Gallbladder issues in dogs most commonly stem from abnormal mucus buildup (called a mucocele), bacterial infection, gallstone formation, or a combination of all three. Unlike humans, where cholesterol-based gallstones dominate, dogs are far more likely to develop problems from thickened bile sludge and overproduction of mucus that turns the gallbladder into a gel-filled sac. Genetics, hormonal disorders, and high blood fat levels all play a role, and certain breeds face significantly higher risk.
Gallbladder Mucocele: The Most Common Culprit
A gallbladder mucocele is the single most recognized gallbladder problem in dogs today. It happens when the gallbladder’s inner lining overproduces a specific type of mucus gel. In a healthy dog, the gallbladder contains mucus dominated by a protein called Muc5b. In dogs with mucoceles, a second mucus protein, Muc5ac, is produced at 17 times the normal level. This shift creates an extremely sticky, viscous mixture that clings tightly to the gallbladder wall rather than flowing out with bile.
The process usually starts with sluggish gallbladder movement. When the gallbladder doesn’t contract and empty properly, bile sits stagnant. Water and electrolytes get absorbed from the stalled bile, concentrating it into thick sludge. Over time, the mucus-producing cells in the gallbladder wall begin to multiply abnormally, pumping out more and more gel. The mucus dehydrates further and transforms into a rubbery, gelatinous mass that can completely fill the gallbladder. On ultrasound, veterinarians look for a distinctive starburst or “stellate” pattern of dense material inside the gallbladder.
If the gallbladder keeps expanding, it can eventually rupture. Early signs of rupture are subtle: loss of appetite and vague belly discomfort. As bile leaks into the abdomen, it triggers a chemical inflammatory reaction, fluid buildup, and visible jaundice (yellowing of the skin, gums, and eyes). Adhesions form quickly, making surgical repair more complicated the longer diagnosis is delayed.
Genetics and Breed Risk
Shetland Sheepdogs carry the strongest known genetic risk. Researchers identified a specific mutation in a gene called ABCB4, which normally helps transport a protective fat molecule into bile. This mutation was found in 14 of 15 Shelties diagnosed with gallbladder mucoceles, but in only 1 of 21 healthy Shelties. The single affected Sheltie without that mutation carried a different mutation in the same gene. In both cases, the gene can’t produce a functional protein, which disrupts the normal protective composition of bile.
The same ABCB4 mutation has been identified in Pomeranians, Cairn Terriers, and Cocker Spaniels with mucoceles, but not in healthy dogs of those breeds. If your dog is one of these breeds, awareness of gallbladder problems is worth keeping in mind, especially as they age.
Hormonal and Metabolic Triggers
Two endocrine disorders are closely linked to gallbladder disease in dogs: Cushing’s disease (where the body produces too much cortisol) and hypothyroidism (an underactive thyroid). Both conditions cause high blood fat levels, and that connection appears to be the key. Elevated cholesterol and triglycerides alter bile composition and promote sludge formation. Dogs with either condition should be monitored for gallbladder changes, particularly if they also belong to a predisposed breed.
Interestingly, diabetes mellitus has not shown the same association. Progesterone-type hormones can also directly stimulate the overgrowth of mucus-producing cells in the gallbladder wall, which has been demonstrated experimentally in dogs.
Bacterial Infection and Cholecystitis
Cholecystitis, or inflammation of the gallbladder, is frequently driven by bacterial infection. The bacteria almost always originate from the intestines, traveling up the bile duct to reach the gallbladder. E. coli is the most commonly isolated species, followed by Klebsiella, Pseudomonas, Salmonella, and Clostridium. Campylobacter, the same bacterium responsible for food poisoning in people, has also been documented causing gallbladder infection and bloodstream infection in dogs.
Gas-producing bacteria like E. coli and Clostridium can cause a particularly dangerous form called emphysematous cholecystitis, where gas pockets form within the gallbladder wall. On ultrasound, an infected gallbladder typically shows thickened walls, sometimes with a distinctive double-layered appearance.
Mucus buildup and infection often feed each other. The thick mucus layer inside the gallbladder creates a biofilm, essentially a sheltered environment where bacteria can persist and resist clearance. This makes dogs with existing sludge or early mucoceles more vulnerable to recurring infections, and infected gallbladders produce more mucus in response to inflammation.
Gallstones in Dogs
Gallstones (cholelithiasis) are less common in dogs than mucoceles, but they do occur. Unlike human gallstones, which are usually made of cholesterol, dog gallstones are pigment stones composed primarily of calcium carbonate and calcium bilirubinate. They fall into two types: black pigment stones, which form during prolonged periods of elevated bilirubin, and brown pigment stones, which are linked to bacterial infections and stagnant bile.
The formation process involves mucus trapping calcium and bilirubin particles into clumps, a process amplified by inflammation and poor gallbladder motility. Gallstones and infection can also create a vicious cycle: stones irritate the gallbladder lining and promote infection, while infection changes bile chemistry in ways that promote stone growth. On ultrasound, stones appear as bright structures that cast a shadow behind them, distinct from the mobile sludge that doesn’t shadow.
Gallbladder Tumors
Though less common than mucoceles or infection, masses can develop within the gallbladder wall. These may appear on ultrasound as irregularly shaped growths protruding into the gallbladder. Tumors can obstruct bile flow and mimic or coexist with other gallbladder conditions, so tissue biopsy is often necessary to determine whether an underlying cancer is driving the problem.
What Gallbladder Surgery Looks Like
When gallbladder disease progresses to the point of rupture, obstruction, or severe infection, surgical removal (cholecystectomy) is the standard treatment. Survival outcomes have improved in recent years, especially when surgery is performed before the gallbladder ruptures. However, the procedure still carries significant risk in dogs. Perioperative mortality ranges from 20% to over 40% in cases involving rupture or systemic illness, with bile leakage, sepsis, pancreatitis, and clotting problems being the main complications. Older dogs and those with concurrent endocrine disease face the highest risk.
This is why early detection matters so much. Dogs caught at the sludge or early mucocele stage, before rupture occurs, have substantially better outcomes than those rushed to surgery in crisis. For breeds at genetic risk or dogs with Cushing’s disease or hypothyroidism, periodic abdominal ultrasound can catch gallbladder changes while they’re still manageable.