Dermocystidium sp. infection in cultured juvenile southern catfish Silurus meridionalis in China.

A species of Dermocystidium was discovered on the skin and fins of reared southern catfish Silurus meridionalis Chen. The parasite only appeared and caused disease in juvenile catfish at a water temperature of 18 to 23 degrees C. Marked external macroscopical clinical signs of the disease were sluggish movement of the fish, and the appearance of white filiform dermal cysts varying in size (3-20 mm in length and 0.15-0.35 mm in width). Examination of both fixed and fresh mounts for light microscopy and of samples for transmission electron microscopy (TEM) showed spherical spores (3.2-15 microm in diameter) with a peripheral nucleus (1.1-1.8 x 0.5-1.6 microm in diameter) and a prominent refractile body (2.08-10.83 microm in diameter) which occupied most of the volume of a mature spore. Three types of spore were identified, and are presumed to represent various developmental stages. Meanwhile, TEM showed the remnant nuclei in the residual plasmodium of a cyst, revealing its degenerative process. This paper represents the first observation and description of Dermocystidium sp. parasitizing catfishes.