Fish Diseases and Management

2.2.21 Lateral line (sense organ)

It is liable to damage by detergents, heavy metals, petroleum hydrocarbons, pesticides and other pollutants. Severe oedema, spongiosis, necrosis and sloughing are the usual acute responses with hyperplasia and metaplasia of sensory epithelium occurring in the longer standing cases.

iii) Eye

Lesions of the eye are numerous and varied in their aetiology. The most frequent clinically apparent eye lesion involves swelling of the orbit or discoloration of the cornea. Blind fish usually darken in colour because of their loss of external stimuli to colour control.

Ulcers in cornea can give way to secondary infections such as parasitic, fungal or bacterial. Corneal damage is also associated with deficiency of the B group vitamin riboflavin, which is essential for the respiration of the avascularised cornea. Deficiency leads to corneal oedema and sloughing of the outer epithelium.

The lens is affected by a number of pathological processes, all leading to progressive degenerative cataract formation. One of the most common eye infection seen in cultured fish is cataract of farmed salmonids caused by the metacercarial stages of the various strigeoid metacercaria.

Nodavirus is known to cause vacuolar retinal degeneration in a number of fin fishes mainly involving the cellular components of the retina especially the bipolar and ganglionic nuclear layers. However small lesions can be found in rod and cone layers.

a) Exophthalmia

Bulging out of the eyes from the eye socket. This may be due to virus, bacterial toxins or due to pollutants.

b) Pop eye/ Gas bubble disease:- One of the most important and frequently reported eye condition in fish is gas bubble disease associated with super saturation of the water with air.

G. The musculoskeletal system

1. Muscle

Muscle is made up of individual units termed as myofibrils and a bundle of muscle fibre is surrounded by a delicate endomysium which remains bathed in lymphoid intracellular fluid. Metabolic differences characterising the red and white muscle are also reflected in the degree of their pathology. One of the first sign of pathological changes associated with muscle is the central migration of nuclei of myofibrils.

a) Ulcer

Lesions developing either due to traumatic injury, bacterial infection, or parasitic infection if not resolved may take the form of ulcers.

Wound healing in fish muscle is critically dependent on vitamin C and its ascorbic acid deficiency has been linked with failure of wound healing in many cases.

Fungal infections (Ichthyophonus spp.) lead to granuloma formation which if uncontained can spread to deeper layer of muscles.

2. Skeleton

Skeletal damages has frequently been associated with vitamin C deficiency. Deficiency of collagen metabolism induced by vitamin C deficiency, particularly in rapidly growing cultured fish produces spinal deformity. Skeletal deformities may also arise due to exposure to pollutants such as heavy metals and chemicals.

a) Scoliosis

Lateral deformation in the vertebral column.  Where in the vertebral column is bent

b) Lordosis - Inward curvature of vetebral column is referred an lordosis 

This may be due to vitamin-C deficiency or deficiency of amino acids like tryptophan and methionine. This may also be due to the pollution of heavy metals, pesticides like organo phosphates and organo sulphates.

H. The Reproductive system

Fish reproductive system shows only a few specific histopathological changes .

Pathology

a) Tumours may be produced due to aflatoxin present in the feed. Ovarian tumours are less common in fishes compared to testicular tumours.

b) Plistophora ovariae invades the ovary, causing necrosis, cyst formation and ultimately fibrosis, resulting in sterility.

c) Malnutrition or disease condition at times may inhibit the normal progressive changes occurring in gonads during maturation leading to follicular atresia or failure to develop primary oocytes .

I. Endocrine system

The endocrine system pathologies are not yet fully investigated and there is limited information available on this aspect.

a) Corynebacterial disease or chronic furunculosis

This is due to bacterial granulomata found in adrenal cortex and involving the corpuscles of stannius and a collar of leukocyte infiltrate frequently found around the pituitary.

b) Degranulation of the β cells of islets of langerhans in the pancreas resulting in diabetes meilletus has been shown to be the main feature of Sekoke disease.

c) Thyroid hyperplasia and neoplasia associated with iodine deficiency has been reported in wide range of fishes.