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.