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Ichthyophthirius multifiliis , a ciliated parasite causing ichthyophthiriasis (white spot disease) in freshwater fishes, results in significant economic loss to the aquaculture sector. One of the important predisposing factors for ichthyophthiriasis is low water temperature (i.e., below 20°C), which affects the health and makes freshwater fishes more susceptible to parasitic infections. During ichthyophthiriasis, fishes are stressed and acute immune reactions are compromised, which enables the aquatic bacterial pathogens to simultaneously infect the host and increase the severity of disease. In the present work, we aimed to understand the parasite-bacteria co-infection mechanism in fish. Later, Curcuma longa (turmeric) essential oil was used as a promising management strategy to improve immunity and control co-infections in fish. A natural outbreak of I. multifiliis was reported (validated by 16S rRNA PCR and sequencing method) in Pangasianodon hypophthalmus from a culture facility of ICAR-CIFRI, India. The fish showed clinical signs including hemorrhage, ulcer, discoloration, and redness in the body surface. Further microbiological analysis revealed that Aeromonas hydrophila was associated (validated by 16S rRNA PCR and sequencing method) with the infection and mortality of P. hypophthalmus , confirmed by hemolysin and survival assay. This created a scenario of co-infections, where both infectious agents are active together, causing ichthyophthiriasis and motile Aeromonas septicemia (MAS) in P. hypophthalmus . Interestingly, turmeric oil supplementation induced protective immunity in P. hypophthalmus against the co-infection condition. The study showed that P. hypophthalmus fingerlings supplemented with turmeric oil, at an optimum concentration (10 ppm), exhibited significantly increased survival against co-infection. The optimum concentration induced anti-stress and antioxidative response in fingerlings, marked by a significant decrease in cortisol and elevated levels of superoxide dismutase (SOD) and catalase (CAT) in treated animals as compared with the controls. Furthermore, the study indicated that supplementation of turmeric oil increases both non-specific and specific immune response, and significantly higher values of immune genes (interleukin-1β, transferrin, and C3), HSP70, HSP90, and IgM were observed in P. hypophthalmus treatment groups. Our findings suggest that C. longa (turmeric) oil modulates stress, antioxidant, and immunological responses, probably contributing to enhanced protection in P. hypophthalmus . Hence, the application of turmeric oil treatment in aquaculture might become a management strategy to control co-infections in fishes. However, this hypothesis needs further validation., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Kumar, Das, Swain, Chowdhury, Roy, Bera, Das, Parida, Dhar, Jana and Behera.)

Cytotoxic T cells are known to have the ability to kill microbe-infected host cells, which makes them essential in the adaptive immunity processes of various vertebrates. In this study, we demonstrated innate cell-mediated cytotoxicity of CD8 + T cells against protozoan parasites found in the ginbuna crucian carp. When isolated effector cells such as CD8 + , CD4 + (CD4-1 + ), or CD8 - CD4 - (double-negative, DN), from naïve ginbuna crucian carp were co-incubated with target parasites (Ichthyophthirius multifiliis), CD8 + cells from the kidney and gill showed the highest cytotoxic activity. On the other hand, DN cells, which include macrophages and CD4 - CD8 - lymphocytes, showed the lowest cytotoxic activity against I. multifiliis. Additionally, the cytotoxic activity of CD8 + cells was found to significantly decrease in the presence of a membrane separating the effector cells from I. multifiliis. Furthermore, the serine protease inhibitor 3,4-dichloroisocoumarin and perforin inhibitor concanamycin A significantly inhibited the cytotoxic activity of CD8 + cells. These results demonstrate that CD8 + T cells of ginbuna crucian carp can kill extracellular parasites in a contact-dependent manner via serine proteases and perforin. Therefore, we conclude that CD8 + T cells play an essential role in anti-parasite innate immunity of teleost fish., (Copyright © 2020 Elsevier Ltd. All rights reserved.)