Distribution of X‐cell disease in common dab, Limanda limanda L., in the North Sea, and ultrastructural observations of previously undescribed developmental stages

Abstract. The distribution and prevalence of common dab, Limanda limanda L., affected with X‐cell lesions was investigated at various localities around the British Isles. Extreme variability in the prevalence of affected gills was recorded in the Moray Firth (NE Scotland), where samples obtained from hauls taken in close proximity ranged from nil to 60% of the collected dab. The condition was not detected in dab from the Irish Sea, and was found only sporadically in samples in the southern North Sea and the northwest coast of Scotland, Areas of high prevalence are typically coastal or relatively shallow habitats. The patchy distribution of the disease throughout dab populations suggests that an infectious agent may be involved in its aetiology. A method is described for rapid, accurate screening of fish samples for X‐cell disease, which utilizes the autofluorescence of the X‐cell. Ultrastructural studies of lesions from external and internal organs in dab disclosed some previously undescribed features of X‐cells. Stages of the X‐cell development which occurred in‘nests', and possibly preceded the commonly found X‐cells, were found in gill lesions. Electron dense inclusions found in X‐cells in the spleen and kidney were rod‐ or dumbbell‐shaped structures and possibly represented secretory granules.