Most people are either unfamiliar with the term "shunt" or are used to its application as a treatment to drain excess fluid from the brain following a stroke.

There is another type, however, that can occur in animals, including dogs. Specifically, a portosystemic shunt is an abnormal connection between the hepatic portal vein and the rest of the circulatory system. This vein connects the gastrointestinal tract to the liver. This abnormal connection in turn causes blood from the gastrointestinal tract to bypass the liver, depriving it of oxygen and vital nutrients. When this occurs, the liver is unable to completely perform its own vital functions, including assisting with the body's metabolism and eliminating toxins from the system. The final effect is to expose the affected animal's body to toxic byproducts from its own digestive system.

These types of shunts can be congenital, meaning they are present at birth. They also can appear at the result of other diseases that strike later in the dog's life. About 75 percent of all cases are congenital, and these cases appear more often in purebred than mixed-breed dogs. Particularly affected are Irish Wolfhounds, Miniature Schnauzers, and Yorkshire Terriers.

Portosystemic shunts are subdivided into two categories, based upon where the malformed blood vessel is located. When it is outside the liver, the shunt is termed extrahepatic; when it's within the liver tissue, it's classified as intrahepatic. Somewhere between 61 and 94 percent of all congenital shunts are extrahepatic, and are usually seen in smaller breeds of dog. Intrahepatic shunts, meanwhile, only make up 6 to 39 percent of congenital shunts, and are seen more often in large and giant breeds of dog, including the Irish Wolfhound and Golden Retriever.

In some cases, shunts appear as a complication of other liver disease that causes sustained pressure on the portal vein. These cases are seen most often in older dogs that have cirrhosis, hepatitis, or other major liver disease. Unlike congenital cases, more than one blood vessel is usually affected.

Dogs with portosystemic shunts will display a variety of symptoms, as their liver is unable to eliminate toxins, drugs, and bacteria absorbed by the gastrointestinal tract. These are reabsorbed into the body, where they can damage the central nervous system and cause a condition known as hepatic encephalopathy. Since the liver itself is deprived of blood, it also usually is underdeveloped, further impairing its ability to function. The dog itself will appear small and underdeveloped, with unhealthy-looking skin and hair.

Affected dogs experience frequent vomiting and diarrhea, plus more subtle problems like lethargy, depression, anorexia, and nervous system-related symptoms like disorientation, temporary blindness, weakness, excess saliva, seizures, head pressing, and a circling walk. These symptoms tend to appear and ebb, and may occur after the dog has eaten a protein-rich meal, causing a high level of neurotoxins to be dumped into the dog's digestive system.

Portosystemic shunts can be treated through diet, medication, antibiotics, and/or surgery, and the choice of treatment is made on a case-by-case basis. Factors under consideration are the dog's age and physical condition, the severity of its symptoms, and the type and location of the shunt itself. The best results are seen in dogs that are surgically treated. However, the procedure is delicate and expensive, and may not be financially feasible for many pet owners. Dogs that are surgically treated, however, often live for many years with a reasonably high quality of life.