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Open Access Research

Population structure of the fish pathogen Flavobacterium psychrophilum at whole-country and model river levels in Japan

Erina Fujiwara-Nagata12*, Céline Chantry-Darmon23, Jean-François Bernardet2, Mitsuru Eguchi1, Eric Duchaud2 and Pierre Nicolas3

Author Affiliations

1 Department of Fisheries, Kinki University, Nara, Japan

2 INRA, Virologie et Immunologie Moléculaires UR892, Jouy-en-Josas, France

3 INRA, Mathématique Informatique et Génome UR1077, Jouy-en-Josas, France

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Veterinary Research 2013, 44:34  doi:10.1186/1297-9716-44-34

Published: 17 May 2013

Abstract

The bacterium Flavobacterium psychrophilum is a serious problem for salmonid farming worldwide. This study investigates by multilocus sequence typing (MLST) the population structure of this pathogen in Japan where it is also a major concern for ayu, a popular game fish related to salmoniforms. A total of 34 isolates collected across the country and 80 isolates sampled in a single model river by electrofishing were genotyped. The data accounting for 15 fish species allowed identifying 35 distinct sequence types (ST) in Japan. These ST are distinct from those reported elsewhere, except for some ST found in rainbow trout and coho salmon, two fish that have been the subject of intensive international trade. The pattern of polymorphism is, however, strikingly similar across geographical scales (model river, Japan, world) in terms of the fraction of molecular variance linked to the fish host (~50%) and of pairwise nucleotide diversity between ST (~5 Kbp-1). These observations go against the hypothesis of a recent introduction of F. psychrophilum in Japan. Two findings were made that are important for disease control: 1) at least two independent F. psychrophilum lineages infect ayu and 2) co-infections of the same individual fish by different strains occur.