Investigations on white spot disease reveal high genetic diversity of the fish parasite, Ichthyophthirius multifiliis (Fouquet, 1876) in China

Ichthyophthirius multifiliis (Fouquet, 1876) is a highly pathogenic ciliate of freshwater fish that threatens the global aquaculture. During the investigations of ichthyophthiriasis in China, a total of sixteen I. multifiliis isolates were collected from four provinces, Hubei, Hunan, Guangdong, and Shanghai. The purpose of this study was to compare the epidemic information of the different isolates and determine the genetic diversity of I. multifiliis in China. The clinical signs and histopathological changes caused by I. multifiliis infection were presented in all the collected fish samples and no obvious differences were observed among the fish or tissues. However, significant genetic variation was found in the cytochrome c oxidase subunit I (cox-1, 0.07%–2.19%,) and NADH dehydrogenase subunit I (nad1_b, 0.00%–3.23%) sequences of the sixteen isolates. Based on the concatenated cox-1 and nad1_b sequences, all the sixteen isolates were defined as novel genotypes. The phylogenetic analyses of these genotypes suggested at least three genetically distinct groups exist in China. This was confirmed by further analyses, which divided the all the forty reported I. multifiliis isolates into nine genetic groups, while the isolates from this study were distributed among seven of these groups. Additionally, isolates from the same sources shared a closer phylogenetic relationship. However, some groups, containing isolates from China, the United States, and Turkey, also reflected the frequently interaction of I. multifiliis between these countries. These data demonstrated a higher level of genetic diversity in I. multifiliis in China, which facilitates a better understanding of the migration and evolution of this parasite.