A gallbladder mucocele is an enlarged gallbladder that contains an excessive amount of mucus. Histologically, the gallbladder
mucosa is characterized by cystic mucosal hyperplasia, with or without inflammation or necrosis. Dogs with gallbladder mucoceles
can be asymptomatic early in the course of disease. Clinical and biochemical abnormalities occur when mucoceles are complicated
by secondary bacterial infection, extrahepatic biliary obstruction (from bile-laden mucus accumulation in the cystic, hepatic
or common bile ducts), or marked distention of the gallbladder leading to ischemic necrosis, gallbladder rupture, and bile
peritonitis. Gallbladder rupture is a common life-threatening complication. The incidence of gallbladder mucocele appears
to be increasing and it is one of the most common causes of extrahepatic biliary tract disease in dogs.
The cause of gallbladder mucocele formation in dogs is unknown. Hyperlipidemia/hypercholesterolemia appears to be a risk factor
and may be idiopathic (Shetland sheepdog, Miniature schnauzer) or secondary to pancreatitis, nephrotic syndrome, endocrinopathies
(hyperadrenocorticism, hypothyroidism) or feeding a high fat diet. Administration of corticosteroids may be a contributing
factor. A primary or secondary motility disorder of the gallbladder has also been proposed. In humans, gallbladder mucoceles
form secondary to functional or mechanical biliary obstruction associated with cholecystitis, cholangitits, or cholelithiasis.
However, predisposing disorders causing mechanical biliary obstruction (infiltrative disease of cystic duct, cholelithiasia)
are not typically identified in dogs. A primary bacterial or inflammatory disorder of the gallbladder and biliary tract appear
unlikely, since aerobic and anaerobic cultures are frequently negative and gallbladder inflammation is inconsistent. A recent
report suggests that affected dogs (Shetland sheepdogs and other breeds with gallbladder mucocele) may have a disorder of
gallbladder mucin secretion associated with a mutation in canine ABCB4 (phospholipid translocator protein). A dominant inheritance
with incomplete penetrance is suspected. Genotyping for the ABCB4 mutation could allow early identification of at risk individuals,
allowing for monitoring and early medical, dietary, or surgical intervention.
Gallbladder mucocele appears to be more likely in older (median age of 10 years) small to medium size dogs. No sex predilection
is apparent. Shetland Sheepdogs, Cocker spaniels, and Miniature schnauzers appear to be at increased risk. Common clinical
signs include anorexia, lethargy, vomiting, icterus, diarrhea, weight loss, PU/PD, abdominal discomfort, and abdominal distention.
Signs are usually acute to subacute and less than three weeks in duration. Physical examination findings include depression,
weakness, lethargy, abdominal pain, icterus, fever, hepatomegaly, tachypnea, and tachycardia. Most dogs with gallbladder
rupture have abdominal pain. Some dogs with gallbladder mucocele are clinically (and biochemically) normal.
Common hematologic findings in symptomatic dogs include leukocytosis (mature neutrophilia or neutrophilic with left shift),
and increased liver enzyme activity (ALP, ALT, AST, and GGT). Findings of hyperbilirubinemia, hypercholesterolemia, and hypertriglyceridemia
are less consistent. Biochemical findings may be normal in some dogs with early gallbladder mucocele formation detected ultrasonographically.
The ultrasonographic appearance of a gallbladder mucocele is characteristic. The gallbladder bile is echogenic and organized
in a stellate or finely striated ("kiwi") pattern. As opposed to billiary sludge, a gallbladder mucocele is not gravity dependent.
Ultrasonographic findings suggestive of secondary gallbladder rupture include loss of gallbladder wall continuity, hyperechoic
fat or fluid around the gallbladder, free abdominal fluid, and striated or stellate echogenic material outside the gallbladder.
Additional findings may include extrahepatic biliary obstruction and pancreatitis.