Spatial and temporal distribution of Neoparamoeba perurans in a tank recirculation system during experimental AGD challenge

Amoebic gill disease (AGD) is the major disease negatively impacting Atlantic salmon aquaculture in Tasmania, Australia. From an epidemiological perspective, it is essential to determine the reservoirs of AGD etiological agent Neoparamoeba perurans. During tank-based experimental infections it was demonstrated that the concentration of N. perurans was significantly higher in the water column (13±7cells/L after 22days) than on the interface surface air–water-tank (0.01±0.1cells/L) in a recirculation system with Atlantic salmon in a stocking density of 7.5kg/m3 and weekly water changes. These are lower numbers compared to those found on farmed Atlantic salmon chronically affected with AGD that can reach 100 times more cells/swab. This suggests that fish themselves and not water are reservoirs of this pathogen. A similar concentration was observed in a different system, the recirculating seawater system where N. perurans infection is perpetuated with 1.7kg/m3 stocking density of Atlantic salmon but no water changes. This suggests that a critical maximum concentration of the amoeba in seawater and time exposure for fish are what need to be corroborated in field studies.