For the Health of It
Liver Disorders in Horses
Reprinted with permission from the July 2005 issue of Equine Disease Quarterly
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 com-
monly 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 10year 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.
– Dr. Neil Williams
(859) 253-0571, [email protected]
University of Kentucky
Livestock Disease Diagnostic Center
Lexington, Kentucky