, right after (or even during) fluids. There are various ways to convince cats that getting fluids is fun, and a little patience and creativity on the part of the human will accomplish that objective. I don’t have any personal experience with dogs and fluid admin, but know that many people have trained their dogs to accept it readily (not surprising, since dogs are generally more trainable than cats).
PS: As for “costly”, treatment through the critical stage is indeed costly, but long term fluid admin isn’t, if you buy supplies in bulk from the least expensive sources (which is almost never a vet). At 150-200mL of lactated ringer’s nightly, along with needles and admin lines, I’m spending about $20 a month.