Is between-farm water-borne pathogen dissemination an important driver in the epidemiology of salmonid rickettsial septicaemia in Chile?

Abstract

Although the bacterium Piscirickettsia salmonis has been detected in many salmon-producing countries around the world, losses caused by salmonid rickettsial septicaemia (SRS) are mostly occurring in the Chilean aquaculture industry. Horizontal transmission of SRS between salmonid farms was suggested, based on the existence of spatiotemporal correlation in the level of disease between neighbouring sea farms. However, it remains unclear to which extent between-farm water-borne pathogen dissemination is important in the epidemiology of SRS in Chile. Such information is critical to assess the level of risk of transmission of SRS from one farm to another at different mortality incidence levels and to apply appropriate and cost-effective mitigation measures. In this study, we used weekly SRS mortality data from all salmonid farms in the Los Lagos region between January 2012 and September 2018 to model the spatiotemporal autocorrelation in the SRS-attributed mortality in the study area. A generalized additive regression modelling framework was adopted, using a linear functional component to model the influence of other farms on the target farm. Several nested statistical models were built to compare the significance of different covariates. Predicted values of SRS mortality on the target farm, conditional on different distance, time lag and mortality values from the source farms were estimated from the best model. The results showed that there was a statistically significant association between the weekly mortality incidence at source farms and the mortality incidence at target farms during the same week and during the previous weeks. This study did not provide evidence that the spatiotemporal correlation observed in SRS mortality may be due to water-borne pathogen dissemination between farms and alternative explanatory mechanisms should be investigated. It remains possible that the patterns of lagged correlation observed between source and target farm mortality may be due to a model artefact. In addition, there was no evidence of a threshold effect above which farms pose a substantially larger health risk to their neighbours. Stronger evidence for or against between-farm transmission of P. salmonis may be obtained by different methods.