Lethargy and lack of appetite in dogs are caused by a variety of serious conditions, according to WebMD. Lethargy and weakness could be caused by medication, pain, disease or a life-threatening condition. Changes in a dog's eating behavior is also a cause for concern, according to HowStuffWorks. If a dog remains listless and refuses to eat any food for more than two days, a veterinarian should be consulted.
Infection is one common reason for chronic lethargy in dogs, according to WebMD. Parvovirus, a canine infection spread through feces, causes lethargy and abdominal pain. Lethargic dogs may have distemper, a condition that causes discharge of fluids from the eyes and nose. A dog infected with heartworm disease may also show signs of weakness, depression and fever. Another common infection that afflicts dogs that are boarded is kennel cough. This condition causes dogs to become feverish and lethargic.
There are a variety of canine diseases that cause lethargy and changes in appetite. These include heart disease, liver disease, diabetes mellitus and hypoglycemia. Owners who notice dogs acting listless with no interest in eating should monitor their pets closely. Sometimes, dogs instinctively try to hide any signs of being ill, according to HowStuffWorks. This is natural for the dog, but it requires owners to be vigilant about detecting serious health problems.