Prospective Longitudinal Study of Putative Agents Involved in Complex Gill Disorder in Atlantic salmon (Salmo salar)

Abstract

Complex gill disorder (CGD) is an important condition in Atlantic salmon aquaculture, but the roles of the putative aetiological agents in the pathogenesis are uncertain. A longitudinal study was undertaken on two salmon farms in Scotland to determine the variations in loads of CGD-associated pathogens (Desmozoon lepeophtherii, Candidatus Branchiomonas cysticola, salmon gill pox virus (SGPV) and Neoparamoeba perurans) estimated by quantitative PCR. In freshwater, Ca. B. cysticola and SGPV were detected in both populations, but all four pathogens were detected on both farms during the marine stage. Candidatus B. cysticola and D. lepeophtherii were detected frequently, with SGPV detected sporadically. In the marine phase, increased N. perurans loads associated significantly (p < 0.05) with increases in semi-quantitative histological gill-score (HGS). Increased Ca. B. cysticola load associated significantly (p < 0.05) with increased HGS when only Farm B was analysed. Higher loads of D. lepeophtherii were associated significantly (p < 0.05) with increased HGS on Farm B despite the absence of D. lepeophtherii-type microvesicles. Variations in SGPV were not associated significantly (p > 0.05) with changes in HSG. This study also showed that water temperature (season) and certain management factors were associated with higher HGS. This increase in histological gill lesions will have a deleterious impact on fish health and welfare, and production performance.

Keywords: Candidatus Branchiomonas cysticola; Desmozoon lepeophtherii; Neoparamoeba perurans; Paranucleospora theridion; amoebic gill disease; aquatic animals; complex gill disease; emerging diseases; pathogens; salmon gill poxvirus.

Figure 1

Variations of Ct values of Desmozoon lepeophtherii in the gills of salmon across weeks. Points represent raw data and the lines and shaded areas represent estimates from GAM and 95% confidence interval.

Figure 2

Variations in the load (Ct values) of Candidatus Branchiomonas cysticola in the gills of fish by week of sampling. Points represent raw data and the lines and shaded areas represent estimates from GAM and 95% confidence interval.

Figure 3

Variations in load (Ct values) of Neoparamoeba perurans in the gills of salmon plotted by week of sampling. Points represent raw data and the lines and shaded areas represent estimates from GAM and 95% confidence interval.

Figure 4

Variations in load (Ct values) of salmon gill poxvirus. Points represent raw data and the lines and shaded areas represent estimates from GAM and 95% confidence interval.

Figure 5

Representation of linear model 2 (LM2) with the histological gill score (HGS). (a) Increase of Desmozoon lepeophtherii load was significantly associated with the HGSs on Farm B (p < 0.05) but not on Farm A (p ≥ 0.05). (b) The increase in of Neoparamoeba perurans load was significantly (p < 0.05) associated with an increase in histological gill score in both Farm A and Farm B.

Figure 6

Representation of linear regression models with the histological gill score (HGS) on Farm B. (a) Linear model 3 (LM3) and linear model 4 (LM4), note the strong association between season and HGS, the points show raw data; small grey points show the raw HGS data, while large red points with error bars show predictions from models and 95% CI. (b) Linear model 5, when temperature was used instead of season, the increase of temperature was significantly (p < 0.05) associated with increased HGSs. Line and shaded area show predicted HGS and 95% CI.

Figure 7

Variations in pathogen Ct values, epidemiology, histological gill score (HGS) and water temperatures in each sampling week of Farm A. Cells in the colour-coded matrix represent HGS results of individual sampled fish. FW = Freshwater; Week 1 is sampling point before transfer to Farm A. AGD = amoebic gill disease; H₂O₂ = hydrogen peroxide.

Figure 8

Variations in pathogens Ct value, epidemiology, histological gill score (HGS) and water temperatures in each sampling week of Farm B. Cells in the colour-coded matrix represent HGS results of individual sampled fish. FW = Freshwater; Week 6 is sampling point before transfer to Farm B. AGD = amoebic gill disease. H₂O₂ = hydrogen peroxide.

Figure 9

Histologic sections of gills from farmed Atlantic salmon stained with haematoxylin and eosin. (a) Mild focal lamellar epithelial hyperplasia and fusion (box). (b) Two foci of moderate amoebic gill disease lesions (box). (c) Mild focal lamellar epithelial lymphocytic branchitis (arrow). (d) Presence of a multinucleated cell among the proliferated lamellar tissue (box). (e) Lamellar sub-epithelial infiltration of macrophages (arrow). (f) Proliferation of the distal part of a single shortened filament (box). (g) Cartilage dysplasia of the filament (arrow).

Figure 10

Histologic sections of gills from farmed Atlantic salmon stained with haematoxylin and eosin. (a) Moderate multifocal lamellar telangiectasias (arrows). (b) Epithelial necrosis of the lamellar outer margins (arrows). (c) Mild focal lamellar haemorrhages (boxes). (d) Two foci of lamellar tissue disruption and haemorrhage (boxes). (e) Lamellar epithelial hyperplasia and fusion and multifocal lamellar thrombi. (f) Mild multifocal lamellar thrombi with variable hyperplasia of the surrounding epithelium and lamellar shortening (arrows).

Figure 11

Histologic sections of gills from farmed Atlantic salmon stained with haematoxylin and eosin. (a) Unidentified metazoan between two lamellae with epithelial hyperplasia (arrow). (b) Amoebic gill disease lesion with presence of amoebae (arrows). (c) Epitheliocyst in the base of the lamellae suggestive of Candidatus Clavochlamydia salmonicola (arrow). (d) Epitheliocysts in the distal part of the lamellae suggestive of Candidatus Branchiomonas cysticola (arrows).