The liver

Regional Anatomy

In the domestic animals the whole of the liver, when normal, is situated in the concavity of the diaphragm, and it does not extend sufficiently beyond the costal arch, even on the right side, for its border to be palpable.

Horse. The greatest part of the liver is situated on the right of the median plane. The parietal surface is in contact with the diaphragm, and the visceral surface is moulded to some extent by the organs which lie against it. The highest point of the liver is level with the right kidney, and its lowest about 7-10 cm from the abdominal floor, opposite the sternal extremity of the seventh or eighth rib on the left side. The right border, which is thin, extends backwards to the sixteenth rib, just below its middle. At this point the liver is separated from the ribs by the diaphragm.

Cattle. The liver is situated almost entirely on the right of the median plane. The parietal surface is mainly in contact with the right portion of the diaphragm but a small area is in direct contact with the last two or three ribs, occasionally even with the flank.

Sheep. The liver is situated entirely to the right of the median plane. The right border may project beyond the costal arch at its lower part.

Pig. The greater part of the liver lies to the right of the median plane. A small part of the parietal surface is in contact with the abdominal floor in the xiphoid region and also at a point ventral to the right costal arch. The ventral border of the two central lobes extends 2-5 cm behind the xiphoid cartilage.

Dog. The parietal surface conforms to the concavity of the diaphragm and the contiguous parts of the ventral abdominal wall. The ventral border of the lobes is related to the abdominal floor a variable distance behind the xiphoid cartilage above the rectus abdominis muscle.

The internal structure of the liver comprises a large number of morphologically identical lobules each of which is capable of performing all the functions of the liver. Assessment of liver disease and of disturbed liver function is not only made complex by the many metabolic activities performed, but also because the organ possesses considerable powers of regeneration. In the latter regard, clinical signs of hepatic dysfunction only appear when about 75 % of all lobules have been rendered inactive, in spite of which complete regeneration can occur within a few weeks.

The main functions of the liver include secre­tion and excretion of bile, protein, fat and carbohydrate metabolism, detoxication, fibrino-gen, prothrombin and heparin production, vitamin A formation from carotene, blood volume regulation and iron and copper storage.

Disturbance of hepatic function is much more likely to occur in diffuse diseases of the liver than with focal lesions, which produce their effects by locally formed toxins or by pressure on other organs, or on the liver itself. The majority of liver diseases in animals are secon­dary to generalized infection or spread from another organ; primary hepatic disease, which is rare, is usually the result of poisoning by toxic agents of chemical or plant origin. In any diffuse disease the various functions of the liver are disordered to the same degree, consequently the clinical manifestations vary in intensity according to the severity of the hepatic injury. There are some functions which, however, when disturbed are more likely to be responsible for clinical signs. These include the maintenance of normal blood sugar levels through the medium of glycogen, formation of some of the plasma proteins, maintenance of bile production and excretion, formation of prothrombin and detoxi­fication and excretion of many toxic substances.

Clinical Examination

The evidence obtained during the general clinical examination may suggest disease of the liver. Special attention can be given to the liver through the medium of a physical examination coupled, when necessary, with biochemical tests, biopsy and occasionally radiographic examination.

The position of the liver in most species of animals is such that physical methods of exami­nation are of limited value. In cattle, dogs and cats, gross enlargement of the liver (hepato­megaly) causes the right or the ventral border to project beyond the costal arch or xiphoid cartilage, and the edge is usually uniformly thickened and rounded, in contradistinction to the sharp edge of the normal liver. Local enlarge­ment, such as occurs in neoplasia or hydatidosis, etc., when it impinges on the border, causes it to be irregular. Palpation of the liver is unrewarding in horses and pigs because of the rigidity and thickness of the abdominal wall.

Gross enlargement of the liver occurs as the result of severe congestion arising from chronic congestive heart failure, and also particularly in the horse, in those forms of plant poisoning in which fibrosis is a prominent feature, in multiple abscessation, in metastatic neoplasia and in occasional cases of hydatidosis. The increase in size of the liver in acute diffuse hepatitis is not sufficient to be appreciated by physical exami­nation. In terminal fibrosis, the reduction in the size of the liver is often not appreciated unless the right or the ventral border is distorted, when palpation may reveal the rather rigid irregular line.

