Desmozoon lepeophtherii (microsporidian) infections and pancreas disease (PD) outbreaks in farmed Atlantic salmon (Salmo salar L.)

An association between infections with the ubiquitous microsporidian Desmozoon lepeophtherii (D. lepeophtherii) and pancreas disease (PD) outbreaks has previously been suggested. We followed infections with D. lepeophtherii and salmonid alpha virus (SAV) in thirteen Atlantic salmon farms in western Norway during the first production year at sea. The purpose of the survey was to examine if the presence and density of infection with the microsporidian could be linked to severity of PD-infection. Since the microsporidian develops both in salmon and in salmon lice (Lepeophtheirus salmonis) and louse abundance varies geographically, we also examined the spatial and temporal distribution and density of D. lepeophtherii infections in farmed Atlantic salmon. The prevalence of D. lepeophtherii was 100% in all samples from all farms. D. lepeophtherii densities varied with season, peaking in autumn followed by a marked decrease in winter. Geographical differences between areas were observed in August, between the outer coast (salinity 23.5 ppt.) and the Hardangerfjord (inner area) (16.1 ppt.) with higher infection rates in the outer coastal areas. We likewise found that D. lepeophtherii density was significantly associated with the host condition factors; D. lepeophtherii density being significantly higher in salmon with low status. SAV infections occurred in six farms. Five of these developed and were diagnosed with PD. No relationship was detected between D. lepeophtherii density, the detection of SAV or PD outbreaks over time. Statement of relevance: This is an epizootiological study of Atlantic salmon, P. theridion and SAV. There is much known about the pathogen in connection with disease outbreaks, but less is known about the occurrence of potential pathogen in healthy farmed salmon or how such infections develop over time. P. theridion has been connected to SAV infection and PD outbreak. The result from this study does not support such connection. It appears that the SAV virus can persist in a farmed population at least 10 months prior to a PD outbreak. There is a connection between the salmon condition and density of P. theridion where salmon with low condition has elevated density. This could identify P. theridion as the agent possibly causing this condition. (C) 2016 Published by Elsevier B.V.