It's most unsavory to you, but coprophagia, the act of eating feces, is not uncommon to dogs. No one is sure why a healthy, well-fed dog who eats a balanced diet resorts to coprophagia. In some cases, a digestive disorder may be to blame, but generally, dogs just like to snack on whatever seems like food. including eating poop of other animals, such as cats.
The first domesticated dogs served as a type of primitive waste disposal system for people, eating their garbage and wastes while guarding the tribe. Fifteen thousand years later, dogs in many parts of the world still clean up after people and other pets by ingesting feces. Dogs, in fact, often eat the feces of herbivorous pets and wildlife such as rabbits. This may actually provide some nutrients to dogs. It also is common, however, for dogs to eat the feces of carnivores such as cats.
A well-fed dog who raids the litter box is probably just snacking, and you can put a stop to it with deterrents or blockades. A dog who's underfed or malnourished, or suffers from a malabsorption disorder such as an inflamed bowel or an intestinal blockage that keeps him from getting nutrients from his food, may turn to coprophagia to supplement his diet. Warning signs of malabsorption problems include greasy or watery stool, stool containing undigested food, diarrhea, vomiting and abdominal pain. Take him to your veterinarian if you suspect medical problems.
Training a dog with a taste for cat poop is a Herculean task that probably won't work anyway. Easier and more effective is to simply make it too difficult for your dog to get to the litter box. This also gives your cat some needed personal space. Placing the litter box too high for your dog to reach helps, provided your cat can get to it. You could also put the box behind a door with an opening only the cat can fit through, or try a dog-proof litter box lid.
If you can't move the litter box out of your dog's reach or provide cat-only access to it, try using a taste deterrent. Most dogs dislike hot pepper sauces and bitter apple flavoring. Put one of these on cat poop and let your dog continue to eat it. You must do this for at least a few weeks so the dog comes to associate the feces with a terrible taste. If this still doesn't work, call an animal behavioral therapist for help.