Snacking out of the catbox is perhaps the most disgusting habit a dog can have, in my opinion– and that’s a pretty competitive category. Unfortunately, at least a substantial minority, and probably a vast majority of dogs living with cats have rummaged in the litterbox for a snack at some point. This self-rewarding behavior is difficult to eliminate completely. Dogs that can access litterboxes and that enjoy snacking on cat poop will continue to do as long as they are able.
Why Punishment Doesn’t Fix It
Most dog owners with this problem try yelling, swatting, or squirting their dogs in an effort to keep them out of the litterbox. Unfortunately, this usually just leads to litterbox raids the moment the owner’s back is turned. Dogs are simple creatures, but they’re not stupid. If you are the punisher of a self-rewarding behavior, they learn only not to perform that behavior when you’re paying attention. Litterbox snacking is highly rewarding to most dogs, and they may even ignore punishment completely and continue to do it while you’re staring right at them.
Punishing dogs after the fact when you discover evidence all over the floor is even less effective than immediately punishing this behavior. All you’ll teach a dog who is punished hours later for a behavior he doesn’t even remember by that time is that, “My owner can’t be trusted not to hurt or berate me.”
What Does Work
So what does keep dogs out of litterboxes? The best solution is to make it impossible for the dog to access the litterbox. Try walling it off with a baby gate that has a cat door, if your cat is smaller than your dog by a large enough margin. If that doesn’t work, consider getting a top entry litterbox. If the cat won’t use that, try spraying Bitter Apple in the cat box daily as a taste deterrent.
As a last resort, there are various control collars designed to create “forbidden zones” inside a home. These are usually called “boundary zone” systems and involve an electric shock when the dog passes a boundary. Some such collars use a scent deterrent and/or sound deterrent instead of or in addition to a shock. I personally would rather simply deal with a dog with bad breath and a gross habit than use a shock collar on any dog, if all other options had failed, but if this is a total deal-breaker for your household, I’d rather see a dog wear a boundary zone collar than end up in a shelter for eating cat poop.
Most dogs that eat out of the catbox simply enjoy it, but some are suffering from a nutritional deficiency. Try some basic bloodwork, a multivitamin supplement, and a probiotic, in addition to training and behavior modification methods.