Fish diseases and parasites

Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens.

This gizzard shad has VHS, a deadly infectious disease which causes bleeding. It affects over 50 species of freshwater and marine fish in the northern hemisphere.

This flatfish Limanda limanda has an outgrowth called a xenoma. It is caused by a microsporidian fungal parasite in its intestines.

Specific defences are specialised responses to particular pathogens recognised by the fish's body, that is adaptative immune responses. In recent years, vaccines have become widely used in aquaculture and ornamental fish, for example vaccines for commercial food fishes like Aeromonas salmonicida, furunculosis in salmon and Lactococcosis\Streptococcosis in farmed grey mullet, Tilapia and koi herpes virus in koi.

Some commercially important fish diseases are VHS, ICH, and whirling disease.

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