The Effect of Farms Spatial Distribution in Salmon Rickettsial Syndrome Mortality in Chilean Salmon Industry

1Joaquin Escobar-Dodero; 1Amy C. Kinsley; 2Fernando O Mardones; 1Andres Perez

1Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota; 2Escuela de Medicina Veterinaria, Pontificia Universidad Catolica de Chile

ABSTRACT

Salmon Rickettsial Syndrome (SRS), caused by Piscirickettsia salmonis, is an infectious disease that causes major health and economic impacts in salmon farming industry. Detected in Chile in 1989, is currently the most prevalent disease in the Chilean salmon industry, causing millions of dollars in losses due to high mortality and prevention strategies. Previous assessments have indicated that demographics and environmental conditions can be associated with the occurrence and outbreaks of SRS during the sea rearing phase, however the potential effect neighboring SRS-infected farms have not been evaluated so far. Aquatic infectious diseases spread among neighboring farms have been studied in different farming contexts and it have been stablished that pathogens can spread around close location by currents, wildlife or fomites. This study aimed to assess the association between farm-level SRS mortality and the occurrence of neighboring SRS-infected farms. To achieve this, we retrospectively gather Atlantic salmon farms production weekly data comprising farm demographics, SRS mortality and sea lice occurrence. To account for SRS contact between salmon farms, we built a spatio-temporal farm contact matrix using farm geolocation and considering a 5 km sea-distance and a week threshold as an active contact and further subset it to account for only neighboring SRS-infected farms. Data was consolidated on a monthly basis; SRS mortality was categorized as high if it reached a 0.01% and sea lice presence was categorized as high burden if it surpassed 3 females per fish. The association between farm SRS mortality and the number of close SRS-infected farms was evaluated using a generalized estimating equations approach to account for correlation between the SRS mortality across months, also including seasonality, farm initial population and sea lice presence as predictors. We obtained longitudinal data from 640 farms production cycles for up to 29 months (range = 11-29 months) stocked at sea between 2012 and 2017, with an average stock of 960.000 fish per farm. The model suggests that being in close contact with SRS-infected farms increases the risk of experiencing a high SRS mortality (OR = 1.25; CI = 1.16 – 1.35) and that high sea lice burden also increases the risk of experiencing a high SRS mortality (OR = 1.36; CI = 1.2 – 1.62). Farm initial population does not associate with an increased risk of high SRS mortality (OR = 1; CI = 1-1), however winter (OR = 0.51; CI = 0.45 – 0.59) and spring (OR = 0.49; CI = 0.42 – 0.56) seasons decrease the risk of experiencing SRS mortality when compared to fall. These results suggest that while accounting for with-in farm factors that can influence the SRS mortality in an Atlantic salmon farm, the presence of nearby infected farms can be associated with higher SRS mortality, which follows the evidence of previous studies that being closed to infected farms increase the risk of experiencing high mortality or outbreaks. Additionally, we showed evidence that sea lice play an important role in the SRS dynamic.