First diagnosis of ectoparasitic ciliates (Trichodina and Chilodonella) on farmed juvenile yellow catfish, Tachysurus fulvidraco in China

Diseases caused by ectoparasitic ciliates can be a threat to yellow catfish, Tachysurus fulvidraco, an economically important fish in China. However, limited data are available regarding ectoparasitic ciliates from T. fulvidraco. In June 2015, a mass mortality event which may have been caused by ectoparasitic ciliates occurred in a population of juvenile T. fulvidraco cultured in Chidong Lake. Air‐dried smears isolated from the body surface of fish were impregnated with silver nitrate for morphological identification. In total, six known ciliate species, including four species belonging to the genus Trichodina (Trichodina acuta, Trichodina maritinkae, Trichodina nobilis, and Trichodina heterodentata) and two chilodonellids, namely, Chilodonella uncinata and Chilodonella hexasticha, and unidentified sessilids were found. Morphological descriptions of the known trichodinids and chilodonellids are provided based on silver nitrate‐impregnated specimens. Histopathological analysis revealed that trichodinids and chilodonellids could adhere to the epithelium firmly by adhesive discs and the ventral bodies, respectively, resulting in massive mucus production and epithelial hyperplasia and desquamation. This study confirmed that T. fulvidraco is a previously unrecognized host for T. acuta, T. heterodentata, T. nobilis and C. hexasticha. Although C. uncinata was also present in this study, histopathological analysis suggests that C. uncinata may be a commensal and not a true pathogen in T. fulvidraco. To our knowledge, this work is the first investigation regarding ectoparasitic ciliates on cultured T. fulvidraco in China.