Introduction
Hepatocytes are the epithelial cells of the liver, and these are the main targets of most liver diseases. They can ultimately undergo necrosis, a form of cell death, after lethal injury. This can occur following severe metabolic disturbances, toxic insults, nutritional deficiencies and through the action of micro-organisms.
Despite many types of injury that the liver is subjected to, the resultant necrosis occurs in one of three patterns:
Random Necrosis (Focal)
In this form of necrosis, there is no distinct pattern of lesions - small foci of necrosis are randomly scattered throughout the liver, and may be either microscopic or just visible to the naked eye. They can result from a variety of insults such as - systemic viral, bacterial and parasitic infections, or as a result of bacteria being absorbed from the gut.
Examples of conditions that result in random focal hepatic necrosis include:
- Equine herpes virus 1 infection found in aborted foetuses
- Septicaemia in association with salmonellosis, tularaemia and listeriosis
- Toxoplasmosis in dogs and cats
Zonal Necrosis
Zonal necrosis is necrosis occurring mainly in a part of the lobule and further subdivided according to whether the lesions are situated centrally, peripherally, or in the mid-zone of the lobule:
Periacinar (Centrilobular)
This is the most common type of zonal necrosis and occurs mainly due to the fact that the hepatocytes in this zone are furthest away from the incoming blood supply and are therefore less oxygenated and relatively anoxic. They are reported to contain the greatest number of enzymes responsible for metabolising substances to more toxic metabolites capable of killing the hepatocytes. Therefore hypoxic states and toxic substances predominate in this type of necrosis, although some viral conditions also are the cause, such as Infectious Canine Hepatitis (ICH). This is a highly infectious disease of young dogs which is spread by urine. It is now rare due to vaccination in the UK.
Midzonal
Midzonal necrosis is rare and is mainly seen in pigs with aflatoxicosis. It is also seen in humans suffering from 'Yellow Fever'.
Periportal (Centroacinar)
Centroacinar necrosis is also rare. This has been seen in cases of phosphorous poisoning.
Massive Necrosis
This occurs when there is necrosis of a complete acinus or an even larger area of tissue. It encompasses large areas of liver cells comprising many lobules (complete acinus or several acini) and sometimes involving almost the whole organ.
It can occur is some cases of ICH infection or carbon tetrachloride poisoning, with the severity of the injury replacing the zonal pattern.
Subacute Cytolytic Necrosis
This is a condition in the dog, of which the aetiology is entirely unknown. Clinical symptoms include acute abdominal pain, collapse and invariably jaundice. Pathology shows the liver is normal or reduced in size and that there is severe necrosis.
Hepatosis dietica
This is a similar condition to subacute cytolytic necrosis, but it occurs in rapidly growing pigs. It is thought to be related to diet and occurs in pigs fed on large quantities of grain concentrates, poor quality or low quantity protein supplements. This results in nutritional deficiencies of selenium and Vitamin E, and probably amino acids. It is thought to be triggered by stress.
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References
Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition), Elsevier Science.
Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2), W.B. Saunders Company.
Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition), Mosby Elsevier.
Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition), Mosby Elsevier.