In this article we will discuss about the diseases of fish:- 1. Bacterial Diseases 2. Fungal Diseases 3. Viral Diseases 4. Protozoan Diseases 5. Disease Caused by Trematodes.

Bacterial Diseases:

1. Fin and Tail Rot:

White line on the margin of the fin; fin rays become brittle and start breaking.



1 minute dip treatment in 500 ppm copper sulphate solution.

2. Furunculosis:

Open sores or ulcers on the body.



Dip treatment for 1 minute in 1 : 2000 copper sulphate solution for 3-4 days.

3. Dropsy:

Accumulation of fluid inside the body cavity; scale protrusion; exophthalmic condition.



Dip treatment in 5 ppm potassium permanganate solution for 2 minutes.

4. Eye Disease:

Cornea of eye becomes vascularised and later becomes opaque; eyeball gets decayed.


Chloromycetin (8-10 mg/ litre) bath for 1 hour for 2-3 days.

Fungal Diseases:

Water-Mould Disease (Saprolegniasis):

Dies after ulceration or exfoliation of skin followed by haemorrhage; blindness; tufts of white hair-like out-growth in the affected region.


Dip treatment for 3 seconds in 1 : 10000 solution of malachite green or for 5-10 minutes in 3% common salt solution or 1 : 1000 solution of potassium permanganate.

Viral Diseases:


1. Viral Haemorrhagic Septicaemia (VHS):

Anaemia; pale discolouration of gill; swelling of eyes and belly; oedema of muscles; inflammation of intestine; anus becomes protruded.


No known cure for VHS. Prevention is the best approach to control like other viral diseases. Contaminated water should be thoroughly disinfected before use.

Protozoan Diseases: 

1. Ichthyophthiriasis (White-Spot Disease):

Small whitish-cysts of about 1 mm diameter on the skin, gills and fins.


5 days bathing in 2 ppm methylene blue; hourly dip treatment in 1 : 5000 formalin solution for 7-10 days.

2. Knot Disease:

Small knots in the skin of common carp.


All infected fish should be destroyed or burnt.

3. Boil Disease:

Large boils varying from the size of a nut to that of a hen’s egg on several parts of body.


Bath in 3% common salt solution or in 1 : 2500 formalin solution for 10 minutes.

4. Pox Disease:

Small milky-white spots raised 1-2 mm above the surface of skin.


Intra-peritoneal injection with 1 ml of a 1 % solution of arsenic compound, Arycil.

5. Costiasis:

Bluish-coating on the skin; lesions as irregular patches.


Bath in 3% common salt solution or in 1 : 2500 formalin solution for 10 minutes.

6. Whirling Disease:

Caudal bend, deformity of the oral region and blackening of tail region.


Destroy all infected fish by applying quicklime (pond disinfectant) at the rate of 2 t/ha.

Disease Caused by Trematodes:

1. Gyrodactylosis Dactylogyrosis:

Fading of colours; drooping of scales; peeling of skin.


Dip treatment in 5% common salt solution or in 1 : 5000 formalin solution for 5 minutes.

2. Diplostomiasis (Black-Spot Disease):

Small black nodules of about 1-5 mm diameter in the affected region.


Dip treatment in 3 : 100000 picric acid for 1 hour; Di-n-butyl tin oxide at the rate of 250 mg/kg body weight.

3. Worm Cataract:

Whitish-eye due to the white dots, i.e. little worms situated between lens and lens capsule. 


Dip treatment in potassium antimonyl tartrate (1.5 mg/litre).