Histopathological parameters, antioxidant enzyme levels, transcriptome, and hematology parameters of Amur minnow (Phoxinus lagowskii) infection with Tetrahymena pyriformis

Phoxinus lagowskii is a widely distributed and predominantly cultured species in the northeast of China. P. lagowskii is highly susceptible to the infection of Tetrahymena in cultivation. Fast increased cell number of Tetrahymena in water for lab farming fish and fish death showed P. lagowskii were seriously infected by T. pyriformis. The fish histological, physiology, and molecular response to T. pyriformis infections is still largely unknown. Therefore, this research was performed to evaluate P. lagowskii infected by T. pyriformis with parameters of mortality, behavior, hematology parameters, antioxidant enzyme activities, and transcription analysis. P. lagowskii showed spin, jump, cluster, sluggish swim, and the big breath after Tetrahymena infection. Histological observation of the gills confirmed that the lamella area was increased, which was beneficial for maintaining respiration. WBC, RBC, Hct, and Hb were generally lower in the infection group when compared to the control group, demonstrated the impairment in the generation of erythrocytes, thus altering the red blood cells' proper functioning. An increase in the gill lamella surface area observed in the present study suggests the hypoxia-like condition in P. lagowski after ciliate infection. Enzyme activity of SOD and GST in the gills was significantly increased compared to control but not in the tissue of liver and spleen, demonstrating the gills playing key roles in the anti-oxidation. To understand the knowledge of immune response against T. pyriformis, transcriptome analysis of gill in P. lagowskii at 48 h post-infection was performed. After the challenge of T. pyriformis, 87 genes were identified as remarkably upregulated, and 76 genes were downregulated. DEGs are associated with innate immune molecules, including oxidation-reduction, innate immune molecules, anti-inflammation, apoptosis-related molecules, and chemokine signaling pathways. Moreover, the expression regulation of seven genes was further validated by qRT-PCR. As such, we documented for the first time that T. pyriformis infected freshwater fish P. lagowskii, the comprehensive results provided more information for the research of ciliates disease and the controlling T. pyriformis in a culture of freshwater fish.

Automated summary

Phoxinus lagowskii, a widely distributed and predominantly cultured species in northeast China, is highly susceptible to Tetrahymena infection in cultivation. Infected P. lagowskii showed abnormal behavior and hematological changes, with increased antioxidant enzyme activities in the gills. Transcriptome analysis revealed upregulation of genes associated with innate immune responses and downregulation of genes related to inflammation and apoptosis upon T. pyriformis infection. This study provides insights into the immune response and potential control measures for ciliate diseases in freshwater fish.