Morphological, histopathological and molecular characterization of parasitic ciliate Ichthyophthirius multifiliis infecting an indigenous and endangered fish, Schizothorax macropogon (Cyprinidae: Schizothoracinae) in high plateau, Tibet, China

Abstract

Ichthyophthirius multifiliis Fouquet, 1876 is an important parasitic pathogen of freshwater fish, with low host specificity and broad geographical distribution. However, the epizootic occurrence of ichthyophthiriasis in high plateau has been rarely reported. In the present study, infection of I. multifiliis was found in juvenile fish of an indigenous and endangered fish Schizothorax macropogon Regan, 1905 (Cyprinidae: Schizothoracinae) in high plateau of Tibet, China, which caused mass mortality of the host. To understand the characteristics of this I. multifiliis isolate in Tibet, a morphological, histopathological and molecular study was performed to compare with another isolate of I. multifiliis from Tachysurus fulvidraco Richardson, 1846 in Guangdong province, China. Despite the distribution in different altitudes, obvious difference was not found in the two isolates in relation with infection site, morphology, ultrastructure and histopathology. The analysis of COX-1 gene, SSU rDNA and ITS1-5.8S rDNA-ITS2 sequences cloned from the two isolates in the present study showed that variable sites were found only in COX-1 gene sequences, which indicated that COX-1 gene was the most effective marker to distinguish different isolates of I. multifiliis. Phylogenetically, all I. multifiliis isolates can be divided into three genetically distinct groups, and the two isolates identified in the present study were placed in different groups. In consideration of the high plateau and low water temperature in Tibet, the possible differentiation of I. multifiliis is of interest for further investigation.