Puppy Not Eating from Upset Stomach
The technical name for an upset stomach is "dyspepsia," or more commonly known as "indigestion". Upset stomach causes abdominal pain and bloating, belching, nausea, abdominal swelling (distention) and, in some cases, vomiting. Upset stomach can also cause early satiety which gives the sensation of being full after eating very little. These symptoms are unpleasant for puppies, so they are enough to cause the puppy to eat very little or nothing at all. Upset stomach typically occurs after meals, usually from eating too fast (taking in too much air), eating foods that don't agree with the dog's system (i.e. table scraps, allergens such as wheat, corn, dairy), or eating while excited or nervous.
Loss of Appetite from Injury or Illness
One of the first signs of injury or illness in puppies, whether minor or serious, is loss of appetite. If your puppy has been physically hurt, he may stop eating out of fear or discomfort from the injury. If your dog has an illness or disease of the kidneys, nose, the blood, eyes, the digestive system, or the skin, the discomfort and pain from the disease will cause your puppy to stop eating.
Loss of Appetite from Parasites
If your puppy is not eating, it may be a sign of a parasite infection or "dog worms." Most puppies are born with Toxicara canis or "roundworm" and it can cause everything from diarrhea and weight loss to appetite loss. Other types of dog worms include tapeworm, hookworm, whipworm, and heartworm. All types of dog worms cause appetite loss.
Less Serious Reasons for Loss of Appetite
A puppy with no appetite isn't always a sign of a serious problem or even a problem that requires medical attention. Some puppies actually refuse food simply because they do not like the taste, or they may be experiencing anxiety from being in a new place, meeting new people or getting to know a new pet.
How to Get Your Puppy to Eat Again
It is dangerous to try to diagnose your puppy on your own. Your puppy could be experiencing much more than anxiety or an upset stomach, so you should take your dog to a vet to get an accurate diagnosis from a professional, especially if he hasn't eaten in the last 12-24 hours.