By Jerold S Bell, DVM, Tufts University School of Veterinary Medicine
(This article originally appeared in the “Healthy Dog” section of the April, 2005 AKC Gazette)
Five out of every 1,000 dogs in the general population are born with an inherited liver shunt. This condition, also called a porto-systemic shunt (PSS), is an abnormal blood vessel that bypasses the liver. The portal vein carries blood drainage from the gastrointestinal system. This blood contains nutrients, but also the waste products from digestion. Because of these waste products, the portal blood is kept separate from the rest of the (systemic) blood supply, and sent to the liver for processing.
A major portion of the waste product from protein digestion is ammonia. The liver metabolizes ammonia into urea, which is then sent to the systemic circulation through the vena cava (the major vein in the body). The kidneys filter the urea into the urine for disposal.
A porto-systemic shunt bypasses the liver, placing the portal blood into the vena cava instead. The high level of unprocessed blood ammonia and waste products can affect the brain, causing drooling, mental dullness, and seizures. This condition, called hepatic encephalopathy, can worsen after a high-protein meal. Because of the lifelong effect of waste products on all cells in the body, affected dogs can be stunted in size.
A liver shunt can be diagnosed by measuring elevated blood bile acid and ammonia levels. Most affected dogs showing clinical signs of hepatic encephalopathy can be diagnosed through a single (non-fasted) blood level. Pre-breeding screening for subclinically or mildly affected dogs requires paired, fasted, and post-meal values to make a diagnosis.
The type of liver shunt varies between affected dogs. The shunt may be a single large blood vessel, or may involve many small vessels. It can run along the outside of the liver, or be totally encased in the body of the liver. Large shunts can be diagnosed by an experienced ultrasonographer, or through a surgical contrast radiograph called a portogram. Some dogs have a disorder called hepatic microvascular dysplasia (HMD), in which the defective vessels connecting systemic and portal circulations are not large enough to be identified by ultrasound. Because of this, the definitive diagnosis for the condition requires blood measurements.
The best way to treat a young dog with a liver shunt is through surgery. This is possible if the shunt is a large blood vessel, either inside or outside the liver. A surgeon narrows the shunt, so that a majority of the portal blood will now enter the liver. The shunt cannot be completely closed, or the liver – which because of the shunt is not accustomed to receive a high volume of portal blood – will swell and back up. This can cause fluid to build up in the abdomen (ascites), and cause liver failure. By narrowing the shunt, most affected dogs will go on to live normal lives.
If the liver shunt is microvascular, or if surgery is not a possibility, medical management may reverse the signs of hepatic encephalopathy. One third of affected dogs do well with medical management, living an average of seven years. Diets with high levels of crude protein should be avoided. Depending on the extent of clinical signs, dogs should be fed either a senior diet, or a special prescription diet. A medication called lactulose will bind to ammonia in the intestines. This prevents ammonia from being absorbed into the portal circulation, so it is eliminated in the stool. Certain antibiotics will also reduce the amount of ammonia producing bacteria in the intestines.
Thirty-three breeds are significantly more likely to have a liver shunt than the general dog population. The breeds with the highest risk include: the Havanese, Yorkshire Terrier, Maltese, Dandie Dinmont Terrier, Pug, Miniature Schnauzer, Standard Schnauzer, Shih Tzu, Bernese Mountain Dog, and Bichon Frise. The majority are small-sized breeds, but the condition is also seen in large breeds, such as the Irish Wolfhound, Scottish Deerhound, and Old English Sheepdog.
Dr. Karen Tobias, a surgeon at the University of Tennessee College of Veterinary Medicine, has been performing epidemiological studies on congenital canine liver shunts. She has found that the incidence of liver shunts in Yorkshire Terriers has increased more than 11 times over the past 20 years. On average, approximately three percent of all Yorkshire Terriers have a porto-systemic shunt. The genes causing liver shunts in the breed are old and widespread, as inbreeding does not significantly increase the incidence of the disorder.
Tobias has found that mating two (surgically corrected) affected Yorkshire Terriers produced normal offspring – which eliminates simple autosomal recessive as the mode of inheritance. Tobias has also found that if two Cairn Terriers with microvascular shunts are bred together, they can also produce affected offspring with single, large liver shunts. This finding suggests than liver shunts can show variable expressivity, and that the single vessel and microvascular shunts may be caused by the same genes.
Genetic studies into liver shunts in Yorkshire Terriers, Cairn Terriers, Irish Wolfhounds, and Maltese have all proven a hereditary basis. It appears to be autosomal, as there is an equal ratio between affected male and female dogs. It is probable that the condition is polygenic, or controlled by more than one gene pair.
Tobias recommends testing fasting and post-feeding bile acids and blood ammonia levels on all prospective breeding stock, and to refrain from breeding dogs with elevated levels. This will eliminate phenotypically affected dogs from breeding. There is an increased risk for phenotypically normal siblings and first-degree relatives of affected dogs to be carriers of liver shunt liability genes. Therefore, breadth of pedigree normality (normal- testing littermates) is important in selecting breeding stock.
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