Fish Diseases Caused by Parasites and Pathogens:
1. Bacterial Diseases:
Two bacterial diseases are very important.
(i) Abdominal dropsy of Carps is caused by Aeromonas punctuata.
(ii) Furunculoris of Salmons and trout’s is caused by Aeromonas salminicida.
2. Viral Diseases:
Economically most important is the viral haemorrhage septicaemia (VHS) of rainbow trouts.
3. Protozoan Diseases:
Main protozoan diseases are caused by Costia, Myxobolus and Trypanosoma.
4. Fungal Diseases:
The gill rot (branchyomyces) of carps involves the attack of Saprolegnia on the gills of carps.
5. Worm Diseases:
Worms of four groups are parasites on fish. The flatworms (trematodes), tapeworms (cestodes), round worms (nematodes) and thorny-headed worms (acanthocephalans).
6. Common Ectoparasites:
Two ectoparasites of fish are most important, the fish lice (Argulus, Lernaea and Ergasilus) and the fish leech (Piscicold). Both parasites weaken fish by feeding on its blood.
Like other animals, fishes are also affected by various diseases. These diseases can be broadly classified under bacterial, viral, fungal, protozoan and those caused by large parasites like tapeworms and roundworms. Diseases like tuberculosis, eye infection, kidney diseases and various types of tumours – benign and malignant are reported from both freshwater as well as marine fish.
Furunculosis, a common disease of fresh water fish is caused by bacteria and can be treated with tetracyclines and the vibrio infections respond to sulphonamide therapy.
Tail rot, another common disease caused by bacteria can be controlled by administering a mixture of penicillin and streptomycin.
Food poisoning caused by fish and fishery products may be due to a variety of causes. Some fish and shellfish (prawns, shrinps etc.) possess poisons or biotoxins. These biotoxins are not destroyed by cooking and cause illness when fish containing them are eaten. Some people are allergic to fish, molluscs or crustaceans.
Some poisons affect the central nervous system, while others cause gastrointestinal and skin disorders. Food poisoning can also be caused by eating spoiled or rotting fish. Toxic principles in the flesh of such fish cause gastroenteritis. Bites and strings caused by varieties of fishes and invertebrates like jelly fish result in serious inflammatory conditions. Dermatitis can be caused by irritants found in the skin of fishes belonging to the tuna fish group.
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