Preliminary findings of ophthalmological abnormalities in farmed halibut

Ocular Surface Biology and Disease in Fish

Ensuring the clarity of the ocular surface of fish species with which we interact is of great importance. There is still much more to learn about the ocular surface of fish species. A better understanding of the anatomy, physiology, and pathology of the ocular surface is thus vital for fish welfare, as well as being a fascinating subject in its own right.

Aggravating factors in the development of ocular abnormalities in farmed Atlantic halibut (Hippoglossus hippoglossus)

Cystic lesions in the eyes of farmed halibut with a number of ocular sequelae have been reported previously with evidence that the condition can be associated with increased activity of choroidal carbonic anhydrase and elevated oxygen tension in the aqueous humour of affected fish. These changes may be compared to the 'bends' in human divers which are characterized by elevated circulating levels of nitrogen together with aggravating factors such as physical exercise. It is postulated that bubble formation and subsequent development of cysts in halibut require not only high aqueous humour oxygen tension but also the imposition of factors such as handling or intraspecific aggression which lead to bubble formation. A significantly higher incidence of lesions was noted in fish handled, weighed and measured every two weeks compared with a control group of unhandled fish, suggesting that physical activity associated with handled and possibly also intraspecific aggression is a factor in generation of ocular lesions in these fish.

Ultrasonographic examination of ocular lesions in farmed halibut

This paper describes the use of ultrasound to document ocular abnormalities occurring in farmed halibut. A cystic condition of the posterior segment of farmed halibut with a number of ocular sequelae has been reported previously. Further research to elucidate the aetiology and pathogenesis of this condition requires in vivo determination of ocular health over and above that provided by ophthalmoscopy alone. Ocular ultrasonography would appear to be an optimal technique for evaluating the presence of gas- or fluid-filled cysts in the posterior segment and retrobulbar space. The aim of this study was to evaluate this technique for investigation of the piscine eye and specifically to define the role of ultrasonography to detect gas bubble formation in the choroid of farmed halibut, a factor considered central to ocular pathology in many of these fish. In particular the study sought to assess the minimum size of halibut in which intraocular cystic lesions could be identified by ultrasonography and the minimum size of lesion identified by ocular ultrasonography. A second objective was to correlate ultrasonographic findings with lesions found at post mortem ocular examination. A number of halibut of varying sizes, with and without clinically apparent ocular lesions, were subjected to examination under anaesthesia. A proportion of these fish were subsequently sacrificed to allow post mortem ocular evaluation with the aim of correlating ultrasonographic findings with intraocular pathological defects.

Intraocular oxygen tensions in normal and diseased eyes of farmed halibut

We have previously documented ocular abnormalities in farmed halibut and reported that the production of gas-filled posterior segment cysts appears to be central in the pathogenesis of many, if not all of the signs in these eyes. In a number of fish, gas bubbles may be seen in the anterior chamber of the eye, especially after trauma or strenuous exertion associated with handling. A knowledge of the composition of this gas is important in understanding fully the mechanism of gas and cyst production. Here we report investigations into the composition of gas in the globes of normal fish, fish in which intraocular cysts have been documented previously by ultrasonography and fish where post-mortem examination demonstrated both gas- and fluid-filled choroidal cysts. High levels of oxygen were demonstrated in samples of aqueous from all fish but fish with affected eyes had a statistically significantly higher partial pressure of oxygen than did fish with normal eyes.

Ophthalmic disease of fish

Ocular disorders in fish are common and can occur as primary or secondary manifestations of systemic disease. Because most fish are sight feeders, visual impairments usually have fatal consequences for the individual fish and can lead to substantial economic losses for the client. This article encourages practitioners to perform a complete ocular examination as part of routine physical examinations in fish by providing a review of the functional anatomy of the teleostean eye, clinical work-up, ocular examination, ocular diseases, and treatment.

Unilateral eye abnormalities in reared Mediterranean gilthead sea bream

Eye abnormalities in reared gilthead sea bream (Sparus aurata) were investigated clinically and by histological techniques. A significant number of fish had a cataract in one eye and ocular inflammation including gross exophthalmos. In a small number of fish gas bubbles were observed in the anterior chamber of the eye. Histological examination provided no evidence of an infectious process and a possible behavioural aetiology is discussed.

Levels of choroidal body carbonic anhydrase activity and glycogen in farmed halibut

We postulate that ocular abnormalities in farmed Atlantic halibut occur following aberrant gas production in the choroidal body associated with the enzyme carbonic anhydrase. We investigated choroidal body carbonic anhydrase activity in affected and normal halibut. Immunohistochemical detection of carbonic anhydrase allowed neither quantification of enzyme levels nor determination of enzyme activity. Histochemical detection of carbonic anhydrase, however, allowed microdensitometric quantification of enzyme activity. Mean enzyme activity in affected individuals was over twice that in normal fish. Glycogen, which we suggest is used to fuel oxygen production in the choroidal body, was evaluated by Best's carmine staining. Microdensitometry demonstrated glycogen levels of be significantly higher in affected fish. These findings show that posterior segment gas bubble and cyst formation is associated with aberrantly high choroidal enzyme activity and high choroidal glycogen concentrations.