By means of percussion it is possible to obtain some idea of the extent of the dull area overlying the liver, although it is unlikely that hepatic enlargement would regularly be recognized in this way. Strong percussion or firm palpation over the liver area is of value in recognizing the presence and extent of hepatic pain. The entire area should be subjected to percussion in order to avoid missing focal pain associated with a discrete lesion.

Clinical Signs of Liver Disease

Jaundice

Secretion of bile is one of the most important functions of the liver. Bile is composed of bile salts (derived from the metabolism of choles­terol), bile pigments (excretory products derived from the catabolism of haemoglobin liberated from effete erythrocytes), alkaline phosphatase, water and various lipids (cholesterol, lecithin, etc.). Bile salts are involved in the digestion and absorption of fats; about 90% recycles from the small intestine back to the liver and is resecreted in the bile. The initially formed free bilirubin (indirect or unconjugated bilirubin) is conjugated in the liver cells with glycuronic acid thus forming bilirubin glycuronide (direct or conju­gated bilirubin). When the liver is damaged, in primary and secondary diseases, the quantities of bile pigment in blood, urine, faeces and body tissues will change as the result of deranged excretion.

Jaundice, which is caused by the accumulation of bilirubin, is manifested clinically by orange-yellow discoloration of the mucous membranes, submucosal tissues, unpigmented skin and other body structures, often occurs in diseases of the liver and biliary system, but also in diseases in which these organs are not significantly damaged; conversely, it may not develop even when the liver is extensively involved as in acute hepatitis. The intensity of the tissue staining is much more pronounced with direct than with indirect bilirubin, so that jaundice is more severe in cases of obstructive jaundice than in haemolytic jaundice. The staining is due to accumulation of bilirubin in the tissues, especially elastic tissue, so that it is usually most obvious in the sclera.

Jaundice has been classified in many ways and for clinical purposes the most simple system is the best. Perhaps the most basic differentiation is between jaundice with and without biliary obstruction.

1. Mechanical or obstructive jaundice. This is jaundice arising from obstruction of the bile ducts or common bile duct (cholestasis). The discolored tissues are greenish yellow, and when biliary obstruction is complete bile pigments are absent from the faeces, the serum levels of direct bilirubin become markedly elevated so that large amounts are excreted in the urine which is free of urobilinogen because no bile pigment is available in the intestine for its formation. In partial biliary obstruction similar variations occur in the serum and urine, but some bile pigments occur in the faeces and urobilinogen in the urine.

Obstruction or compression occlusion of the larger bile ducts or common bile duct can be caused by nematodes, trematodes or inflamma­tion of the ducts themselves. In pigs, Ascaris suum is an important cause of biliary obstruction and secondary cholangitis, especially in animals which have been subjected to transportation for prolonged periods. Cattle and sheep occasionally develop cholangitis and cholecystitis as the result of infestation by Fasciola hepatica or Dicrocoelium dendriticum. Occasionally, also, ascending cholangitis, developing secondarily to catarrhal enteritis, may be sufficiently severe to cause obstruction of the common bile duct at its termination. This form of obstruction has been observed in horses with parasitic enteritis. Obstruction of the bile ducts or common bile duct by calculi or compression by neoplasms is a rare event in animals. In all these conditions, the cause is extrahepatic in origin.

Mechanical bile stasis can also occur when there is constriction and obliteration of the intrahepatic biliary canaliculi after hepatitis, and in certain forms of hepatic fibrosis. The changes in the blood, urine and faeces are the same as those which occur in extrahepatic biliary obstruc­tion.

