Heart disease, liver disease and diabetes mellitus all cause loss of appetite and lethargy in dogs, reports WebMD. Pet owners should examine their pets for other symptoms and seek the advice of a veterinarian before deciding on a cause for these symptoms.
Parvovirus can also cause lethargy and reduced appetite, states 1800PetMeds. Parvovirus is a bacterial infection that often affects puppies and young dogs. If caught in time, parvovirus can be treated with antibiotics.
Heartworms are another cause of weakness and disinterest in food. Heartworms are parasites transmitted primarily by mosquitoes, notes the American Heartworm Society.
Only a veterinarian can determine the reason for a dog's health symptoms. Fecal tests, blood tests and other diagnostic exams help veterinarians isolate a particular problem.