Introduction

The liver plays a central role in the metabolism of food and detoxification of waste products and can be seen as both the 'power house' and 'filter' for the body. Liver disease is common in both dogs and cats, although they typically suffer from very different conditions with very different nursing needs (which contrasts with many other organs). The liver has a very big 'functional reserve', which means that a large amount of liver mass is lost before clinical signs are seen. Unusually, it also has a tremendous regenerative capacity, setting it apart from many other organs such as the kidney, which cannot regenerate. The liver will do all it can to 'grow back' after an insult, if it is given the chance. Part of 'giving it a chance' is appropriate feeding: feeding is a very important part of treatment for liver disease. The most severe cases of liver disease may be in the intensive care unit with severe neurological signs, so the nurse has a very important role to play in the treatment of cats and dogs with liver disease.

Overview of the Role of the Liver

The liver has an amazing array of functions, many (or all) of which are disrupted in liver disease, explaining many of the problems and clinical signs seen. The liver is joined to the duodenum by the bile duct, which excretes bile into the small intestine, not only to get rid of waste products of red blood cell breakdown but also, importantly, to allow effective fat digestion by allowing emulsification of fat in the gut with bile acids. All of the portal blood draining the guts passes through the liver first before it reaches the general circulation. It is carefully filtered in the sinusoids before exiting the liver again. In a normal animal, none of the blood draining the intestines escapes this 'filtering' process. The liver is also central for metabolism: it is important for glucose and fat metabolism and storage and production of energy; it is the place where amino acids are built up into proteins, or broken down and where ammonia (which is a waste product of protein metabolism and is very toxic) is detoxified by converting it to urea to be excreted in the kidneys; it is the place where albumin and many other plasma proteins are made and it is central to the activation, synthesis and storage of many vitamins and the synthesis of most coagulation factors and anti-coagulant factors. The liver breaks down many hormones as they pass through (such as insulin and cortisol) and is also very important in detoxifying exogenous toxins (such as drugs we administer or alcohol). It plays an important role in the immune system, with large macrophage-like cells called Kupffer cells clearing blood-borne infections, particularly from the gut, and it plays an important role in making new blood cells in utero and in times of extreme need in the adult.

Liver Diseases

Liver diseases in dogs and cats can be acute or chronic. The commonest diseases in cats are both acute and chronic disease of the biliary tract (cholangitis) and hepatic lipidosis (massive fat build-up in the liver). Hepatic lipidosis used to be thought to be uncommon in the UK but it is becoming increasingly recognised. Cats rarely suffer from severe liver fibrosis or cirrhosis, unlike dogs. The commonest diseases seen in dogs are chronic hepatitis (often progressing insidiously to cirrhosis) and copper storage disease. Acute hepatitis also occurs in dogs, although much less commonly and usually due to toxicity or infections (such as leptospirosis). Both dogs and cats can also suffer from congenital portosystemic shunts (PSS) and various forms of hepatic neoplasia (primary and secondary).

Chronic hepatitis is common in dogs, but in most cases the cause remains unknown and the treatment is non-specific and supportive; consequently, these dogs often die of their disease. True copper storage disease is not recognised in cats, but is seen in certain breeds of dog. It is still very common in Bedlington Terriers and it has more recently been reported in some Dalmatians in the USA (and one or two are now being recognised in the UK) and Labrador Retrievers in the USA and Holland (but not to the author's knowledge yet in the UK). In true 'copper storage disease', early treatment with a low-copper diet, increased zinc and (where necessary) copper chelators can lead to a good long-term outcome.

Biliary tract disease in cats is often associated with concurrent pancreatitis and/or inflammatory bowel disease (leading to the term 'triaditis'). These concurrent diseases obviously increase the challenges of diagnosis and treatment.

