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Diseases of wild and farmed Finfish

Some fish and shellfish diseases of particular significance in Scotland.

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Introduction

Diplostomum spathaceum has a complex life cycle, which includes sexual reproduction in the intestine of a gull or other fish-eating bird and represents the definitive host. Eggs are shed and passed in the bird faeces to the water, where they hatch into free-swimming ciliated miracidia; this is followed by asexual reproduction in the snail intermediate host (Lymnaea sp.) and finally metacercarial stage in the eyes of freshwater fish, the second intermediate host. The life cycle is characterised by high parasite fecundity in the definitive host, high reproductive potential in the snail and long life-span of metacercariae in fish. Infection with Diplostomum can lead to severe ocular pathology and mortality in commercial fish farming.

Diplostomum spathaceum metacercaria within the lens of rainbow trout

Gross signs

In recently infected fish, petechiae and exophthalmia are observed. Chronically infected fish frequently show dark colouration, exophthalmia with emaciation and loss of condition, lens dislocation and capsular rupture. The site of entry is marked by tiny capsular perforation through which cortical lens fibres exude.

Histopathology

Acute infections cause subcapsular cataract formation with varying capsular change. Occasionally, the perforations lead to lens rupture and severe endophthalmitis, followed by a generalised cortical liquefaction as flukes migrate to the anterior cortex with proliferation of the lens epithelium.

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