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.

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.

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.

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.

Preliminary findings of ophthalmological abnormalities in farmed halibut.

Ocular abnormalities occurring in farmed halibut at the Sea Fish Industry Authority Marine Farming Unit, Ardtoe, Argyll were investigated clinically and post mortem. A significant number of fish were found to have posterior polar cystic changes either causing scleral ectasia or gross degenerative posterior segment abnormalities with chorioretinal atrophy. Other ocular abnormalities included cataract formation and intraocular inflammation. In a small number of fish gas bubble formation in the anterior chamber was observed during capture and examination. It is possible that the lesions seen in these fish are a form of gas bubble disease although super-saturation, the cause of gas bubble disease previously reported in farmed fish, does not occur in the tanks in which these fish are housed. While this paper provides no answers with regard to the aetiopathogenesis of the lesions, it is hoped that it will stimulate discussion, leading to resolution of these questions through a multidisciplinary approach to the problem.

Some hepatic neoplasms in non-domesticated birds.

A metastasising hepatocellular carcinoma in a Lesser flamingo (Phoeniconaias minor), a metastasising bile duct carcinoma in a Chilean flamingo (Phoenicopterus ruber chilensis) and a liver cell adenoma in a Luzon hornbill (Buceros hydrocerox hydrocerox) are described. Hepatic neoplasia in birds is discussed.