An animal can lose his appetite for many of the same reasons humans do. Diseases or infections can make eating painful or cause nausea.

Illnesses that cause anorexia – or an unwillingness to eat - include cancers, kidney malfunction and stomach ulcers and they need treatment, FAST.

Diseases that make animals refuse their food include the following.

Gastrointestinal Problems

If the esophagus (the tube in the throat that connects the mouth to the stomach), the stomach, or the intestine, is inflamed by disease, eating can become uncomfortable or nauseating. Gastrointestinal diseases can also cause increased salivation, vomiting, diarrhea, and sometimes lethargy and sluggishness.

Some of these diseases include parasites like worms, viruses that attack the intestines and cat distemper (panleukopenia), as well as bacterial and fungal infections, ulcers, food allergies, inflammation of unknown cause and certain cancers.

Blockages of the digestive tract can also make a cat unwilling to eat. Cancers are a common cause. So is blockage caused by swallowing an object that gets stuck part way down the digestive tract.

Diseases of the Liver

The liver filters many of the body's waste products and toxins from the bloodstream; if these substances accumulate in the blood as a result of inadequate liver function, the brain will be affected – and the sense of hunger blunted.

Cirrhosis and cancer are some of the most common liver diseases that cause loss of appetite in both cats and dogs. Often, liver disease in general will cause increased salivation, vomiting, lethargy and sluggishness.

Diseases of the Pancreas

The pancreas secretes many of the digestive juices that dissolve food into particles that the intestine can absorb. If it is inflamed, the pancreas will release some of those powerful dissolving substances into the internal organs rather than on food in the intestine. These corrosive juices may irritate and erode the pancreas itself – as well as surrounding tissues. This is a painful process that often makes an animal completely unwilling to eat and frequently causes vomiting and lethargy. Pancreatic cancer is another reason for loss of appetite.

Diseases of the Kidneys and Urinary Tract

Anorexia is a hallmark of kidney disease. Mouth and stomach ulcers caused by accumulation of waste in the bloodstream (uremia) may cause both loss of appetite and discomfort. In an effort to make up for the amount of fluid lost by the sick kidneys through the urine, your pet may increase his consumption of water. Vomiting and listlessness are also common symptoms that occur. Not all types of urinary disease affect the appetite, however: Most bladder infections, for example, don't.

Diseases of the Blood

Generally, blood diseases that lead to loss of appetite also cause lethargy and sluggishness, and possibly signs of weakness such as intermittent collapse. Eating rat poison can cause a bleeding disorder and loss of appetite; other disorders include severe anemia, leukemia and stomach ulcers and a condition in which the body makes excessive red blood cells (polycythemia).

Diseases of the Eyes, Mouth, Nose and Throat

Dental problems or blockages in the mouth or throat can cause pain during chewing. Nasal infection or tumors can weaken an animal's sense of smell, which is essential in recognizing – and accepting – food. Conjunctivitis and glaucoma can cause pain or discomfort of the eyes, which will also make an animal not want to eat.

Infections Under the Skin

This is a common problem of outdoor cats, whose fighting with other cats causes bite or scratch wounds that become deeply infected. The result is an abscess, which is often difficult to see because the hair coat covers the wound. Since pus is trapped under the skin, it continually seeps into the bloodstream, causing a sense of illness and unwillingness to eat.