Experimentally induced hemorrhagic thelohanellosis of carp caused by Thelohanellus hovorkai (Myxosporea: Myxozoa)

Abstract

Hemorrhagic thelohanellosis in two varieties of carp (Cyprinus carpio) caused by Thelohanellus hovorkai was investigated by experimental infection. One-year old coloured carp (koi-carp) and common carp were kept for five weeks in tanks with oligochaetes (Branchiura sowerbyi) collected from a pond where hemorrhagic thelohanellosis was enzootic. Another group of coloured carp was exposed only to the effluent from a tank containing B. sowerbyi. Four weeks post-exposure, both coloured carp and common carp maintained with B. sowerbyi showed clinical signs of infection and chronic mortalities. On the other hand, neither mortalities nor clinical signs were observed in the effluent group, though light infections with T. hovorkai were detected. Control groups of coloured carp and common carp maintained in clean tanks showed no signs of infection at all. Through a standardized trypsin digestion method, the intensity of infection with T. hovorkai in carp maintained with B. sowerbyi was determined to be more than 50,000 spores/g-tissue. Histological observations revealed that T. hovorkai plasmodia were most frequently detected in connective tissues of the intestine and the ventral skin in the cohabitation group, whereas they were found most frequently in the gills of the fish of the effluent group. In conclusion, hemorrhagic thelohanellosis was experimentally induced in carp by maintaining hosts with B. sowerbyi regardless of host's varietal difference, and the ingestion of B. sowerbyi infected with the actinosporean may be an important factor inducing the disease conditions.