2. Haemolytic or overproduction jaundice. In this form the affected tissues are distinctly yellow and there is haemoglobinuria and anaemia. The urine contains no bilirubin but there is an increased amount of urobilinogen. The indirect bilirubin content of the serum is greatly elevated. This type of jaundice is caused by excessive intravascular haemolysis which results in an abnormally large quantity of bile being produced. Haemolytic jaundice is very common in animals and may be caused by bacterial toxins (bacillary haemoglobinuria, leptospirosis), protozoa (babe-siasis, anaplasmosis, eperythrozoonosis), viruses (infectious equine anaemia), inorganic and organic poisons (chronic copper poisoning, phenothiazine poisoning in horses), vegetable poisons (rape and other cruciferous plants), snake venoms and immunological (iso-immunization haemolytic anaemia of foals, puppies and piglets) and other blood incompatability reac­tions.

3. Jaundice due to hepatic cell degeneration (toxic jaundice). This occurs when the paren­chyma of the liver has been damaged to a sufficient degree to interfere with the normal formation and elimination of bile. The serum bilirubin level is elevated and the urine contains an increased amount of direct bilirubin and urobilinogen. The body tissues including the unpigmented skin areas are reddish-yellow or brownish-yellow in colour. The cause may be any disease associated with acute or chronic diffuse hepatitis.

In an animal with complete obstructive jaundice, because of the absence of bile, the faeces are light-grey, clay-like and firm (acholic or bile-free faeces). They also contain a propor­tion of undigested fat. In haemolytic jaundice the faeces are usually normal in colour, or otherwise somewhat yellow. The differences are explained by the fact that in obstructive jaundice, when occlusion is complete, the intestine is deprived of bile pigments which normally impart colour to the intestinal contents. In haemolytic jaundice, in contra-distinction, an excessive quantity of bile is released into the intestine.

Neonatal jaundice, which is rarely, if ever, observed clinically in young animals, is probably caused by biliary retention rather than exces­sively rapid breakdown of fetal erythrocytes. Otherwise, severe jaundice developing in young animals, particularly foals and puppies, soon after birth, is usually caused by iso-immunization of the dam during pregnancy. The disease results from the natural occurrence of inherited blood groups with anti-erythrocyte antigens, in species such as horses, mules, pigs and dogs. In this situation the fetus has an equal chance, in matings between parents of incompatible blood groups, of inheriting erythrocyte antigens from the sire. The antigens may then traverse the placenta and, if not already part of the dam's complement of antigens, they will stimulate the production of erythrocyte antibodies. These antibodies, although they may be present in high titre in the blood of the pregnant mare from the eighth month of pregnancy onwards, are unable to reach the foetus because they are molecularly too large to diffuse across the placenta, but they are transferred to the newborn animal in the colostrum, causing massive destruction of its erythrocytes with severe haemolytic anaemia, haemoglobinuria and jaundice.

False jaundice. This is a situation in which yellow colouration of the body tissues occurs which is not caused by bile pigments but by other colouring substances, the commonest of which is carotene, a complex pigment found in young green plants. Deposition of carotene in the tissues is most marked in certain breeds of cattle, particularly the Guernsey; it also occurs to a moderate extent in horses eating green plant foods.

Nervous and Other Signs

In animals with diffuse hepatic disease, apart from jaundice, which is by no means a constant feature, other signs may be observed indicating involvement of the nervous system, or oedema and emaciation, deranged intestinal function, haemorrhagic diathesis and abdominal pain. The main nervous signs include dullness, anorexia, muscle tremor and weakness, compulsive walking, head-pressing, hyperexcitability, failure to res­pond to customary signals, and convulsions. Biochemical aspects which, it has been suggested, might be responsible for these changes in behaviour include hypoglycaemia, accumulation of excess amino acids or of acetylcholine due to failure of the usual hepatic detoxification mechanisms.

