Introduction, expansion and coexistence of epidemic Flavobacterium psychrophilum lineages in Chilean fish farms

Chile is one of the countries where the development of salmonid farming has been the most successful. The first importation of salmonids in Chile from the northern hemisphere dates back to the late 19th century and the country now ranks as the world second largest producer of farmed salmon. However, the fast increase of infections caused by the bacterium Flavobacterium psychrophilum is a growing concern for this local industry. This pathogen, also recognized as an important problem worldwide, has been first reported in Chile in 1993 and is currently affecting all three cultivated salmonid species: Atlantic salmon (Salmo salar), Coho salmon (Oncorhynchus kisutch) and rainbow trout (O. mykiss). Here we conducted a MLST (multi-locus sequence typing) analysis of the local genetic diversity of F. psychrophilum to better understand its origin and propagation in the country, and to suggest practices that could contribute to its control in the future. A total of 94 bacterial isolates, collected from the main production zones, were analyzed and compared to those of other origins already available. The data reveal the country-wide distribution of several genotypes closely related to those that are the most prevalent in European and North American fish farms, and overlapping host fish species of the different lineages. This population structure is probably the direct consequence of local fish farming practices that relied until recently on massive import of fish eggs (e.g., 78 million of eggs in 2012) and where mixed-species farms and fish transportation across the country are common.