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Background: Myxozoan parasites pose emerging health issues for wild and farmed salmonid fish. Rainbow trout 13 (Oncorhynchus mykiss) is a particularly susceptible species to Tetracapsuloides bryosalmonae (Malacosporea), the 14 etiological agent of Proliferative Kidney Disease (PKD), and to Myxobolus cerebralis (Myxosporea), the etiological 15 agent of Whirling Disease (WD). The objective of this study was to investigate the impact of concurrent myxozoan 16 co-infections on the pathogenesis of PKD and WD in the rainbow trout. 17 Methods: Two groups of rainbow trout (96 fish each) were primarily infected with T. bryosalmonae and triactinomyxons 18 of M. cerebralis; after 30 days half of the fish in each group were co-infected with these parasites vice versa and 19 remaining half was continued as single infection. Mortalities and clinical signs were recorded at different time points. 20 Histopathology and immunohistochemistry were performed to assess the extent of each infection and estimate the 21 parasite burden between groups. 22 Results: Fish firstly infected with M. cerebralis and co-infected with T. bryosalmonae exhibited exacerbated pathological 23 changes of both parasitic diseases and elicited a higher mortality rate. A higher kidney swelling index (grade 4) appeared 24 together with more severe cartilage destruction and displacement, when compared to the pathological changes in fish 25 upon single infections with T. bryosalmonae or M. cerebralis. Conversely, fish firstly infected with T. bryosalmonae and 26 co-infected with M. cerebralis also exhibited typical pathological changes of both parasitic diseases, but with a 27 lower mortality rate, similar as caused by the single T. bryosalmonae or M. cerebralis infection. WD clinical signs were 28 milder, without skeletal deformities, while kidney swelling index was similar to single infection with T. bryosalmonae 29 (grade 2 to 3). 30 Conclusions: In this study, a concurrent co-infection between myxozoan parasites was for the first time successfully 31 achieved in the laboratory under controlled conditions. The impact of co-infections in concurrent myxozoan infections 32 mainly depends on the primary pathogen infecting the host, which could alter the outcomes of the secondary pathogen 33 infection. The primary M. cerebralis infection followed by T. bryosalmonae had a much more serious impact and elicited a 34 synergistic interaction. Contrasting results were instead seen in rainbow trout primarily infected with T. bryosalmonae and 35 then co-infected with M. cerebralis.

Background: Myxozoan parasites pose emerging health issues for wild and farmed salmonid fish. Rainbow trout 13 (Oncorhynchus mykiss) is a particularly susceptible species to Tetracapsuloides bryosalmonae (Malacosporea), the 14 etiological agent of Proliferative Kidney Disease (PKD), and to Myxobolus cerebralis (Myxosporea), the etiological 15 agent of Whirling Disease (WD). The objective of this study was to investigate the impact of concurrent myxozoan 16 co-infections on the pathogenesis of PKD and WD in the rainbow trout. 17 Methods: Two groups of rainbow trout (96 fish each) were primarily infected with T. bryosalmonae and triactinomyxons 18 of M. cerebralis; after 30 days half of the fish in each group were co-infected with these parasites vice versa and 19 remaining half was continued as single infection. Mortalities and clinical signs were recorded at different time points. 20 Histopathology and immunohistochemistry were performed to assess the extent of each infection and estimate the 21 parasite burden between groups. 22 Results: Fish firstly infected with M. cerebralis and co-infected with T. bryosalmonae exhibited exacerbated pathological 23 changes of both parasitic diseases and elicited a higher mortality rate. A higher kidney swelling index (grade 4) appeared 24 together with more severe cartilage destruction and displacement, when compared to the pathological changes in fish 25 upon single infections with T. bryosalmonae or M. cerebralis. Conversely, fish firstly infected with T. bryosalmonae and 26 co-infected with M. cerebralis also exhibited typical pathological changes of both parasitic diseases, but with a 27 lower mortality rate, similar as caused by the single T. bryosalmonae or M. cerebralis infection. WD clinical signs were 28 milder, without skeletal deformities, while kidney swelling index was similar to single infection with T. bryosalmonae 29 (grade 2 to 3). 30 Conclusions: In this study, a concurrent co-infection between myxozoan parasites was for the first time successfully 31 achieved in the laboratory under controlled conditions. The impact of co-infections in concurrent myxozoan infections 32 mainly depends on the primary pathogen infecting the host, which could alter the outcomes of the secondary pathogen 33 infection. The primary M. cerebralis infection followed by T. bryosalmonae had a much more serious impact and elicited a 34 synergistic interaction. Contrasting results were instead seen in rainbow trout primarily infected with T. bryosalmonae and 35 then co-infected with M. cerebralis.