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Summary
The liver is an organ that functions to 1) aid in the digestive process by helping to break down food into digestible nutrients, 2) help the immune system, 3) produce important blood chemicals and 4) filter toxins through enzymatic reactions that eliminate their poisonous effects (among other wonderful functions).
It's a multi-purpose organ that we think of mostly as one that secretes bile for digestion and biochemically breaks down bad stuff so that animals can survive the effects of toxins they're regularly exposed to in their environments.
Some dogs, unfortunately, (and some humans too) have a congenital, hereditary malformation that leads blood vessels to bypass the liver. It's called a "portosystemic shunt" but is also often referred to as a "hepatic shunt" or "liver shunt." Others have the "acquired" form of the disease, which is typically secondary to severe, diffuse liver ailments (usually seen in older dogs).
Here's what happens: The abnormal vessel(s) allow blood to go around or through the liver without stopping to clear the blood of its toxins or feed the liver its normal quantity of blood. The toxins then move along to the rest of the body.
Some shunts are "simple." A big vessel leading to the liver completely circumvents it. Instead of driving blood through the liver so that it can be "cleansed," it gets "shunted" completely around it. The blood (in which all the bad stuff goes when it enters the body) just keeps circulating, taking the untreated toxic waste to all the organs and tissues. This is called an "extrahepatic shunt," and it's most common is small breed dogs.
But the liver might well have shunts that travel through it, too. In these cases, called "intrahepatic shunts," one or more blood vessel is located in the liver but does not actually exchange blood with its tissues, effectively circumventing it. It's more common in larger breed dogs with and with acquired portosystemic shunts.
Symptoms and Identification
Animals with portosystemic shunts eventually die of common toxins and infections normal bodies don't stress over. But first, they usually show some or all of the following symptoms:
Abnormal behavior after eating
Pacing and aimless wandering
Pressing the head against the wall
Episodes of apparent blindness
Seizures
Poor weight gain
Stunted growth
Excessive sleeping and lethargy
Abdominal distention (from ascites)
Normally, we see the first sign of a portosystemic shunt in dogs when they're very young--before six months is common--but some less severely affected dogs won't show signs until a year of age or later.
Diagnosis is achieved through basic laboratory tests, bile acid testing, ultrasound exploration of the abdomen (which may or may not reveal the abnormal vessel(s), X-rays (sometimes revealing a small liver because of poor circulation). Sometimes a test called nuclear scintigraphy is done to confirm the diagnosis of a shunt. But in many cases, exploratory surgery is undertaken as a more immediate approach to definitive diagnosis
Affected Breeds
In the case of extrahepatic portosystemic shunts, small breeds are most affected. These include the Yorkshire Terrier, Miniature Schnauzer and, less commonly, Cairn Terriers and Maltese.
For intrahepatic portosystemic shunts, large and giant breeds are predisposed, especially the Irish Wolfhound and, less commonly, Australian Cattle Dogs, Golden Retrievers and Labrador Retrievers. But approximately one third of large dogs with shunts will have extra-hepatic shunts, so large breed predisposition is not limited to the intrahepatic form of the disease.
Treatment
Surgery to clamp off the abnormal vessel(s) is typically undertaken in the case of extrahepatic portosystemic shunts (for which many patients experience complete remission of symptoms and can live normal lifespans). Unfortunately, intrahepatic shunts may not be so easy to isolate and clamp. Furthermore, when many shunted vessels are present, it may be impossible to identify and ligate all of them.
Non-surgical treatment of shunts is undertaken when shunt ligation is impossible or when surgery is insufficiently curative (as when many shunts are present). In these cases, the goal is to minimize the symptoms of abnormal blood flow through the liver: to reduce toxins in the body, support the liver with supplements, relieve fluid-build-up in the abdomen and help the animal fight off infections.
Veterinary Cost
The expense of diagnosis and surgery can easily run into the mid-thousands, though simple clamping of single extrahepatic shunts will sometimes cost as little as $2,000-$3,000.
Medical therapy can be expensive depending on the array of drugs and supplements employed. It's not unusual for patients' owners to spend $100-$200 every month in medications and substantially more in frequent testing aimed at constantly monitoring the appropriate dose of these drugs.
Prevention
Because the mode of inheritance of portosystemic shunts is not clear, at the present time veterinarians recommend that affected dogs, their parents and their siblings not be bred.
References
Kerr, M.G. and van Doorn, T. 1999. Mass screening of Irish wolfhound puppies for portosystemic shunts by the dynamic bile acid test. Veterinary Record 144:693-696.