Oedema and emaciation are the result of failure by the liver to anabolize amino acids and proteins. This leads to a fall in plasma protein and thus a reduction in plasma osmotic pressure with loss of fluid into the tissues. The oedema is most marked in the intermandibular space. The alimentary tract syndrome consisting of anorexia and constipation with intermittent attacks of diarrhoea is the result of the partial, or complete absence of bile salts in the intestines (bile salts possess both laxative and disinfectant properties). In these circumstances absorption of fat-soluble vitamins, especially vitamin K, is seriously reduced. The formation of prothrombin, fibrinogen and thromboplastin is impaired in all severe, diffuse diseases of the liver. The deficiency of prothrombin, which along with other similar factors requires the presence of vitamin К for its synthesis, leads to prolongation of the blood clotting time and, therefore, the appearance of tissue haemorrhages. Abdominal pain in diseases of the liver arises from two mechanisms: tension of the capsule from increase in the size of the liver and lesions involving the capsule itself. Distension of the liver occurs in acute inflam­mation and in congestive heart failure, and is due to vascular engorgement. The pain results from stimulation of the subcapsular pain end-organs by inflammatory or neoplastic involvement of the capsule or the subcapsular parenchyma of the liver. Clinically, pain of hepatic origin causes arching of the back, disinclination to move and tenseness of the abdominal wall.

Liver Function Tests

An appreciation of the tests that would most effectively measure the degree of liver derange­ment in disease can be obtained from a considera­tion of those hepatic functions, which, when deranged, are responsible for clinical signs. The tests can be classified into five groups: those that measure secretory and excretory functions, those that measure the metabolic activity of the liver, those which measure protein, lipid and carbo­hydrate metabolism, those which test detoxifi­cation mechanisms and finally serum enzyme tests.

As yet, laboratory tests for assessing hepatic function, because they are exacting, time-consuming and somewhat expensive, have not been generally applied in clinical work; normal criteria for most of the selected parameters have been established, but the significance of the variations which can occur have not been exactly determined in all cases. The accurate determina­tion of particular body constituents may supply much information relating to liver function, provided it is realized that they vary from other than hepatic causes.

Secretory and excretory functions. These are measured by determining whether there is any alteration in the quantities of bile pigment in blood, urine and faeces, and also by measuring the excretion rate of bile pigment, or more usually bromsulphalein, following intravenous injection. Bile pigments and bile salts are normally present in the blood in such low concentrations that none, or only a trace in the case of bile pigments, appears in the urine. Consequently the state of bile metabolism in an individual animal can be investigated by per­forming suitable tests on a urine sample. Unfortunately up to 25 % of normal cattle have traces of bilirubin in the urine, so that the interpretation of positive results in this species requires careful evaluation, paying particular attention to the clinical examination and the results of other more meaningful liver function tests.

A variety of tests are available for the detection of bilirubin in urine, including Fouchet's test for which two solutions are required: (a) a 10% solution of barium chloride and (b) Fouchet's reagent which is made up as follows:

Trichloracetic acid 25 ml

Distilled water 100 ml

Ferric chloride 10% solution 10 ml

The urine should be boiled and filtered if protein is present and, in the event that the urine is alkaline, acidified with a little acetic acid. Add 5 ml of the barium chloride solution to about 10 ml of the urine, mix thoroughly and filter. The urine damped filter paper is then laid flat on a dry filter paper and one drop of Fouchet's reagent is placed on the precipitate. The presence of bile pigment is indicated by the appearance of a green or blue colour.

A proprietary tablet (Ictotest, Ames Co.), containing a stable diazonium compound, sulphosalicylic acid and sodium carbonate, will give a rapid result, but it may be more difficult to read the colour reactions. Five drops of urine are placed on an asbestos-cellulose fibre mat, and then one of the tablets is put on the urine-moistened area. Two drops of water are deposited on the surface of the tablet. A purple colour developing on the mat around the periphery of the tablet within 30 seconds indicates that bilirubin is present. Delayed colour reactions are of no significance.

The methylene blue test, which is easy to perform, can be made to give a quantitative result. In the presence of bilirubin, methylene blue is changed to a green colour; this is not a specific chemical reaction. Methylene blue solution at a concentration of 0-2% is added drop by drop to a measured quantity of urine with continuous mixing. Each milligram of bilirubin requires 200 drops of the methylene blue solution to bring about the colour conver­sion.

