Congenital portosystemic shunt

There are many liver problems that can affect cats but most of these affect older animals. One that tends to occur in young animals (often a few months old) is a congenital portosystemic shunt. A portosystemic shunt is an abnormal blood supply which bypasses the liver allowing toxic substances into the general circulation and preventing full use of the nutrients in the diet. Unfortunately, the signs of portosystemic shunts are often very vague and it can be hard to put all the signs together as a single illness. If you notice any odd signs in your kitten always report them to your veterinarian, even if they come and go without treatment. Early recognition and treatment of most conditions can prevent more serious problems later on.

©Larissa Barbosa / Pexels

A portosystemic shunt (often referred to simply as a ‘shunt’) is an abnormality of the blood supply to the liver. In a normal animal blood passing through the walls of the intestine picks up nutrients and toxins that have been absorbed from the gut. These blood vessels join together and drain into a large vein which delivers this blood to the liver where it is processed – useful elements (such as nutrients) are removed and packaged for delivery to other tissues in the body and toxic waste products and bacteria absorbed from the gut are filtered out to prevent them entering the rest of the body.

When a kitten is in the uterus the function of the liver, ie ‘cleaning’ the blood and processing nutrients is performed by the mother’s placenta. Since nutrition is not passing through the intestine there is no need for the blood to be delivered from the intestinal circulation through the liver and therefore it is diverted around the liver and straight to the heart. After birth the large blood vessel diverting blood around the liver quickly shuts down forcing the blood supply back through the liver. In some animals, however, this vessel does not close down properly, and blood continues to bypass the liver – this is known as a congenital portosystemic shunt.

Shunts can also develop in later life – these are called acquired portosystemic shunts. An acquired shunt develops when there is long term damage to the liver which results in scarring of the liver. As scar tissue develops in the liver the liver gets smaller and the blood pressure inside the liver increases. Blood tends to divert around the liver through multiple small existing vessels which stretch to accommodate the new blood flow. With time it becomes progressively easier for blood to flow around the liver than to pass through it and more blood bypasses the liver.

This factsheet focuses on the diagnosis and management of congenital portosystemic shunts (CPSS).

Congenital shunts can be inherited and are more common in some breeds particularly Siamese, Himalayans and Persians are more likely to be affected. Shunts that develop later in life due to liver disease can occur in any animal and are obviously secondary to liver damage that leads to scarring (fibrosis) of the liver.

The signs of a portosystemic shunt may begin at any age from a few weeks of age up to many years. A number of cats with portosystemic shunts have unusually copper colored irises.

Most animals with a shunt are poorly grown compared to their litter mates. However, if you have just one pet you may not recognize that it is not growing as fast as it should. Vomiting, diarrhea, eating abnormal substances (pica) and anorexia are also common in cats with shunts but these signs tend to be intermittent and may be put down to food intolerances, or scavenging at first.

Since there is more blood flow from the gut after eating, signs in cats with shunts are often most noticeable within a few hours of a meal. The unexpected substances (that would normally be removed by the liver) circulating in the blood can cause effects in the brain. Affected cats may develop odd behavior, signs of brain derangement (staring into space, circling, blindness or seizures) or signs of a headache, ie depression or pressing their head against a wall.

Cats with portosystemic shunts are at increased risk of developing stones in the bladder. These stones can cause discomfort in urination and blood in the urine.

The signs are very varied and may be very difficult to recognize in some animals. Since the signs are so different (and most animals have a different combination of signs) it is often hard to put all the signs together and recognize that they are caused by a single illness. Many animals with shunts have visited their veterinarian with many different problems on a regular basis before a diagnosis is made.

Cats with shunts may have poor appetites and often salivate excessively (possibly due to nausea). Another common problem in animals with shunts is a susceptibility to repeated bacterial infections. Because the bacteria that enter the blood from the gut are not being removed by the liver, they can enter the general circulation and can spread throughout the body. One telling sign is a prolonged recovery from anesthesia or sedation. Signs may be first noticed when the affected cat is neutered and takes several days to fully recover from the anesthetic.

Your veterinarian may suspect that your cat has a shunt from the history you report, or they may be able to recognize that your kitten is not growing well. However, since many of the signs of a shunt are so vague it can be very difficult to suspect the diagnosis of a shunt immediately. Suspicions may be raised if you have repeated visits to the veterinary practice and the veterinarian can look at a longer-term history of signs ranging over months. Cats with congenital shunts also often have heart murmurs so investigation of this should be considered.

Once a shunt is suspected diagnosis can usually easily be made on a blood sample to test liver function (usually two samples taken before and after eating). Your veterinarian may also want to do some imaging tests, particularly ultrasound of the liver. It is usually possible to see the abnormal blood vessel in the liver on an ultrasound examination although your veterinarian may need to refer your pet to an experienced ultrasonographer for this examination.

Since a portosystemic shunt usually occurs due to a failure of a blood vessel to close off at birth the best treatment is a surgical procedure to tie off the blood vessel. However, this is a complex operation and in some cases the blood vessel runs through the liver tissue and therefore cannot be accessed easily at surgery. A further complication arises because in animals with congenital shunts the liver fails to develop properly as the kitten grows and so sometimes when the vessel is closed off the liver is unable to deal with all the blood flow redirected through it. Complications around the time of surgery are not uncommon and can be very serious so this procedure is best done in a special surgery center where the veterinarians have experience of dealing with these cases. Post-operative care for these patients can be complex and quite prolonged. In the majority of cases surgery is successful in achieving full or partial closure of the CPSS, clinical signs improve, and most cats become normal following successful surgery.

Medical management of cases is used to stabilize animals before surgery and in those cases where surgery is not possible for whatever reason. This involves the feeding of a low protein diet (to minimize the toxic substances being produced in the gut and therefore reaching the blood stream). Antibiotics and lactulose may also be prescribed – these help to reduce the numbers of bacteria in the gut that produce the toxins. Some cats do quite well on special diets whereas in others the signs of disease may persist (although often at a lower level) despite treatment.