Pathogen coinfections in wild and farmed salmonids: A systematic review

Coinfections have been reported in a wide range of organisms, including wild and farmed salmon and trout. However, the effects of coinfections on host resistance to pathogens remain poorly understood. A primary pathogen can modulate the host's immune response to subsequent infections, either by suppressing or priming the immune system. However, in some cases, pathogens may compete within the same host, potentially reducing or neutralizing the severity and impact of the disease. This systematic review examines the impacts of coinfections on farmed and wild salmonids, synthesizing evidence from 146 studies that met the eligibility criteria. Field evidence from 90 studies demonstrates that pathogen coinfections are widespread in salmonids across diverse geographic regions and aquatic environments. Coinfections were most frequently reported in Atlantic salmon and rainbow trout but were also documented in multiple wild salmonid species, indicating that coinfection is a pervasive phenomenon across the Salmonidae family. Complex coinfections involving three or more pathogens were common, and among pairwise interactions, parasite-parasite, virus-virus and bacteria-bacteria coinfections predominated. Experimental evidence shows that both intra- and interspecific coinfections strongly influence disease outcomes in salmonids. Notably, strategies to mitigate coinfection remain poorly explored, and there is still limited understanding of how host immune responses shape pathogen-pathogen interactions. Overall, coinfection emerges as a context-dependent and underappreciated driver of disease risk across wild and farmed salmonids, highlighting the need to strengthen disease surveillance frameworks through explicit coinfection reporting, as current monitoring approaches likely underestimate its frequency and epidemiological relevance.