Changes in the quantities of bile pigments in blood can be measured by means of the diazo test for bilirubin (van den Bergh test). In performing the test an acidic diazobenzene sulphonic acid solution is added to serum. In obstructive jaundice a reddish-blue or violet colour appears at once; giving the so-called 'direct reaction'. With sera from cases of haemolytic jaundice, the colour only develops after the addition of alcohol following the diazo reagent—the 'indirect reaction'. In cases of hepatocellular jaundice, a biphasic reaction occurs consisting of the rapid appearance of a reddish colour which intensifies on standing.

Variations in serum bilirubin and, therefore, in the intensity of jaundice can be determined by measuring the 'yellowness' or icteric index. This is an empirical method and consists of adding saline solution to 1 ml serum until it matches a standard solution of 1 in 10 000 potassium dichromate. The icteric index is the volume of saline in millilitres. In animals, more particularly horses and cattle, the presence of lipochromes or carotenoids causes yellow colour­ation which reduces the efficiency of the test.

Of the tests measuring excretory efficiency that employing phenoltetrabromophthalein (brom­sulphalein, BSP) has been extensively employed. It is of limited diagnostic value in cattle, but gives a fair indication of hepatic dysfunction in horses, and is regarded as being sensitive and reliable in dogs. Bromsulphalein, following intravenous injection of an amount proportional to bodyweight (5 mg/kg), is largely eliminated from the body through the liver, so that hepatic dysfunction is indicated by delayed blood clearance. The principle of the test in the dog is that a sample of heparinized blood is collected 30 minutes after the injection, the plasma is removed and treated with sodium hydroxide solution. The colour reaction is compared against a similarly treated sample of plasma collected prior to the injection of the bromsulphalein. The percentage dye retention is calculated by multi­plying the 30-minute concentration (mg/100 ml) by the factor 10; in healthy dogs it is 5 % or less. For large animals 1 g bromsulphalein is injected intravenously. Then two heparinized blood samples are taken at an arbitrary interval— usually 4 minutes—before 12 minutes have elapsed. The BSP clearance rate is determined by estimating the concentration of the dye in the two samples and plotting the results against time on semilog graph paper. The half clearance time can be read off.

Bile salts. In intrahepatic and extrahepatic biliary obstruction the bile salts, sodium glycocholate and sodium taurocholate, are returned to the blood and excreted by the kidneys. By reason of their detergent properties bile salts reduce the surface tension in the gut and in the urine. When a sample of urine containing a significant ' amount of bile salts is shaken a persistent foam is produced. Reduction in surface tension can also be recognized by means of Hay's sulphur test which is satisfactory for the urine of all species except cattle. For the test a small amount of sulphur is sprinkled on the surface of a motionless column of urine. If the sulphur immediately sinks through the urine it may be concluded that bile salts are present, provided that synthetic detergents can be excluded as a cause of a false positive reaction.

Metabolic functions. A large proportion (90-95 %) of the serum proteins including albumin, globulin and fibrinogen are synthesized by the liver. Serum protein estimations, which may be carried out by empirical turbidity and flocculation tests or by electrophoresis, have been found to have a limited application in veterinary diagnosis because changes, which may not involve the liver, do not appear until a disease process is somewhat advanced. Measurement of the prothrombin time has been found of use in the diagnosis of liver disease.

Measurements of lipid metabolism, particu­larly cholesterol, provides a useful index of liver function in animals. The method is, however, a complex one. In the case of carbohydrates, liver function can be assessed by means of the galactose tolerance test. Galactose utilization is restricted to the liver and its metabolism appears to impose a considerable burden on the hepatic cells.

