Infection of juvenile salmon Salmo salar with a Dermocystidium-like organism in Sweden

Abstract

Free spores of a Dermocystidium-like organism were recovered from the epidermis and covering mucus of gills and fins of moribund farmed salmon Salmo salar. The parasite appeared in juvenile fish only and at low water temperatures (≤5°C). The most prominent external macroscopical clinical signs of disease were thickened fins that gave the tips a pronounced greyish opaque appearance often in combination with signs of fin rot/fin erosion. The gills were swollen and pale and could also be necrotic. Examination of fresh mounts and tissues prepared for light and electron microscopy showed vacuolated spherical spores typical for parasites of the genus Dermocystidium. Irregularly vacuolated spores with 1 or multiple nuclei were also observed. Histological examination of infected salmon indicated concurrent Flexibacter sp. infection that was verified in Gram-negative stained imprints. The present finding is the first observation of Dermocystidium in Sweden. In addition, this is the first record of a Dermocystidium-like agent that occurs freely in the mucus and epidermis of freshwater teleosts.