Abstract
Streptococcus iniae is an emerging zoonotic pathogen of increasing concern for aquaculture and has caused several epizootics in reef fishes from the Caribbean, the Red Sea and the Indian Ocean. To study the population structure, introduction pathways and evolution of S. iniae over recurring epizootics on Reunion Island, we developed and validated a Multi Locus Sequence Typing (MLST) panel using genomic data obtained from 89 isolates sampled during epizootics occurring over the past 40years in Australia, Asia, the United States, Israel and Reunion Island. We selected eight housekeeping loci, which resulted in the greatest variation across the main S. iniae phylogenetic clades highlighted by the whole genomic dataset. We then applied the developed MLST to investigate the origin of S. iniae responsible for four epizootics on Reunion Island, first in inland aquaculture and then on the reefs from 1996 to 2014. Results suggest at least two independent S. iniae emergence events occurred on the island. Molecular data support that the first epizootic resulted from an introduction, with inland freshwater aquaculture facilities acting as a stepping-stone. Such an event may have been facilitated by the ecological flexibility of S. iniae, able to survive in both fresh and marine waters and the ability of the pathogen to infect multiple host species. By contrast, the second epizootic was associated with a distinct ST of cosmopolitan distribution that may have emerged as a result of environment disturbance. This novel tool will be effective at investigating recurrent epizootics occurring within a given environment or country that is despite the fact that S. iniae appears to have low genetic diversity within its lineage.
Highlights
Streptococcal infections underlie disease outbreaks in numerous farmed and wild fish species, causing septicemia, central nervous system damage and meningoencephalitis (Eldar et al, 1994; Toranzo et al, 2005)
LIAN analysis of linkage disequilibrium based on the eight housekeeping genes suggested that the bacterial population analyzed in this study is in linkage disequilibrium when considering all 89 strains (IAS = 0.1348; p < 0.01), or a subset consisting of the 13 unique sequence type (ST) (IAS = 0.1839; p < 0.01)
Multi Locus Sequence Typing (MLST) based on the sequencing of a few housekeeping genes was developed almost 20 years ago, it is still a rapid, convenient and relevant method to shed light on the origin and long term evolution of bacteria (Pérez-Losada et al, 2013; Jolley and Maiden, 2014)
Summary
Streptococcal infections underlie disease outbreaks in numerous farmed and wild fish species, causing septicemia, central nervous system damage and meningoencephalitis (Eldar et al, 1994; Toranzo et al, 2005). The pathologic agent, Streptococcus iniae has been directly linked to massive economic losses in both marine and freshwater aquaculture environments, with mortality rates reaching 75% in tilapia farms for example (Perera et al, 1994; Eldar et al, 1997a; Francis et al, 2014). First isolated in the 1970s (from abscesses in captive freshwater dolphins), the first recorded S. iniae outbreaks in farmed fish were documented in the 1980s throughout Japan, the United States, Israel, Australia and Asia (Eldar et al, 1994; Perera et al, 1994; Stoffregen et al, 1996; Bromage et al, 1999; Nguyen et al, 2002). The efficacy of vaccination has been challenged as reinfection of vaccinated stock are known to occur, most notably following the emergence of new serotypes bypassing vaccine protection through spontaneous point mutations in genes involved in capsule biosynthesis (Bachrach et al, 2001; Eyngor et al, 2008; Millard et al, 2012; Barnes and Silayeva, 2016)
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