Serum enzymes. The concentration of certain serum enzymes varies as the result of three processes involving the liver: (a) elevation due to disruption of liver parenchymal cells because of necrosis, or greater membrane permeability, e.g. glutamic pyruvic transaminase (SGPT), glutamic oxalacetic transaminase (SGOT), arginase, isocitric dehydrogenase (SICD), sorbitol dehydro-genase (SD), glutamic dehydrogenase (GD), ornithine carbamyl transferase (ОСТ), and lactic dehydrogenase (LDH); (b) elevation because of lack of biliary excretion in obstructive jaundice, e.g. alkalinephosphatase (SAP); and (c) decrease due to impaired synthesis by the liver, e.g. choline esterase. Certain of these enzymes exist in high concentrations in hepatic tissue and might be regarded as being 'liver-specific', e.g. SGPT in the dog, cat and primates; SD and GD in sheep and cattle; SD in horses; arginase and ОСТ in all ureotelic animals. The other enzymes men­tioned occur in high concentrations in other tissues than the liver so that unless the disease process is localized to the liver, measurement of these enzymes is likely to be confusing in diagnosis.

The technique for measuring serum enzymes are, in most instances, complex; 'kit' tests are commercially available and are reliable and relatively simple to perform but they require a colorimeter or spectrophotometer on which to read the results. Serum enzyme values for various species of animals are given in Table 17 (p. 297).

Detoxification mechanisms. The liver deals with toxic substances by biotransformation, conjugation and destruction. Conjugation is with amino acids and substances such as glucuronic acid. Substances such as benzoic acid and chloral hydrate are detoxified by this mechanism and, therefore, form the basis of a method for measuring liver function. The risk associated with introducing a toxic substance into the body of an animal considered to have severe or extensive liver disease should not be overlooked.

Biopsy of the Liver

Biopsy of the liver provides material suitable for histological and chemical examination. In the former case the small proportion of tissue recovered may not reveal significant changes, except when the liver is diffusely affected. Chemical determinations of value include those for copper, glycogen and vitamin A. Success in performing the operation requires anatomical knowledge and experience, and is more easily achieved in cattle than in horses or other species. The essential equipment consists of a sharp-pointed, small-bore trocar and cannula about 30-40 cm long with a screw thread at the blunt end to which can be attached a syringe of sufficient capacity to produce the necessary negative pressure for withdrawing the sample of liver. The pointed end of the assembled instru­ment is inserted through the desensitized skin in the upper part of the intercostal space appropri­ate for the species, on the right hand side, directed towards the left elbow region and advanced across the pleural cavity. The instru­ment is inserted with a rotating action, until the point is considered to have reached the surface of the liver, when the trocar is withdrawn; the syringe is then attached and insertion continued for about 2-5 cm. Suction is then applied, the instrument vigorously rotated and slowly with­drawn, during which time strong suction is maintained. The syringe is useful in discharging the core of liver from the cannula when the biopsy has been successful. Although the tech­nique has been repeated on many occasions in the one animal without untowards effect, it is not free from attendant risk. The main danger arises from misdirection of the instrument, causing damage to large blood vessels or bile ducts in the portal area. Other recognized sequelae include suppurative peritonitis as the result of penetrating an active abscess or fatal haemoperitoneum if there is a defect in the blood clotting mechanism. Failure to obtain a sample may arise when the liver is shrunken or the instrument not inserted at the correct point or in the right direction. The technique is suitable for horses, cattle, sheep and dogs.

Radiological Examination

The application of radiological examination to the liver and biliary system is almost entirely confined to the dog and cat although it is possible, on an experimental basis, to extend the method to some species of farm animals. The gall-bladder can be visualized through the medium of cholecystography following the oral administration of a halogenated phenolphthalein compound. Cholelithiasis is a rare condition in animals, occurring very occasionally in the dog. The concretions are usually soft in consistency, and would be unlikely to be revealed by direct radiography. Visualization of the intrahepatic biliary system cannot be satisfactorily achieved by means of intravenous injection of a radiopaque phenolphthalein compound. It may be stated that radiographic examination of the liver has a low level of diagnostic efficiency, frequently failing to reveal any abnormality even when hepatic disease is advanced. Distortion of the outline of the liver may be noted in radiographs when the organ is extensively involved in neoplasia, by which time the presence of the abnormality is usually indicated by clinical signs and may be palpable. Primary neoplasms of the liver include adenoma and adenocarcinoma; occasionally the gall­bladder is the initial site. Metastatic liver tumours are more common, consisting of a wide variety of types.