Chronic hepatitis is a poorly understood, progressive disease characterized by inflammation, irreversible scarring and death of liver tissue. The causes of chronic canine hepatitis are not known. Some people think that a weakened or overactive immune system plays a leading role. Drugs, toxins, cancer or infection may also contribute to chronic hepatitis. Whatever the cause, the damage from this disease affects the entire liver and usually is fatal. The liver has an enormous reserve capacity; up to 75% of liver tissue must be destroyed before the liver will fail. At first, most dogs with chronic hepatitis don’t show signs of disease. Their liver swells and becomes rubbery and firm, as cells die and are replaced by scar tissue. Once most of the liver is destroyed, the dog will start vomiting and have diarrhea, weight loss, weakness, abdominal distention, depression, disorientation, jaundice, poor body condition and pain. The dog’s liver is now partitioned by scar tissue into a mass of firm, irregular nodules, and the damage is irreversible.