The reasons for which animals refuse to eat may be grouped into two major categories: Psychological and Medical Psychological causes imply that something in the animal's environment has caused him to lose his appetite. Examples include moving to a new home, having a new person or new animal in the home, and switching to a new pet food.
Medical causes are disease processes that result in the loss of appetite.
A major difference between psychological loss of appetite and disease-related loss of appetite is that when there is disease, additional symptoms are usually present. These symptoms can include the new development of excessive salivation (drooling), vomiting, diarrhea, lethargy or sluggishness, weight loss, labored breathing, signs of infection such as discharge of pus or blood, or sudden changes in behavior.
Common diseases that make animals unwilling to eat include the following:
If the esophagus (tube in the throat that connects the mouth to the stomach), the stomach, or the intestine, is inflamed (irritated) by disease, eating can become uncomfortable or nauseating, resulting in anorexia. Diseases that can cause this kind of irritation include parasites (worms), viruses such as parvovirus and coronavirus, other infections such as bacterial and fungal infections, ulcers, food allergy, inflammation of unknown cause ("idiopathic"), and certain infiltrative cancers. A complete or partial blockage of the digestive tract can also cause unwillingness to eat. This most often occurs with foreign bodies (objects that are swallowed and become stuck partway down the digestive tract) and cancers of either a benign or malignant nature.
Gastrointestinal diseases in general often will also cause increased salivation, vomiting, diarrhea, and sometimes (particularly when more severe) lethargy and sluggishness.
Diseases of the Liver
The liver filters many of the body's waste products and toxins from the bloodstream, so that accumulation of these substances as a result of inadequate liver function affects the brain, and blunts the sense of hunger. Common diseases of the liver in dogs include chronic hepatitis (not the same as human hepatitis A, B, or C, and NOT contagious), portosystemic shunt (a defect in the blood circulation through the liver), cirrhosis (severe scarring of the liver), liver cancer, and adverse reaction to certain drugs (e.g. carprofen, trimethoprim-sulfa, others).
Liver diseases in general often will also cause increased salivation, vomiting, and lethargy and sluggishness.
Diseases of the Pancreas
The pancreas secretes many of the digestive juices that dissolve food into tiny particles the intestine can absorb. If inflamed ("pancreatitis"), the pancreas releases some of those powerful dissolving substances into the internal organs rather than on food in the intestine. These corrosive juices may severely inflame and erode the pancreas itself and other surrounding tissues, a painful process that often makes an animal completely unwilling to eat and frequently also causes vomiting and lethargy. Another disease of the pancreas that can cause loss of appetite is pancreatic cancer.
Diseases of the Urinary Tract
Anorexia is a hallmark of kidney disease. There may be both a loss of appetite and discomfort caused by ulcers in the mouth and stomach associated with uremia (accumulation of waste products in the blood stream). Your pet's consumption of water may be the same or even greater than usual. This is an effort to make up for the tremendous amount of fluid lost by the sick kidneys through the urine. Also, vomiting and listlessness are common symptoms that occur along with loss of appetite in kidney disease. Not all types of urinary disease affect the appetite, however. For instance, most cases of bacterial cystitis (bladder infection) do not affect the appetite.
Diseases of the Blood
Generally, diseases of the blood that lead to loss of appetite also cause lethargy and sluggishness, and possibly signs of weakness such as intermittent collapse. Blood disorders causing loss of appetite include severe anemia of different causes (immune-mediated hemolytic anemia, leukemia-related anemia, blood loss due to ulcers of the stomach or rat bait poisoning), cancer of the blood (leukemia), and polycythemia (excessive red blood cells – the opposite of anemia).
Diseases of the Eyes, Mouth, Nose and Throat
These can cause unwillingness to eat as a result of pain in the mouth during chewing (dental disease, foreign object caught in the mouth or throat), inability to smell the food, which is essential in animals for recognition and acceptance of food (nasal infections or tumors), or pain or discomfort of the eyes (conjunctivitis, uveitis, glaucoma).
Essentially any disease process, when severe enough, can cause an animal to stop eating. Loss of appetite is one of the first and most common symptoms of "not feeling well" in animals. Don't hesitate to take your pet to the veterinarian when he is anorexic.