Extreme pathological symptom generated by Dermocystidium koi infection of common carp (Cyprinus carpio L.) in Hungary

Dermocystidiosis in common carp (Cyprinus carpio L.) as a skin infection in the form of small, pea‐sized bulging nodules is well‐known in Hungary. Its occurrence is sporadic, and the pathogenic effect is generally moderate. Nonetheless, here we report an unusual case of the infection when a mass of dermocystid spore‐filled hyphae‐like cysts formed a large tumour‐like nodule under the skin intruding deeply in the red muscle on one side occipital region of the fish body. Histological sections showed that the tumour‐like nodule was composed of two layers, a pale peripheral part with a high mass of cysts surrounded by some cell debris of muscle cells and a dark central part with a less dense meshwork of tubular cysts which were embedded in the partially damaged red muscle tissue. No infection was found in the white muscle. Sequences of 18S rDNA of spores from the tumour‐like nodule showed a complete identity to sequences from typical bulging nodules in skin. They were also highly similar to the sequences of D. anguillae clones (99.5%–99.4%), to some samples of D. salmonis (99.7% and 99.4%), and to the Dermocystidium sp. CM‐2002 (99.4%) from European perch.