Sometimes we don't know why a particular dog starts eating poop, but certain conditions can trigger the behavior. Since some of these indicate a dog who needs help, you'll want to consider them as possibilities for what is going on with your dog.
1. A dog with a physical problem that causes excessive hunger, pain, or other sensations may resort to eating feces. If your adult dog who has not previously had this habit suddenly develops it, take the dog to your veterinarian for a check-up.
2. A dog who is not getting enough to eat or is going too long between meals may eat feces. Your veterinarian can help you evaluate the dog's weight and can suggest a feeding schedule and amount. Sometimes it takes experimentation to see what works best for a particular dog.
3. A dog with intestinal parasites or other condition that creates blood or other fecal changes may eat feces. One dog may eat the feces of another dog who is shedding something like this in the stools. A fresh fecal specimen to your veterinarian for evaluation can detect some of these problems.
4. Sometimes a change of diet helps. There doesn't seem to be any one food that is right for all dogs, and your dog may need something different than you're currently feeding. Be sure to make any changes of diet gradual, mixing the new food in with the old over a period of several days or weeks, to give the dog's intestines time to adjust and avoid diarrhea from the change.
5. Some dogs develop a mental connection that they will be punished if their humans find them in the same room with feces. Dogs react to this fearful situation in various ways, and one way is to eat the feces so it will not be there to make the human angry. This is one of many reasons not to use punishment when housetraining a dog.
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