Hepatic lipidosis in cats is worthy of a special mention. It can occur either as a primary disease (usually in a previously 'fat' cat which for some reason goes off its food, e.g., a change of diet from its usual) or it can occur secondary to another disease (such as pancreatitis or diabetes), which both reduce the cat's food intake and increase its mobilisation of fat from fat stores in the body. In the 'secondary' cases, the cat may not be noticeably 'fat' at the start. The underlying cause is still incompletely understood, but it occurs because of a combination of increased peripheral fat breakdown, which floods into the liver, combined with a reduced ability to mobilise the fat again from the liver and a concurrent drop in appetite (rather like anorexia nervosa in people). Dogs do not suffer from this, but in cats it is a very serious disease and usually fatal unless treated. The liver becomes clogged up with fat and literally can't function. The central treatment for this condition is feeding, which usually requires tube feeding because of the concurrent anorexia. With intensive feeding, the mortality can be greatly reduced. It is very important to be aware of the potential of cats to get hepatic lipidosis and never to advise owners of fat cats (or any sick cats!) that it is acceptable for them to stop eating for a period of time: any new diet should be introduced carefully and gradually in these cats and any anorexia taken very seriously.

Treatment of Liver Disease

Although dogs and cats suffer from different types of liver disease, the overall aims of treatment are the same in both:

 Treat the underlying condition/cause, if we know what it is (which we often don't!)

 If the cause is not known, at least try to slow the progression of the disease

 Avoid overworking the liver (i.e., avoid foods, drugs etc. which will be hard work for it to metabolise or detoxify)

 Support the liver and give it sufficient food to regenerate

 Treat the clinical signs of liver disease: particularly ascites; hepatic encephalopathy (neurological signs associated with congenital PSS and severe chronic liver disease in dogs); clotting problems and gut ulceration

Achieving these aims usually involves using antibiotics and anti-oxidants. Drugs which help bile flow (ursodeoxycholic acid) and steroids are also used in many cases. However, worryingly, there is very little 'evidence base' for any of the drug therapy we use in liver disease in dogs and cats and most of it is based on experience, anecdote and extrapolation from humans. We need many more clinical studies before we can define the 'ideal' treatments for liver diseases in dogs and cats. Careful avoidance of drugs which are metabolised in the liver or increase the clinical signs of liver disease is also important; this particularly applies to the choice of anaesthetics, for example, for animals with chronic liver disease. Dogs and cats with severe hepatic encephalopathy (particularly dogs and cats with PSS) may fit or be comatose and these animals will need intensive management with fluid therapy, sedatives as necessary and enemas and careful monitoring of blood electrolyte and glucose levels.

Feeding Dogs and Cats with Liver Disease

Appropriate and careful feeding is central to the treatment of liver disease. It used to be thought that animals (and humans) with liver disease should have the protein content of their diet quite markedly reduced to avoid increased ammonia production and liver 'work'. However, there is now plentiful evidence in dogs and people that not only should the protein level of the diet be normal, but also that marked protein restriction may actually increase ammonia production and neurological signs by increasing breakdown of the animal's own protein (i.e., muscle) because of protein-calorie malnutrition. Enough protein in the diet is important to allow hepatic regeneration, but too much protein or a poor-quality protein will increase liver 'work'. Therefore, in many cases we advise a normal level of a high-quality, highly digestible protein. For many dogs and cats with liver disease, diets marketed for intestinal disease are ideal, particularly for cats with cholangitis and concurrent inflammatory bowel disease. The hepatic/liver diets are also useful because they contain other beneficial ingredients, but the author strongly advocates adding extra high-quality protein in most patients because the 'liver' diets are rather protein restricted. Cats with hepatic lipidosis are a special case, as they need a high-protein diet: which usually means tube feeding an intensive care diet.

References

1.  Watson PJ. Liver disease. In: Hall, EJ; Simpson, JW; Williams, DA. eds. BSAVA manual of small animal gastroenterology (second edition). Gloucester: BSAVA Publications, 2005; 240-268.

2.  Watson PJ. The pathophysiology of liver disease part I: Normal hepatic structure and function: UK Vet 2004: 9(5): 26-31.

3.  Watson PJ. Pathophysiology of liver disease part 2: Acute and chronic liver disease. UK Vet 2004: 9(6): 31-38. Watson PJ. Treatment of liver disease part 1: general principles. UK Vet 2004; 9(7): 39-45.

4.  Watson PJ. Treatment of liver disease part 2: treatment of specific canine and feline diseases. UK Vet 2004; 9(8): 39-48.