Immersion challenge of naïve Atlantic salmon with cultured Nolandella sp. and Pseudoparamoeba sp. did not increase the severity of Neoparamoeba perurans-induced amoebic gill disease (AGD)

<p>Amoebic gill disease (AGD) is one of the main health issues impacting farmed Atlantic salmon. <i>Neoparamoeba perurans</i> causes AGD; however, a diversity of other amoeba species colonizes the gills and there is little understanding of whether they are commensal or potentially involved in different stages of gill disease development. Here, we conduct in vivo challenges of naïve Atlantic salmon with cultured <i>Nolandella</i> sp. and <i>Pseudoparamoeba</i> sp. to investigate their pathogenicity to Atlantic salmon gills. Additionally, we assessed whether the presence of <i>Nolandella</i> sp. and <i>Pseudoparamoeba</i> sp. influences the onset and/or severity of <i>N. perurans</i>‐induced AGD. All three strains attached and multiplied on the gills according to qPCR analysis. Furthermore, minor gross gill lesions and histological changes were observed post‐exposure. While <i>N. perurans</i> was found associated with classical AGD lesions, <i>Nolandella</i> sp. and <i>Pseudoparamoeba</i> sp. were not found associated with lesion sites and these lesions did not meet the expected composite of histopathological changes for AGD. Moreover, the presence of these non‐<i>N. perurans</i> species did not significantly increase the severity of AGD. This trial provides evidence that cultured <i>Nolandella</i> sp. and <i>Pseudoparamoeba</i> sp. do not induce AGD and do not influence the severity of AGD during the early stages of development.</p>