Cataracts: Etiology, Pathology, Diagnosis & Treatment

Overview

A condition of the eye in which the lens becomes cloudy or opaque, resulting in impaired vision or complete blindness.

Cataracts: Etiology, Pathology, Diagnosis & Treatment

Etiology

Numerous etiologies have been reported to cause unilateral or bilateral cataracts in both wild and captive fish, including nutritional, infectious, intralenticular parasites, trauma, excess ultraviolet light, changes in water temperature and hereditary factors. In addition, poor water quality can cause osmotic changes in the lens.
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Route of transmission

Depends on the specific etiology.

Host range

All fish species are probably susceptible to cataracts depending on the etiology. Both a nutritional deficiency of vitamin A (hypovitaminosis A) and feeding salmon diets containing a high proportion of animal viscera (e.g., offals) can result in the formation of cataracts . A number of freshwater species of fish, such as salmonids and various pan fish (i.e., sunfishes, yellow perch, crappie, croaker, minnows) may serve as the second intermediate (i.e., metacercaria) host for a number of digenetic trematode species (e.g., Diplostomum spp.), which have a predisposition to encyst as a metacercarial stage in the lens of the eye.

Clinical Presentation

Gradual increasing cloudiness
and opacity of the lens of the eye. Behavioral alterations may include a lack of response to predators or shadows/movement. In addition, the fish may lose weight due to the inability to feed or compete for food.
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Pathology

The histologic changes seen in cataracts of fish include hydropic swelling of the lens fibers, lysis of fibers, epithelial hyperplasia and intralenticular migration of surface epithelium.

Differential diagnosis

Need to distinguish cataracts from corneal edema or corneal opacity, which cause cloudy eye. Cataracts should also not be confused with lens opacification that normally occurs postmortem.

Diagnosis

Most commonly diagnosed by visual inspection of the eye along with behavior suggesting decreased vision in one or both eyes. Management/control: Control of cataracts in fish is dependent on the specific etiology with optimal nutrition and diets with adequate vitamin A meeting the nutritional deficiencies, and elimination of the first intermediate host (e.g., snail) of the larval digenetic trematode species preventing the parasitic causes. Several surgical procedures have been used in ornamental or display species to either phacoemulsify and remove the lens from the eye or remove/ replace the entire eye for cosmetic reasons.

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