Fish Diseases and Management

2.2.15 The digestive system

The digestive system consist of (1) oral cavity or mouth (2) oesophagus (3) gut or stomach (4) intestine (5) rectum (6) intestinal glands (liver and pancreas)

Depending upon species, feeding habits and associated structure may vary.

Pathology

1) Oral cavity

Traumatic lesions develop at times but heal quickly. Damage by fishing hooks may cause ulcers and necrotic fungal infections particularly on the upper jaw. Polluted water may cause odontoblastoma and fibrinous traumatic lesions of the lips or of the pharynx.

a) Mechanical injuries leading to ulcer

This is manly due to of hard objects. Mechanical injury leads to damage of skin exposing the animal to attack by secondary pathogen and causing ulcer formation.

b) Enteric red mouth disease

It is caused by the bacteria Yersinia ruckeri, and is associated with septicaemic infection. The mouth gets highly reddened and inflamed and feeding stops, lysis of cell may occur leading to mortality. It can be easily identified by  looking at or observing the animal.

c) Tumours

Perioral tumours may occur. This may be due to virus, environment and non-infectious pathogens or may be due to system failure.

2) Oesophagus

a) Oesophagus myopathy

Myo-muscle pathy-pathology. It means pathology in muscle and denotes the disease condition in muscles lining the oesophagus. It affect the swallowing ability of the fish.

b) Calcified plaques and granuloma formation involving large necrotic areas have been described in the stomach wall of rainbow trout in CO₂ mediated nephrocalcinosis.

3) Swim bladder

Obstruction of pneumatic duct or loss of gaseous secretion is the most commonly reported idiopathic condition in swim bladder of fish. It may sometimes results in clinical bloat, impairing the ability of the fish to maintain position in water. It may be due to nutritional reasons or due to fungal or parasitic infections.

Due to genetic reasons in certain fishes (cyprinodonts) swim bladder fails to develop properly and is present as a vestigial organ and thus impairing the ability of the fish to position in the water. Affected fish show characteristic ‘belly sliding’ movement which gives the condition its name.

Swim bladder inflammation

This is found in cyprinids, it develops as a necrotizing chronic inflammatory lesion of one or both the chambers of the bladder in the farmed carp and eventually bladder wall collapses and reddish brown fluid exudates which seen in the lumen.

4) Stomach

Not much of pathology due to the presence of gastric epithelium and the acidic pH which prevents  pathogen attack. Fewer pathologies which are noticed occur in the muscules of the stomach wall.