Immune response and apoptosis of gibel carp (Carassius auratus gibelio) gills to Chilodonella hexasticha infection: Insights from histopathological and multi-omics analyses

Gibel carp (Carassius auratus gibelio) is an important economically farmed fish in China. Chilodonella hexasticha parasitizes on the gills and fins of host fish, causing disruption to their normal respiration and movement, ultimately resulting in death of the fish. In this study, a combination of histopathological, immunohistochemical, transferase dUTP nick end labeling (TUNEL), multi-omics, and molecular approaches were employed to identify the immune reaction and cell apoptosis in gill tissue in response to C. hexasticha infection. Significant lamellae fusion, hyperplasia, hyperemia, necrosis, and desquamation of infected gibel carp gills were observed. In total, the expression of 3619 genes was higher, and 3143 lower, for gills in the infected group compared to the control group. Furthermore, 76 metabolites were significantly increased and 105 were significantly decreased in the infected group compared with the control group. From the qRT-PCR analysis results, immune system-related genes encoding IL-8, CXCL8a, and CXC11 were significantly up-regulated in infected gibel carp, while ZAP70 was significantly down-regulated. Immunohistochemical results also showed the down-regulated ZAP70 in the infected group. Apoptosis-related genes encoding CASP3 and Mcl-1b were up-regulated in response to C. hexasticha infection. These genes indicate the activation of CASP family-related apoptosis and Bim-mediated mitochondrial apoptotic pathways. TUNEL assays also revealed severe apoptosis in the infected group. Based on this study's results, it can be concluded that C. hexasticha infection leads to histopathological changes in the gills of infected fish, and induces both a significant immune response and apoptosis.

Epidemiology and identification of two species of Chilodonella affecting farmed fishes in China

The genus Chilodonella includes free-living ciliated protozoa as well as pathogenic species for freshwater fish, with Chilodonella hexasticha and Chilodonella piscicola being the most important ones. These parasites cause outbreaks with high mortalities among farmed freshwater fishes with great economic losses. There are few reports of these species in China, and their identification has been based mostly on their morphological characteristics. In the present work, the parasites causing five outbreaks occurring in China between 2014 and 2017 have been identified by morphological and genetic analysis. We provide the first records of Ctenopharingodon idella and Siniperca chuatsi as hosts of C. hexasticha, and of Procypris rabaudi and Schizothorax wangchiachii as hosts of C. piscicola. There are no differences in the gross pathological findings produced by C. hexasticha and C. piscicola, consisting in desquamation and necrosis of epithelial cells in the skin and gills and in severe fusion of gill lamellae. However, both species differ in their geographic distribution: C. piscicola was found in farms located at altitudes over 1500 m above sea level and with a water temperature ≤18 °C, while C. hexasticha was found in farms located at altitudes under 50 m above sea level and with a water temperature ≥21 °C. Present results confirm that C. hexasticha and C. piscicola are two different species that can be differenced by their morphology; however, their biological variability may lead to erroneous identifications and the diagnosis should be preferably based in genetic analysis including nuclear LSU rDNA and mitochondrial SSU rDNA sequences.