PSS in dogs stands for portosystemic shunt, a condition where an abnormal blood vessel allows blood from the digestive tract to bypass the liver entirely. Normally, everything a dog absorbs from food travels through the liver first for filtering and detoxification. With a PSS, toxins like ammonia skip that step and circulate directly through the body, affecting the brain and other organs. Most cases are congenital, meaning the dog is born with the abnormal vessel.
How a Portosystemic Shunt Works
In a healthy dog, blood from the stomach, intestines, pancreas, and spleen flows through the portal vein into the liver. There, specialized liver cells capture and neutralize toxins, process nutrients, and produce essential proteins. The liver acts as a biological filter for everything absorbed during digestion.
When a shunt is present, an extra vessel creates a shortcut that routes blood around the liver and directly into general circulation. This has two major consequences. First, toxins that should be neutralized, especially ammonia, build up in the bloodstream. Second, the liver never receives the blood flow and growth signals it needs to develop properly. Over time, the liver stays small (a condition called liver hypoplasia) and becomes increasingly unable to do its job, even for the blood that does reach it. This creates a worsening cycle of poor liver function and rising toxin levels.
Two Types Based on Location
Congenital shunts come in two forms. Extrahepatic shunts sit outside the liver and are far more common in small and toy breeds. Intrahepatic shunts run through the liver itself and tend to affect large breeds. In a study of over 200 dogs, 92% of operable shunts in large breeds were intrahepatic, while only 7% of shunts in small breeds were. The distinction matters because it influences which surgical approach a veterinarian will use.
Breeds at Higher Risk
PSS has a strong genetic component, and certain breeds are significantly overrepresented. Among small breeds, Maltese, Silky Terriers, Bichon Frises, Shih Tzus, Miniature Schnauzers, and Jack Russell Terriers are diagnosed more often than expected. For larger breeds, Irish Wolfhounds and Border Collies show higher rates. Australian Cattle Dogs also carry increased risk.
Interestingly, dogs from breeds not typically predisposed to PSS are more likely to have unusual or inoperable shunt anatomy if they do develop one. In predisposed breeds, the shunt anatomy tends to be more predictable and surgically accessible.
Signs and Symptoms to Watch For
The hallmark of PSS is hepatic encephalopathy, a set of neurological symptoms caused by ammonia and other toxins reaching the brain. Affected dogs may seem disoriented, stare blankly at walls, circle aimlessly, press their heads against surfaces, or have seizures. These episodes often worsen after meals, especially protein-rich ones, because digesting protein produces more ammonia. Some owners notice their dog seems “spacey” or unusually quiet, then returns to normal, making early signs easy to dismiss.
Beyond the neurological signs, dogs with PSS commonly show poor growth compared to littermates, chronic vomiting or diarrhea, excessive thirst and urination, and a general failure to thrive. Many are noticeably smaller than expected for their breed. Some dogs develop urinary problems first. Because the liver can’t properly convert ammonia and uric acid, these waste products get filtered by the kidneys instead, creating an ideal environment for ammonium urate bladder stones. In some cases, urinary stones or repeated urinary tract issues are what lead to the PSS diagnosis.
Most dogs with congenital PSS show signs within the first year of life, though mild cases can go undetected until adulthood.
How PSS Is Diagnosed
The most common screening tool is a bile acid test, which measures how well the liver processes and recycles bile acids. A blood sample is taken after fasting and again two hours after a meal. In healthy dogs, serum bile acid levels fall below about 25 μmol/L. Dogs with portosystemic shunts often show dramatically elevated levels. In one study, Maltese dogs with confirmed shunts had average bile acid levels of 192 μmol/L, while non-Maltese dogs with shunts averaged 137 μmol/L.
Elevated bile acids alone don’t confirm a shunt, so imaging is the next step. Abdominal ultrasound can often visualize the abnormal vessel directly and will typically show a smaller-than-normal liver. In some cases, more advanced imaging like CT angiography is needed to map the exact shunt anatomy before surgery.
Medical Management
Whether a dog is awaiting surgery or isn’t a surgical candidate, medical management focuses on reducing the toxin load the liver can’t handle. The cornerstone is a diet lower in protein that uses highly digestible protein sources. When protein is easier to digest, less of it reaches the colon where bacteria would break it down into ammonia.
Lactulose, a syrupy liquid given by mouth, plays a key role alongside diet. It works by making the colon more acidic, which converts ammonia into a form that can’t be absorbed back into the bloodstream. It also speeds up gut transit time, giving bacteria less opportunity to produce ammonia in the first place. Antibiotics are sometimes added to further reduce ammonia-producing bacteria in the intestines. Together, these treatments can significantly reduce neurological episodes, but they manage symptoms rather than fix the underlying problem.
Surgical Treatment and Outcomes
Surgery is the definitive treatment for most congenital portosystemic shunts. The goal is to gradually close the abnormal vessel so blood is redirected back through the liver. Gradual closure is critical because the liver and its blood vessels need time to adapt to handling the full volume of portal blood flow. Two common techniques exist for extrahepatic shunts: one uses a ring made of a material that slowly swells as it absorbs fluid, constricting the shunt over weeks, while the other wraps the vessel in a band that triggers a slow inflammatory response to close it.
Both approaches produce good to excellent outcomes with low risk. In a comparative study of 49 dogs, postoperative complication rates were similar between the two methods (around 23 to 26%), and overall surgical mortality was just 2%. Long-term results were strong: 100% of dogs in both groups achieved either a good or excellent clinical outcome. The ring method did show an advantage in one area, with no residual shunting detected on follow-up ultrasound, compared to about 32% of dogs treated with banding that showed signs of incomplete closure.
The bigger picture strongly favors surgery over long-term medical management alone. A study of 124 dogs found that surgically treated dogs had a dramatically better survival rate, with medically managed dogs facing roughly eight times the risk of death during the follow-up period. The frequency of ongoing clinical signs was also significantly lower in the surgical group, with the difference becoming especially clear four to seven years after treatment. After successful surgery, bile acid levels typically drop significantly, and the liver can begin to grow toward a more normal size.
Living With a Dog After PSS Treatment
Dogs that undergo successful shunt surgery often go on to live normal or near-normal lives. The liver has a remarkable ability to regenerate and grow once it starts receiving proper blood flow. Many owners report that their dog’s energy, appetite, and personality noticeably improve in the weeks and months following surgery.
Dogs managed medically can still have a reasonable quality of life, but they typically require lifelong dietary restrictions and medications. Their neurological episodes may recur, especially during periods of dietary indiscretion or stress. Regular monitoring of blood work helps track liver function over time regardless of which treatment path is taken.