Cataracts are attributed to many factors including infection by the digenean trematode Diplostomum sp., as well as bacterial infections, chronic eye congenital conditions, environmental toxins, excess gas, osmotic changes, nutritional imbalance and trauma. A cataract is defined as opacity of the lens, regardless of cause, size or location and results in a clouding or opacity of the lens, consequently less light rays are transmitted onto the retina and vision is impaired.
Cataracts result from opacity of the crystalline lens that starts in the lens cortex with rarefaction, liquefaction of cortical cells leading to fragmentation of lens fibres and extracellular globule formation. Affected fish may show loss of weight, become dark in colour with chronic inflammatory lesions, enophthalmia, exophthalmia and ulceration. Cataracts can be unilateral and bilateral.
Fish infected with Diplostomum show subcapsular cataracts in heavily infected fish. In chronically infected fish the lens becomes discoloured, with capsular rupture and detachment of the retina.
Cataracts generally develop at different rates depending on cause, and can be loosely described as subcapsular, degenerative, proliferative or vacuolative. Histologically, cataract development consists of proliferation of the lenticular epithelium and capsular cells, vacuolation of lens epithelium and cortex and swelling of lens fibres. The lens nucleus may show a progressive increase in the amount of insoluble proteins. Lens degeneration may also result in liquefaction which escapes through capsular damage