Figure 7: A large cholelith surgically removed from the common bile duct of a horse.
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In horses, choleliths mostly consist of calcium bilirubinate (Figure 7) and are associated with cholangitis, which accounts
for the presence of fever. Choleliths that obstruct bile flow cause increased biliary pressure, which likely is the reason
for abdominal discomfort. If biliary pressure is not resolved, pressure-induced periportal hepatocellular necrosis with subsequent
fibrosis ensues.
Cholelithiasis should be considered in the differential diagnosis in horses with a history of fever, icterus and abdominal
pain, especially if accompanied by signs of hepatic disease. Hematology often reveals leukocytosis due to mature neutrophilia,
especially if cholangitis is present. The most common abnormal laboratory findings that suggest cholestatic liver disease
include markedly increased GGT (greater than 15 times normal) and markedly increased serum bile-acids concentration. Hepatic ultrasonography often is helpful in securing a diagnosis by the presence of visibly dilated bile ducts and/or hyperechoic
foci within the bile ducts that may cast acoustic shadows (Figures 3 and 4). Determination of obstruction of the common bile duct in horses is difficult, as the common bile duct cannot be seen by transabdominal
ultrasonography; thus choledocholiths often are detected only via palpation during exploratory celiotomy. Obtaining a liver biopsy is useful for diagnostic, therapeutic and prognostic purposes. The histopathologic findings of periportal
fibrosis, biliary stasis and hyperplasia and cholangitis are not pathognomonic for cholelithiasis; however, a liver biopsy
may help rule out other causes of biliary stasis. Concentric fibrosis around intrahepatic bile ducts is evidence of occlusion
of the common bile duct. Michelle Henry Barton, DVM, PhD, Dipl. ACVIM is the Josiah Meigs Distinguished Teaching Professor at the University of Georgia's
College of Veterinary Medicine, where she is a large-animal internist in academic practice. She received her DVM from the
University of Illinois in 1985, her PhD in physiology at the University of Georgia in 1990 and became an ACVIM diplomate in
1990. SUGGESTED READING- Johnston J.K., Divers T.J., Reef V.B., et al. Cholelithiasis in horses: Ten cases (1982-1986). J Am Vet Med Assoc 1989;194:405-409.
- Mendle V.E. Pyrrolizidine alkaloid-induced liver disease in horses: An early diagnosis. Am J Vet Res 1988; 49:572-578.
- Moore B.R., Abood S.K., Hinchcliff K.W. Hyperlipemia in Miniature Horses and Miniature Donkeys. JVIM 8(5): 371-381, 1994.
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