Use of regulatory data to describe and identify risk factors for salmonid rickettsial septicaemia in Chile, 2011–2017

Highlights

• Salmonid production species is a key driver of salmonid rickettsial septicaemia (SRS)-attributed mortality outcomes.

• Incidence rates of salmonid rickettsial septicaemia-attributed mortality vary between Chilean regions.

• Hypotheses generated for future investigations of SRS risk factors include the number of seawater phase vaccinations.

• Specific considerations for future research strategies include stratification of analyses by salmonid species.

Abstract

The bacterial disease salmonid rickettsial septicaemia (SRS) has caused substantial economic losses in the salmonid aquaculture industry, particularly in Chile. Increasing understanding of the epidemiology of this disease is important for improving the productivity and sustainability of the industry: by identifying important influences on disease occurrence, interventions can be identified and optimised to minimise disease occurrence and its effect on the industry. This study aimed to describe SRS-attributed mortality across production species, time and space in the Chilean salmonid industry; identify risk factors for mortality attributed to SRS; and generate additional hypotheses for future investigations of SRS epidemiology in the Chilean salmonid industry. Regulatory data held in the Sistema de Fiscalización de la Acuicultura and Informe Ambiental para la Acuicultura databases of Servicio Nacional de Pesca y Acuicultura (‘Sernapesca’, the Chilean National Fisheries and Aquaculture Service) were analysed using the information-theoretic approach for risk factor analysis. Post hoc modelling generated hypotheses for future investigations. Analyses determined associations between the outcomes of SRS-attributed mortality incidence risk and rate across salmonid production cycles with a range of putative risk factors. Results indicate that salmonid production species is a key driver of the risk of a Chilean salmonid production cycle experiencing SRS-attributed mortality, and the SRS-attributed mortality rate amongst affected cycles. Compared to Atlantic salmon, rainbow trout and coho salmon production cycles were less likely to report SRS-attributed mortality (rainbow trout: incidence risk ratio = 0.17, 95% CI 0.07–0.40, p < .001; coho salmon: incidence risk ratio 0.05, 95% CI 0.02–0.11, p < .001). Amongst affected production cycles, coho salmon had a relatively reduced incidence rate of SRS-attributed mortality (incidence rate ratio = 0.32, 95% CI 0.20–0.49, p < .001) whilst rainbow trout had a relatively increased rate (incidence rate ratio = 3.60, 95% CI 2.51–5.17, p < .001). The incidence rate of SRS-attributed mortality was considerably lower in one region (Region XII: incidence rate ratio = 0.000003, 95% CI 0.0000003–0.00002; p < .001) compared to other regions (Regions X and XI). Further hypotheses for investigation generated in this study include associations between aspects of vaccination strategy and improved SRS-attributed mortality outcomes. This study highlights specific considerations for further epidemiological investigations using routinely collected Chilean aquaculture regulatory data.

Introduction

Salmonid rickettsial septicaemia (SRS or piscirickettsiosis) is a potentially severe disease of salmonids caused by infection with Piscirickettsia salmonis bacteria (Rozas and Enríquez, 2014). First described in 1989, SRS has caused considerable economic losses in the salmonid aquaculture industry, particularly in Chile (Rozas and Enríquez, 2014). Vaccines are available against SRS, and certain vaccination strategies increase the time-to-first-outbreak and reduce mortality associated with SRS outbreaks in Chilean farmed Atlantic salmon and rainbow trout (Jakob et al., 2014). Antibiotics are available to treat SRS, but effectiveness can be poor in farmed salmonids and varies with factors such as antibiotic type and time-to-treatment (Almendras and Fuentealba, 1997; Rozas and Enríquez, 2014; Price et al., 2016). Additionally, the use of antibiotics in commercial aquaculture entails concerns about negative impacts on human, animal and environmental health (Millanao et al., 2011; Cabello et al., 2016). Increasing understanding of the epidemiology of SRS in Chilean salmonid aquaculture is important to improve productivity and sustainability of the industry (Mardones et al., 2018). By acquiring information on the drivers of disease occurrence, interventions can be identified and optimised, to effectively minimise disease occurrence and its effect on the industry.

Currently-understood features of SRS epidemiology include that it is almost exclusively a disease of the seawater phase of salmonid production (Rozas and Enríquez, 2014). However, there are few studies of risk factors for the occurrence or severity of SRS. Experimentally, the pathogen is most likely to survive in sea water at 5–10 °C and does not survive in seawater >25 °C (Lannan and Fryer, 1994). These estimates are consistent with anecdotal observations of a seasonal pattern to disease incidence in Chile, with the highest incidence of outbreaks observed in fall and spring (Bravo and Campos, 1989; Cvitanich et al., 1990). Stressors may also be predisposing factors (Garcías et al., 2005). For example, outbreaks have been observed after exposure to fluctuating temperature and non-toxic algae concentrations (Branson and Nieto Díaz-Muñoz, 1991). The length of the fallowing period after a production cycle affected by SRS may affect the occurrence of SRS in the following production cycle (Price et al., 2017).

The objectives of this study were to describe SRS-attributed mortality across production species, time and space in the Chilean salmonid industry; identify risk factors for mortality attributed to SRS; and generate hypotheses for future investigations of SRS epidemiology in the Chilean salmonid industry.