There are many possible reasons that a dog may not eat its food, such as aging, heart failure, medications that upset a dog's stomach, toxins and tumors in the dog's body, according to PetMD. If an owner finds that his or her dog refuses food, the owner should bring the dog into the veterinarian's office for treatment, advises WebMD.
A dog may lose its appetite if it eats something that makes its stomach hurt or if it has a cold. Dogs may also lose their appetites if they are in environments that are too hot, notes Healthline. Dogs also may only eat up to 60 or 70 percent of the manufacturer's recommended feeding amount, so dogs that eat less food may simply not need as much food, reports WebMD. However, if a dog has not eaten any food or eats significantly less over a two- to three-meal period, a veterinarian should examine the dog, explains Healthline.
Other common symptoms that appear with loss of appetite include fever, jaundice, pain, eye changes, shallow breaths and changes in the sizes of the dog's organs. There can also be a lack of sound from the heart or from the lungs, notes PetMD. To diagnose the problem, a veterinarian performs a thorough physical examination and possibly a heartworm exam, blood analysis, retrovirus exam, urinalysis and X-rays or cell tissue samples, according to PetMD.