Fish Diseases and Management

2.2.17 Intestinal glands

i) Liver

Teleost liver consists of two or in some cases three i.e. the parenchymatous hepatic tissue, the biliary drainage tree and where the organ is hepatopancreas, the exocrine and endocrine pancreatic tissues. Biliary or vascular tissue is vestigeal in adult unless there is an anaemia, when compensatory hepatic haematopoiesis occurs. The parenchymatous hepatic tissue involves in intermediary metabolism of protein, carbohydrate and lipids, the synthesis of plasma proteins such as albumins and reproductive protein ovalbumin and also the formation and secretion of bile. It also carries out the detoxification of endogenously derived waste products as well as externally derived toxins, drugs, heavy metals and pesticides. Teleost liver lacks fixed kupffer cells.

a) Hepatic necrosis

May develop as a result of both primary liver disease and in reactive hepatitis in response to systemic infection with primary focus elsewhere.

A) Focal necrosis

Only a small area of liver gets damaged. Small clusters of necrotic hepatocytes may be found within normal hepatic tissue or infiltrate by inflammatory cells.

B) Confluent necrosis

A large area of liver tissue is necrotised. Where the necrosis is large, but does not have an inflammatory infiltrate associated with it, it is generally the result of infarctive process. Infarction follows obstruction of the vascular supply, and is usually the result of thrombosis of hepatic arterial vessels.

b) Lipid infiltration

Lipid gets accumulated in the liver. This happens more in carnivorous fish which has large supplementation of lipid in its diets. Lipid infiltration is commonly reported in farmed fishes but may also occur in wild fishes especially in those who store large amount of lipid during the active feeding stage. Enlargement of liver and loss of sharp edges of its lobes characteristic feature of this pathology. The liver cells are distended by clear, fat vacuoles and some vacuoles may coalesce. This excess accumulation of lipid in the liver is said to be lipoid liver. In case of heavy infiltration liver function is impaired, reduction in circulating protein, increased susceptibility to intoxication, hypoplastic anaemia and nutritional oedema may set in. It may be caused by feeding  highly rancid trash fish, toxic conditions or vitamin deficiencies.

c) Hepatic Granuloma:

Hepatic granuloma are caused by a wide range of pathogens. To prevent the progression of damage by pathogen to adjacent tissue the causative agent and damaged tissues are surrounded by a very narrow flattened bed of epithelioid and fibrous cells this is referred to as granuloma. If the granuloma take place in liver, it is called  hepatic granuloma.

d) Pigment Accumulation

Most commonly reported is the accumulation of ceroid and lipofuscin . These by-products of catabolism are seen extensively in the liver cells and haemopoietic tissue, following starvation or wasting disease.

e) Cirrhosis

Cirrhosis is a diffuse increase in the fibrous tissue of the liver, usually associated with chronic damage and destruction of liver cells ( hepatic cells or hepatocytes). The damage can result from a wide range of stimuli, from long standing biliary obstruction, heavy metal or pesticide poisoning to chronic parasitism which leads to the loss of function of hepatocytes.

f) Hepatic carcinoma (Hepatoma )

Hepatoma may be caused by a variety of pathogens.  It is an increased proliferation of cells in liver. One of the toxin known as Aflatoxin  produced by a fungus known as Aspergillus flavus is the main or primary causative agent of hepatomas.

g) Cystic growth

Unidentified growth of cyst in liver is associated mostly with some of the metazoan parasites. Sometimes these cysts may be so large that it causes cachexia and wasting.

ii) Billary ducts Gall Bladder

Bile empties into the gall bladder through bile ducts and release into gastro intestinal tract. If the stone is formed in the gall bladder it is known as gall stone ( cholelithis). Stone formation is generally termed lithiasis.