Depth distribution of the amoebic gill disease agent, Neoparamoeba perurans, in salmon sea-cages

Abstract

Identifying where and when parasites occur in farming environments is vital to understand transmission dynamics and develop preventative measures that reduce host−parasite encounters. A major parasite concern for Atlantic salmon farming is Neoparamoeba perurans, a marine amoeba that causes the potentially fatal amoebic gill disease (AGD), for which few control options exist. We explored whether free-living N. perurans abundance differs among depths in commercial Atlantic salmon Salmo salar sea-cages. Water samples collected from the surface to 10 m depth at multiple cage sites and times, and subsequently subjected to qPCR analysis, revealed that N. perurans abundance was influenced by depth at the time of year when amoeba numbers were highest, with more amoebae in surface waters. No distinct depth patterns were observed when amoebae were in low abundance. Across all times, temperature and salinity were largely homogeneous throughout cage depths. Possible factors explaining the presence of amoebae at the surface are discussed. Our results suggest that excluding caged salmon from upper cage depths where N. perurans is more abundant could be an effective management strategy to reduce the speed at which initial infections occur and delay the development of AGD outbreaks.

PCR survey for Paramoeba perurans in fauna, environmental samples and fish associated with marine farming sites for Atlantic salmon (Salmo salar L.)

“…According to Wright et al . (), P. perurans inhabits the upper surface layers of sea water at farms (Wright et al . ).…”

“…(), P. perurans inhabits the upper surface layers of sea water at farms (Wright et al . ). Previously, pathogenic salmon virus has also been detected at the surface in fat layers leaking from dead infected salmon (Stene et al .…”

“…Thus, treatments are usually initiated at a point when the salmon population already have facilitated spread of the amoeba between farms. According to Wright et al (2015), P. perurans inhabits the upper surface layers of sea water at farms (Wright et al 2015). Previously, pathogenic salmon virus has also been detected at the surface in fat layers leaking from dead infected salmon (Stene et al 2016) and we +/no = numbers of positive organisms/numbers of tested organisms.…”

Repeated sublethal freshwater exposures reduce the amoebic gill disease parasite, Neoparamoeba perurans, on Atlantic salmon

“…Due to inhibition detected during a preliminary dilution series test, qPCR was performed on 1:20 diluted total nucleic acid extract. Amoebae numbers were estimated based on a copy number of 2,880 per 1 N. perurans cell (Wright, Nowak, Oppedal, Bridle, & Dempster, ; Wright et al., ).…”

Amoebic gill disease: a growing threat

“…Amoebae of the genus Neoparamoeba (Amoebozoa, Dactylopodida) are ubiquitous in the marine environment 7 , and N. perurans specifically have been detected throughout the water column on and near Atlantic salmon (Salmo salar L.) farms 8,9 . All species from the genus Neoparamoeba harbour at least one intracellular endosymbiont known as a Perkinsela amoebae-like organism (PLO) 10 .…”