A liver shunt in dogs is an abnormal blood vessel that diverts blood around the liver instead of through it. Normally, blood from the intestines travels through the liver first, where toxins like ammonia are filtered out before the blood enters general circulation. When a shunt is present, that toxic blood bypasses the liver and flows directly into the bloodstream, causing a buildup of harmful substances that can affect the brain, kidneys, and other organs.
The medical term is portosystemic shunt (PSS). Most cases are congenital, meaning the dog is born with the abnormal vessel, though shunts can also develop later in life. Understanding the type of shunt, the symptoms it causes, and the available treatments can make a significant difference in a dog’s outcome.
How a Liver Shunt Works
In a healthy dog, the portal vein carries nutrient-rich blood from the stomach, intestines, and spleen into the liver. The liver processes that blood, removing toxins (especially ammonia produced during protein digestion), metabolizing medications, and producing proteins the body needs. Clean blood then exits the liver and joins the general circulation.
A shunt is essentially a shortcut vessel that connects the portal vein to the body’s main circulation, skipping the liver entirely. Because the liver never gets a chance to filter the blood, ammonia and other waste products accumulate. Over time, the liver also shrinks because it isn’t receiving its normal blood supply, which further reduces its ability to function.
Congenital vs. Acquired Shunts
Most liver shunts in dogs are congenital, meaning the abnormal vessel was present at birth. These are typically a single vessel and fall into two categories based on location:
- Extrahepatic shunts sit outside the liver and are more common in toy and small breeds, including Yorkshire Terriers, Cairn Terriers, Havanese, Maltese, Pugs, and Miniature Schnauzers.
- Intrahepatic shunts run through the liver itself and are more common in large breeds such as Irish Wolfhounds, Labrador Retrievers, Golden Retrievers, Australian Cattle Dogs, and Old English Sheepdogs.
Acquired shunts are a different situation. These develop later in life, usually as a consequence of chronic liver disease, severe scarring (cirrhosis), or conditions that increase blood pressure in the portal vein. When pressure builds up in the portal system, the body essentially opens up alternative routes for blood to escape, creating multiple small shunt vessels. Acquired shunts are harder to treat because the underlying liver disease is typically advanced.
Signs and Symptoms
Dogs with congenital shunts often show symptoms within the first year of life, though mild cases can go undetected until adulthood. The signs stem primarily from ammonia and other toxins reaching the brain, a condition called hepatic encephalopathy. Symptoms tend to be worse after meals, especially protein-heavy ones, because protein digestion produces the most ammonia.
Early, mild signs can be easy to miss or dismiss. Your dog may seem a little dull, confused, or slow to respond to commands. Some owners notice their dog stares into space or seems “not quite right” after eating. As the condition progresses, the neurological signs become more obvious: disorientation, aimless wandering, walking in circles, pressing their head against walls or furniture, loss of house training, personality changes (including unusual aggression), drooling, and poor coordination.
In severe cases, dogs can develop temporary blindness, seizures, or collapse into a coma. These episodes can come and go, which sometimes delays diagnosis because the dog seems fine between episodes.
Beyond the neurological signs, dogs with liver shunts are often smaller than their littermates, slow to gain weight, and may have a poor appetite. Urinary problems are also common. The liver normally converts ammonia into a harmless compound called urea. When the liver is bypassed, ammonia takes an alternative route and can form crystals or stones in the urinary tract, leading to frequent urination, bloody urine, or difficulty urinating.
How Liver Shunts Are Diagnosed
A bile acid test is the most common screening tool. Bile acids are substances the liver produces and recycles. In a dog with a shunt, the liver can’t recycle them properly, so levels in the blood rise. The test measures bile acids in two blood samples: one taken after fasting and another taken two hours after a meal. In healthy dogs, fasting bile acids are typically below 5 to 15 µmol/L. In dogs with shunts, fasting values exceed 25 µmol/L about 95% of the time, and post-meal values usually climb above 75 µmol/L.
Blood ammonia levels can also be measured directly. Elevated ammonia supports the diagnosis, particularly if neurological symptoms are present.
Once blood work suggests a shunt, imaging is needed to confirm its location and type. Abdominal ultrasound is the most widely used tool. In experienced hands, it can identify shunts with very high accuracy. Intrahepatic shunts are especially easy to spot on ultrasound, with sensitivity and specificity both reaching 100% in some studies. Extrahepatic shunts are slightly trickier, with sensitivity around 90% and specificity around 97%. In some cases, more advanced imaging like CT angiography is used to map the shunt vessel precisely before surgery.
Surgical Treatment
Surgery is the preferred treatment for most congenital shunts because it addresses the root problem: redirecting blood flow back through the liver. The goal is to close off the abnormal vessel so the liver begins receiving its normal blood supply again.
One of the most common techniques involves placing a device called an ameroid constrictor around the shunt vessel. This small ring is made of a material that slowly absorbs moisture from the body and swells over time, gradually squeezing the shunt vessel shut over about 20 to 40 days. The gradual closure is important. If the shunt were closed all at once, the sudden increase in blood flow to the liver could cause dangerously high pressure in the portal vein. The slow approach gives the liver’s blood vessels time to adapt and expand.
A key advantage of this method is that it typically requires only one surgery. Older techniques sometimes involved partially tying off the vessel and then going back in for a second surgery to complete the closure. With an ameroid constrictor, the device does the work on its own over several weeks.
Recovery after surgery generally involves a hospital stay of a few days, followed by several weeks of restricted activity at home. Most dogs continue on a special diet and medications for a period after surgery while the liver regains function. Follow-up bile acid tests help confirm whether the shunt has closed successfully.
Medical Management
Not every dog is a candidate for surgery. Some dogs have shunts that are difficult to access surgically, some have acquired shunts from advanced liver disease, and some owners may not be able to pursue surgery. In these cases, medical management aims to reduce the amount of toxins reaching the brain.
Diet is the cornerstone. Dogs with liver shunts are typically fed a restricted-protein diet, because protein digestion is the primary source of ammonia in the blood. The goal isn’t to eliminate protein entirely, since dogs still need it, but to feed moderate amounts of highly digestible protein sources (like eggs or dairy) that produce less ammonia during digestion. Frequent small meals are better than one or two large ones, because they prevent spikes in ammonia after eating.
Lactulose, a syrup given by mouth, is commonly prescribed alongside the diet. It works in the gut by drawing ammonia into the intestines and trapping it there so it gets excreted in the stool rather than absorbed into the bloodstream. Antibiotics targeting ammonia-producing bacteria in the gut are sometimes added as well.
Medical management can control symptoms effectively in many dogs, but it doesn’t fix the underlying problem. Dogs managed medically generally need lifelong dietary restrictions and medications, and their symptoms may worsen over time as the liver continues to receive inadequate blood flow.
Breeds at Higher Risk
The exact genetic basis of congenital shunts isn’t fully understood, but certain breeds are clearly predisposed. Yorkshire Terriers and Miniature Schnauzers are among the most commonly affected small breeds. Maltese, Cairn Terriers, Havanese, and Pugs also appear frequently. Among large breeds, Irish Wolfhounds, Labrador Retrievers, Golden Retrievers, Australian Cattle Dogs, and Old English Sheepdogs carry higher risk.
If you have a puppy from one of these breeds and notice slow growth, a poor appetite, unusual behavior after meals, or urinary issues, a bile acid test is a straightforward way to screen for a shunt. Early detection matters because younger dogs with smaller, underdeveloped livers tend to respond well to surgery. The liver has a remarkable ability to regenerate once normal blood flow is restored, and dogs diagnosed and treated early often go on to live normal, healthy lives.