The spatiotemporal distribution and risk factors of amoebic gill disease in the Chilean salmon farms 2013–2022

The seasonal presence of Amoebic Gill Disease (AGD) in farmed Salmo salar was detected in preliminary samplings to characterize AGD in relation to other coinfections on the east coast of Chiloé Island. Histopathological and PCR analyses of gills revealed AGD and its etiological agent, Neoparamoeba perurans, in the Hueihue farm in March and April 2013. Therefore, to determine the spread of AGD in the Chilean salmon industry, data from the Passive Surveillance Program by SERNAPESCA were analyzed from 2013 to 2022. The reports of five farms with amebiasis in 2013 increased to 496 in 2022, but particularly, there was an explosive increase in the number of fish with other pathogens in their gills, due to Complex Gill Disease (CGD) sometimes concurrently occurring with AGD. The annual incidence of AGD ranged from 1 week to 37 weeks per farm as discontinuous outbreaks. Infected fish were almost all adults, with S. salar being the most affected species, with the largest production, but showing low correlations between production and the number of affected farms. Positive farms and production were reduced for Oncorhynchus kisutch, and it was almost zero in Oncorhynchus mykiss. More infected farms were identified in autumn and summer for both S. salar and O. kisutch. The impact of AGD is probably low in Chile, due to the focus on S. salar production and the continuous harvesting model. More AGD-infected farms were in insular locations compared to continental areas, more in Los Lagos rather than Aysén Region, but almost zero farms were in Magallanes. Risk factors and oceanographic characteristics that explain the occurrence of disease outbreaks are discussed, as well as gill surveillance in live fish and management methods.