The liver is the largest organ in the body and has a multitude of roles, including metabolism, detoxification, excretion, storage, synthesis, and defense (phagocytosis of microbes). Approximately 60% of its mass consists of hepatocytes.
Liver disease can be the result of a process directly affecting the liver, or it can become secondarily injured by diseases originating in other sites. Clinical signs commonly associated with liver disease include depression, loss of appetite, weight loss, colic, and icterus. In addition, signs of central nervous system dysfunction can occur, which is referred to as hepatic encephalopathy.
Liver disease is classified as acute, chronic, or congenital. Acute diseases are caused by bacteria, viruses, parasites, obstructions, and toxins (plants, chemicals, or drugs). Chronic liver diseases can result from toxins, obstructions, neoplasia, chronic hypoxia, infections (abscesses), and immune system disorders. Congenital liver diseases include vascular shunts that bypass the liver and failure of development of a portion of the liver (atresia).
A review of cases submitted to the University of Kentucky Livestock Disease Diagnostic Center (LDDC) over the past 10 years revealed the most commonly diagnosed liver condition to be hepatitis (6-10 cases/year). Hepatitis indicates inflammation of the liver, often with degeneration or necrosis of hepatocytes. Most cases were seen in foals related to bacterial infection and sepsis caused by salmonellae, rhodococcus, corynebacterium, and actinobacilli. Another common condition in foals was rupture of the liver leading to rapidly fatal internal hemorrhage observed from the day of birth to 4 months of age. Rupture of the liver was typically associated with trauma at delivery, or being stepped on or kicked by another horse.
In foals found dead, necrotizing hepatopathy was often diagnosed. These cases were associated with infection by Clostridium piliformis (Tyzzer’s disease) or equine herpes virus 1 (EHV-1). At the LDDC, an average of 5 cases of Tyzzer’s disease per year were seen in foals 5 days to 5 weeks of age. EHV-1 was diagnosed during the first days of life with an average of one case a year.
An uncommon but severe liver disease seen in adult horses is characterized by acute hepatic necrosis. This has features similar to Theiler’s disease, which has been associated with the use of live virus vaccines and equine-origin antiserum (serum sickness). Acute hepatic necrosis usually occurs sporadically but small outbreaks may occur. Affected horses die unexpectedly, or exhibit a short course of icterus and anorexia, with nervous signs commonly observed. The LDDC typically receives 3-4 cases each year among horses whose ages range from 2-17 years. While the condition is readily diagnosed, the precise cause is obscure. Occasionally, there is a history of recent vaccination; however, with cases occurring most commonly in the summer, possible exposure to toxic plants is a potential explanation. Cases at the LDDC over the last 10 years have involved a variety of breeds. Of 26 cases where breed was known, only two were Thoroughbreds.
Another common cause of liver disease in adult equines is hepatic lipidosis (fatty liver) most commonly seen in pony breeds including Miniature horses and donkeys (91% of cases). Twenty-two cases were diagnosed over 10 years, the ages ranging from 3 to 27 years. Hepatic lipidosis is a sequela to primary hyperlipemia and the animals are often obese and commonly have another illness or form of stress.
Chronic fibrosing hepatitis in adult horses (average age 15 years) was occasionally diagnosed, with 15 cases over the 10-year period. This condition results from chronic liver disease with secondary scarring to the extent that there is cirrhosis, and diminished hepatic function. The cause is usually unknown.
Two rarely diagnosed conditions of the liver included biliary disease (7 cases) and neoplasia. Primary neoplasia of the liver was diagnosed rarely (2 cases). Metastatic neoplasia in the liver was more common and included cases of lymphosarcoma, malignant melanoma, and hemangiosarcoma.
Congenital anomalies were extremely rare, with only a single case of portosystemic shunting in a Rocky Mountain Horse.