Dilatation of the gallbladder due to accumulation of abnormal mucus (mucin) that distends the wall and predisposes to rupture and biliary peritonitis. It is a multifactorial disease, with marked breed predisposition (Shetland Sheepdog, Cocker Spaniel, Pomeranian, Cairn Terrier, Miniature Schnauzer) and familial aggregation in some lines. The genetic component is unresolved: the candidate variant in ABCB4 (MDR3) was initially associated with risk, but the association was not replicated and was refuted; it must not be used as a diagnostic test.

Marked breed predisposition: Shetland Sheepdog (highest relative risk), Cocker Spaniel, Pomeranian, Cairn Terrier and Miniature Schnauzer. There are no reliable carrier frequencies in large series (limited data).

- Abdominal distension
- Intermittent vomiting and anorexia
- Lethargy
- Jaundice if there is obstruction or cholestasis
- Pain in the right hypochondrium
- Fever and shock if there is biliary rupture with peritonitis
- Ultrasound finding of heterogeneous biliary content in a "kiwi" pattern

Canine gallbladder mucocele has been recognised with increasing frequency since the 2000s and has been linked to predisposed breeds. In 2010, Mealey et al. described an insertion in ABCB4 (MDR3) associated with hepatobiliary disease in the Shetland Sheepdog, which suggested a defect in bile composition. In 2014, Cullen et al., with a larger number of animals, found no association between that insertion and the mucocele, so the hypothesis of a monogenic cause was ruled out. The current consensus considers GBM a multifactorial disease with endocrine and environmental factors.

- There is no validated or useful genetic test for GBM; do not use ABCB4 c.1660_1661insG for selection purposes.
- Given the multifactorial nature, prioritise control of risk factors (thyroid, cortisol, weight, diet) and ultrasound monitoring in predisposed breeds.
- Do not breed animals with a clear clinical or familial history of GBM without individualised veterinary assessment.
- Avoid crosses between lines with repeated history, so as not to concentrate risk factors.

Ultrasound diagnosis: heterogeneous biliary content without acoustic shadow ("kiwi"), thickened wall. Differentiate from cholecystitis and from an isolated mucus plug. Frequent comorbidities: hypothyroidism, hyperadrenocorticism and pancreatitis; always assess the endocrine and lipid profile. Cholecystectomy is indicated in symptomatic cases or those with an altered wall, not on a generalised prophylactic basis. The former ABCB4 molecular test has no clinical value.

1. Mealey KL et al. 2010. An insertion mutation in ABCB4 is associated with gallbladder mucocele formation in dogs. Comparative Hepatology. PMID: 20598156
2. Cullen JM et al. 2014. Lack of association of ABCB4 insertion mutation with gallbladder mucoceles in dogs. Journal of Veterinary Diagnostic Investigation. PMID: 24760133
3. Jaffey JA et al. 2019. Effect of clinical signs, endocrinopathies, timing of surgery, hyperlipidemia, and hyperbilirubinemia on outcome in dogs with gallbladder mucocele. The Veterinary Journal. PMID: 31492387
4. OMIA:001524-9615. Gallbladder mucoceles in Canis lupus familiaris. Online Mendelian Inheritance in Animals.