Epidemiological situation and phylogenetic relationship of Vibrio harveyi in marine-cultured fishes in China and Southeast Asia
Introduction
In recent decades, China and Southeast Asia have become the major maricultural regions of the world. In China, total production of dozens of farmed marine fish species reached 1.50 million tons in 2018 (Ministry of Agriculture and Rural Affairs of the People's Republic of China, 2019). For instance, the annual production of each of the large yellow croaker (Pseudosciaena crocea), Japanese seabass (Lateolabrax japonicus) and grouper (Epinephelus spp.) were over 150, 000 tons, and turbot (Scophthalmus maximus) production was over 100, 000 tons in 2018. Asian seabass (Lates calcarifer), Pompanos (Trachinotus spp.), and Epinephelus spp. are the most important commercial species in Southeast Asian countries such as Vietnam, Thailand, Malaysia, etc. (FAO, 2006–2019; FAO, 2016–2019). Marine fish of large-scale cultured is rich in high-quality protein, and contributes substantially to improved food and health quality among Asians and globally. However, the pathogenic Vibrio harveyi which causes scale drop and muscle necrosis were widely reported in commercial fish, such as P. crocea (Liu et al., 2016), Epinephelus spp. (Shen et al., 2017; Zhu et al., 2018), Japanese flounder (Paralichthys olivaceus) (Li et al., 2011), cobia (Rachycentron canadum) (Liu et al., 2004) and L. calcarifer (Ransangan et al., 2012; Dong et al., 2017) in China and Southeast Asian countries.
V. harveyi, a Gram-negative bacterium commonly found in marine aquaculture environments, has been reported as a major pathogen in both vertebrates and invertebrates (Austin and Zhang, 2006). Infection by V. harveyi commonly causes acute mortality in commercial aquatic animals resulting in serious economic losses to mariculture industry (Cano-Gómez et al., 2009). Although reports of V. harveyi causing disease in fish and shrimp had been recorded since 1990 (Kraxberger-Beatty et al., 1990; Lavillapitogo et al., 1990), diseases caused by this bacterium still prevalent not only in China and Southeast Asia, but also in other regions, such as the Mediterranean Sea (Firmino et al., 2019) and India (Stalin and Srinivasan, 2016). In 2017, V. harveyi was firstly reported as a pathogen that caused mass mortality in ark clam (Scapharca broughtonii) in the Bohai Sea in northern China (Wei et al., 2019). Thus, aquatic animals that infected by V. harveyi possibly increased with the rapid development of the aquaculture industry. Although V. harveyi was widely reported as bacterial pathogen in China and Southeast Asia, almost all of the previous studies were focused on specific farms or small areas. There is a lack of epidemiological surveillance of V. harveyi to show its prevalent situation in the main aquatic regions of China and Southeast Asia. In an earlier study based on 16S rDNA analysis, we found that V. harveyi isolates which were isolated from south-east coastal areas of China were close relatives of those from the north-east (Zhu et al., 2018). However, the genetic relationships among V. harveyi isolates, which are widely distributed in the coastal environment of China and around the South China Sea, remain unclear. Thus, epidemiological investigation and phylogenetic analysis of V. harveyi is essential for maricultural environment management and disease prevention in these areas.
In this study, diseased and healthy fish samples belonging to seventeen species were collected from the main maricultural areas in China, Malaysia, and Vietnam. Bacterial species composition and abundance analysis was conducted to investigate the dominant bacterial species. Sequencing and phylogenetic analysis based on 16S rDNA was performed for preliminary identification of isolates. And then multi-locus phylogenetic analysis (MLPA) of five house-keeping genes, encoding RNA polymerase alpha subunit (rpoA), uridylate kinase (pyrH), topoisomerase I (topA), cell division protein (ftsZ) and rod shaping protein (mreB), was carried out to clarify the genetic relationship between V. harveyi isolated from marine fish in China and Southeast Asia. The results will be helpful for comprehensive understanding of the epidemiology of V. harveyi and developing scientific and effective strategies for disease control in China and Southeast Asia.
Section snippets
Sampling and bacteria isolation
Marine-cultured fish samples were collected from nine major culture areas in China, Malaysia and Vietnam from 2015 to 2018, including six marine cage-culture sampling sites: Qingdao City (Shandong Province, SD; abbreviated to SDQD), Raoping County (Guangdong Province, GD; GDRP), Huizhou and Zhuhai Cities (GD; GDHZ and GDZH, respectively), and Wenchang and Danzhou Cities (Hainan Province, HN; HNWC and HNDZ, respectively), where distributed from high to low latitude along the coastline of China;
Characteristic symptoms of diseased fish
A total of 191 marine fish samples, which belonging to 17 species and including diseased and healthy individuals, were collected in this study (Table 2). The number of fish samples ranged from 6 to 75 among nine sampling sites, and those most important commercial fish species in China and Southeast Asian such as E. fuscoguttatus♀ × E. lanceolatus♂, pompano (T. ovatus), L. calcarifer, Lutjanus spp., and R. canadum were all collected (Table 2). Typical symptoms of partial diseased fish were shown
Discussion
Although reports of V. harveyi as a pathogen in vertebrates and invertebrates date back decades (Kraxberger-Beatty et al., 1990; Lavillapitogo et al., 1990), a serious disease with typical scale drop and muscle necrosis symptoms which also caused by V. harveyi still widely occurred in marine cultured fish species in China and Southeast Asian countries recently (Li et al., 2011; Sharma et al., 2014; Liu et al., 2016; Tu et al., 2017; Dong et al., 2017; Shen et al., 2017; Zhu et al., 2018). In
Declaration of Competing Interest
We have read and understood your journal's policies, and we believe that neither the manuscript nor the study violates any of these.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Science and Technology Planning Project of Guangdong Province of China [Grant Number 2018A050506027]; the National Natural Science Foundation of China [Grant Number 31702380]; the Fundamental Research Funds for the Central Universities [Grant Number 18lgpy30]; and the China-ASEAN Center for Joint Research and Promotion of Marine Aquaculture Technology. The authors wish to acknowledge the generous assistance of Goh Siong Tee Marine Product Sdn Bhd (Malaysia) for
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