Amoebic gill disease (AGD) in Atlantic salmon (Salmo salar) farmed in Chile

Abstract

Between May and November 2007, three marine Atlantic salmon farms around Chiloé Island, Chile, reported mortalities in which affected fish presented with Caligus rogercresseyi infections and gross gill lesions characteristic of amoebic gill disease (AGD). Histological examination of the gills from affected fish confirmed the presence of AGD lesions. Trophozoites possessing one or more endosymbiotic Perkinsela amoeba-like organisms (PLOs) were observed in association with hyperplastic tissue. Further analyses were undertaken using a combination of PCR and in situ hybridization and the trophozoites were identified as Neoparamoeba perurans. Thus, our data indicate that N. perurans is a causal agent of AGD in Chile. However, it is possible that AGD was not the single cause of mortalities in the epizootics investigated here. The exceptionally high level of co-infection with Caligus rogercresseyi (maximum mean intensity 34, prevalence 100%), could have contributed to the production losses.

Introduction

Amoebic gill disease (AGD) is a parasitic condition affecting some species of fish farmed in the marine environment (Munday, 1986, Kent et al., 1988, Dyková et al., 1995). AGD is caused by Neoparamoeba perurans (see Young et al., 2007), which is the confirmed aetiological agent of this disease in cases reported from Australia, Ireland, Japan, New Zealand, Norway, USA, Scotland and Spain (Young et al., 2008a, Crosbie et al., 2010, Steinum et al., 2008, Nylund et al., 2008). AGD is grossly characterised by the presence of multifocal lesions in the gills (Munday et al., 1990, Rodger and McArdle, 1996). The presence of these gross lesions allows presumptive diagnoses of AGD in the areas where AGD is enzootic. The final diagnosis is based on histopathology when amoebae that possess one or more endosymbiotic Perkinsela amoeba-like organisms (PLOs) (Dyková et al., 2003, Dyková et al., 2008, Adams and Nowak, 2003) are detected in close association with hyperplastic epithelial-like cells (Dyková and Novoa, 2001). Molecular tools, including PCR and in situ hybridization (ISH) have been developed and used to confirm the identity of the parasite (Young et al., 2007, Young et al., 2008b). In this study, we applied these new diagnostic tools to samples obtained from Atlantic salmon (Salmo salar) that were presumptively diagnosed with AGD in Chile. Our data show that fish were indeed affected by AGD and that Neoparamoeba perurans was the causal agent. However, fish were also co-infected with Caligus rogercresseyi and a causal relationship between either of the parasites and ongoing mortalities could not be established.