Histopathology and ultrastructure of ocular lesions associated with gas bubble disease in salmonids

Ocular lesions associated with natural and experimental outbreaks of gas bubble disease (GBD) in commercial salmonids were assessed histologically and by scanning electron microscopy. Small gas emboli were first detected in the choroid gland of the posterior uvea. In subacute and chronic cases, bubble size increased markedly and localization in retrobulbar and periocular sites was favoured. During the acute phase of GBD, ocular lesions were limited to anatomical displacement of tissue and local degeneration of compressed tissues around the perimeter of bubbles. Subacute sequelae included the formation of anterior synechia, lens cataract, and suppurative panophthalmitis. During chronic stages, when large retrobulbar bubbles had caused severe exophthalmia, there was stretching of the optic nerve and of retinal blood vessels and severe distortion of the posterior aspects of the globe. The sequential development, pathogenesis and persistence of ocular lesions associated with GBD in fish are discussed.