Epidemiological situation and phylogenetic relationship of Vibrio harveyi in marine-cultured fishes in China and Southeast Asia

In recent years, marine fish farms in China and Southeast Asian countries, where are the most important maricultural areas around the world, have frequently suffered severe economic losses due to fish diseases. The objective of this study was to clarify the dominant bacterial pathogen in diseased fish collected from China, Vietnam, and Malaysia from 2015 to 2018, and investigate their genetic relationships by multilocus phylogenetic analysis (MLPA). A total of 645 isolates, 371 from diseased fish and 274 from healthy fish, were collected, and Vibrio harveyi was identified as the dominant bacterial pathogen in the samples. V. harveyi was detected in 88.76% of diseased fish and 36.27% of healthy fish, which accounted for 52.02% and 12.77% of the total isolates from diseased and healthy fish, respectively. The intraand inter-area phylogenetic relationships of 228 V. harveyi isolates were investigated via MLPA based on the concatenated sequences of five house-keeping genes fragments (rpoA-pyrH-topA-ftsZ-mreB, 3, 264 base pairs (bp)). Close genetic relationships were observed among the V. harveyi isolates, including minor difference of nucleotide substitution, and intraand inter-area phylogenetic depths. The phylogenetic tree also showed that isolates from different areas were randomly distributed on clades, and it was difficult to establish an obvious correlation between the genetic relationships and V. harveyi isolation sites. Therefore, closely related V. harveyi are highly prevalent in marine farms in China and Southeast Asian countries, which is a potentially great challenge to maricultural management and disease prevention.