INTRODUCTION
Interest in tilapia is increasing in aquaculture because the species is affordable to grow and maintain, is an inexpensive source of protein and nutrients, and has high tolerance to stress induced by handling, high-density conditions, and a wide range of environmental conditions (Prabu et al., 2019). To date, tilapia, which has been farmed in over 135 countries, is the second most important farmed fish worldwide (FAO, 2018; Prabu et al., 2019). In 2015, global tilapia production was 6.4 million tons, with an estimated value of $9.8 billion and a trade value of $1.8 billion (FAO, 2017). Vietnam is among the top seven tilapia producers worldwide (FAO, 2017). The yield of tilapia in Vietnam reached 255,000 tons in 2018, and its production aims to reach 400,000 tons by 2030 (MARD, 2019). Intensive production and growing numbers of tilapia farms have led to the emergence and rapid spread of infectious diseases, which is likely to have a significant impact on overall tilapia production (Li et al., 2015; Romero et al., 2012).
Edwardsiella ictaluri a gram-negative rod-shaped bacterium belonging to the family Enterobacteriaceae, is the causative pathogen of enteric septicemia in channel catfish (ESC) in the United States (USA) (Hawke et al., 1981). The bacterium is a facultative intracellular pathogen that survives inside channel catfish phagocytes, such as macrophages and neutrophils (Baldwin & Newton, 1993; Waterstrat et al., 1991). To date, the pathogen has been reported to infect other catfish species, including walking catfish and hybrid catfish in Thailand (Kasornchandra et al., 1987; Suanyuk et al., 2014), striped catfish in Vietnam, Indonesia, and Thailand (Dong et al., 2015; Rogge et al., 2013; Yuasa et al., 2003), yellow catfish in China (Liu et al., 2010) and non-catfish species, including zebrafish in the USA (Hawke et al., 2013), tilapia in the USA (Soto et al., 2012), and wild ayu in Japan (Hassan et al., 2012; Sakai et al., 2008). Naturally, disease outbreaks cause 40%–90% mortality (Dong et al., 2019; Dung et al., 2004; Iwanowicz et al., 2006), while experimental infection results in up to 100% mortality (Dong et al., 2019; Ngoc Phuoc et al., 2020; Plumb & Sanchez, 1983; Sakai et al., 2008), indicating that E. ictaluriis a deadly pathogen in multiple freshwater fish species.
The Vietnamese catfish industry has suffered from Edwardsiellosis caused by E. ictaluri for almost two decades (Ferguson et al., 2001). The emergence of natural cases of E. ictaluri in red tilapia in open floating cages in northern Vietnam in 2016 raised alarm for the widespread appearance of this emerging pathogen in this significantly important industry (Dong et al., 2019). A recent comparative genomic analysis of E. ictaluri from different fish hosts revealed four distinct host-specific genotypes, indicating that the E. ictaluristrain from tilapia is an emerging, unique genotype (Machimbirike et al., 2021). However, little is known regarding the arrival, establishment, and spread of this E. ictaluri strain in the tilapia aquaculture industry in Vietnam. This study provides comprehensive information on the establishment and spread of a highly pathogenic E. ictaluri strain in northern Vietnam and highlights the importance of biosecurity measures, especially for imported stocks, to prevent the spread of this emerging pathogen.