Malformed largemouth bass (Micropterus salmoides) are more susceptible and vulnerable to Ichthyophthirius multifiliis infections: Immunological and histopathological study

It is widely acknowledged that the malformed fish shows low resistance to diseases due to abnormal development of body, leading to the decrease of viability. Until now, there is no evidence for the relationship between developmental abnormalities and pathogen infections in fish breeding. In this study, the first record of Ichthyophthirius multifiliis infection in Micropterus salmoides was reported. Importantly, an interesting phenomenon that malformed fish suffered from more serious Ichthyophthiriasis than normal fish was demonstrated with the investigation of differences in the infection rate and the number of trophonts. Furthermore, the immune response, histopathological examination, and efficacy of antiparasitic drug against I. multifiliis infection were determined in normal and malformed fish. The fish affected by malformation resulted in decline of survival rate after infection with I. multifiliis at day 2 in comparison to the fish with normal development (p < 0.001). Malformation also caused significantly low expression of immune-related genes, including immunoglobulin (IgM), T cell receptor (TCR), pro-inflammatory cytokines (IL-10, IL-1 beta), cell mediated-immunity mediators (MHC-I), chemokine (CCL2), complement (C5), cluster of differentiation (CD4-1 and CD8-a), in skin, liver and spleen (p < 0.05). Also, histopathological examination showed distinct lesions in skin, gills, and fins of malformed fish. In addition, the survival rate of normal fish was significantly elevated after treatment with magnolol, an antiparasitic drug, for 7 days compared to malformed fish (p < 0.001). Overall, these data indicate that weak immune response and more tissue damage in malformed fish might increase the susceptibility to pathogens, and the high mortality rate was associated with poor efficacy of magnolol in malformed fish. This study provides evidence for low resistance to pathogens and high mortality rate of fry caused by high malformation rate. Particularly, the malformed fish may act as a source of infection, spreading infectious disease for other healthy fish, causing mass economic losses for M. salmoides industry. Malformation is worth to pay more attention for fish breeding studies.