ABSTRACT
Gill diseases represent one of the most prolific and economically important groups of diseases and disorders of fish. The paucity of research in the area of fish disease physiology can be attributed to the fact that the techniques used to study respiratory physiology in healthy fishes are difficult to transfer to disease compromised fish without extensive mortality. Non-infectious gill diseases are not associated with a pathogen, but arise as a consequence of either physical injury, chemical insult or nutritional deficiency. Mucus cell hyperplasia is a common and non-specific response to many infectious and non-infectious gill diseases. Chloride cell hyperplasia is not often reported as a consequence of infectious diseases and is most likely to occur only when an ionoregulatory or osmotic stress occurs as a consequence of infection. Necrotic diseases include those that cause either focal or extensive necrotic damage to the respiratory epithelium.
