Hydrodynamic connectivity, water temperature, and salinity are major drivers of piscirickettsiosis prevalence and transmission among salmonid farms in Chile

F Bravo,Fo Mardones,Rh Bustamante,F Rizwi,P Bernal,D Jimenez,Adl Steven,S Condie,M Herzfeld,Jps Sidhu,B Gorton

doi:10.3354/aei00368

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https://doi.org/10.3354/aei00368

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Abstract

Piscirickettsiosis is one of the most important diseases affecting farmed salmonid in Chile. Several studies have demonstrated the survival of Piscirickettsia salmonis in seawater and the horizontal transmission from infected to non-infected fish; however, the extent of waterborne transmission between farms has not been quantified. In this study, we used a stochastic hydrodynamic connectivity-based disease spread model to determine the role of hydrodynamic connectivity and the effect of seawater temperature and salinity on the dynamics of piscirickettsiosis in the Los Lagos region of Chile. Results demonstrate that environmental dynamics play a major role in disease prevalence. The strongest determinants of piscirickettsiosis prevalence were the number of infected farms in upstream waters and the extent of disease outbreaks in upstream waters (total mortality), followed by seawater salinity and temperature. In farms downstream from infected farms, observed disease prevalence 25 wk into the farming cycle was close to 100%, while in farms with little or no exposure to upstream, infected farms, prevalence reached only ~10% by the end of the farming cycle (Week 56). No previous studies have quantified the scales of connectivity associated with piscirickettsiosis or provided risk metrics of waterborne transmission of the disease among farms; these are a novel aspect of this research. The above knowledge regarding the use of the epidemiological model will allow industry and regulators to better target disease control strategies for more effective control of piscirickettsiosis in the study area.

Highlights

Aquaculture of seawater salmonid species has expanded rapidly in the last 2 decades to become the second-largest in the world, with an annual production of over 900 000 t (SERNAPESCA 2018a)
Mortality due to piscirickettsiosis was confirmed by certified laboratories through analysis of dead-fish tissue with PCR or indirect fluorescent antibody test (IFAT), following procedures defined by SERNAPESCA (2018d)
The variability in farming cycle length was mostly due to differences in the intrinsic growth rates among salmonid species (e.g. Oncorhynchus kisutch have the shortest production cycle), temperature dependencies, and harvesting due to disease outbreaks

Summary

Introduction

Aquaculture of seawater salmonid species (hereinafter referred to as 'salmon') has expanded rapidly in the last 2 decades to become the second-largest in the world, with an annual production of over 900 000 t (SERNAPESCA 2018a). In Chile, one of the most devastating pathogens of farmed salmon is Piscirickettsia salmonis, a Gramnegative intracellular bacterium that causes salmonid rickettsial syndrome (SRS) or piscirickettsiosis. In salmon-farming countries in the Northern Hemisphere, such as Canada (Cusack et al 2002) and Norway (Olsen et al 1997), the prevalence of piscirickettsiosis is relatively low, whereas in Chile, it is a leading cause of mortality (Rozas & Enriquez 2014, SERNAPESCA 2016, 2018b,c) and high use of antibiotics (Miranda et al 2018). Waterborne transmission of P. salmonis from fish to fish within and across farms has been recognized as the main route of infection (Almendras & Fuentealba 1997, Fryer & Hedrick 2003), but the extent of waterborne transmission between farms has, to date, not been quantified in Chile

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