A cohort study of gill infections, gill pathology and gill-related mortality in sea-farmed Atlantic salmon (Salmo salar L.): A descriptive analysis

Abstract

Gill disease is an important cause of economic losses, fish mortality and reduced animal welfare in salmonid farming. We performed a prospective cohort study, following groups of Atlantic salmon in Western Norway with repeated sampling and data collection from the hatchery phase and throughout the 1st year at sea. The objective was to determine if variation in pathogen prevalence and load, and zoo- and phytoplankton levels had an impact on gill health. Further to describe the temporal development of pathogen prevalence and load, and gill pathology, and how these relate to each other. Neoparamoeba perurans appeared to be the most important cause of gill pathology. No consistent covariation and no or weak associations between the extent of gill pathology and prevalence and load of SGPV, Ca. B. cysticola and D. lepeophtherii were observed. At sea, D. lepeophtherii and Ca. B. cysticola persistently infected all fish groups. Fish groups negative for SGPV at sea transfer were infected at sea and fish groups tested negative before again testing positive. This is suggestive of horizontal transmission of infection at sea and may indicate that previous SGPV infection does not protect against reinfection. Coinfections with three or more putative gill pathogens were found in all fish groups and appear to be the norm in sea-farmed Atlantic salmon in Western Norway.

Keywords: D. lepeophtherii; AGD; Atlantic salmon; Ca. B. cysticola; SGPV; gill disease.

FIGURE 1

Overview of freshwater sites (n = 4), sea sites (n = 8) and fish groups (n = 16). Stocking period (S0 = autumn stock/S1 = spring stock) and water treatment (flow through vs. RAS = circular arrow) is indicated

FIGURE 2

Normal gill tissue and histology lesions recorded as counts and percent. All tissues stained with haematoxylin and eosin. (a) Almost normal gill tissue at low magnification, very few foci of pathology are seen. Bar 2 mm. (b) Normal gill tissue at high magnification. Bar 200 μm. (c) Multifocal vascular lesions and focal segmental hyperplasia, fish group B2, 3% of lamella with vascular lesions. Bar 4 mm. (d) Vascular lesions, high magnification. Aneurysms with associated lamellar epithelial hyperplasia and variable extent of recanalization (arrowhead). Bar 100 μm. (e) Multifocal segmental hyperplasia affecting both proximal and distal aspects of the filaments, fish group G2, 21% of gill tissue affected. Bar 3 mm. (f) Lamellar epithelial hyperplasia with amoeba (arrowheads), also note subepithelial inflammation (arrows) and haemorrhage, fish group G2, same fish as (e). Bar 100 μm. (g) Lamellar epithelial hyperplasia and inflammation of the distal aspects of the filaments – fish group A1, 33% of gill tissue affected. Bar 4 mm. (h) High magnification of inflammation shows loss of lamella (arrowheads) and expansion of the filament by fibrous tissue with mild inflammatory infiltrates (*) and hyperplasia and inflammation (arrow) in surrounding lamellar epithelium. Fish group A2. Bar 200 μm

FIGURE 3

Sea temperature, daily gill‐related mortality (%), percentage of fish with gross gill score >1 and percentage of fish with >5% of gill tissue affected (histopathology) over time at sea per fish group. Daily gill‐related mortality (%) for (a) autumn‐transferred fish and (b) spring‐transferred fish, percentage of fish with gross gill score >1 for (c) autumn‐transferred fish and (d) spring‐transferred fish, percentage of fish with >5% of gill tissue affected (histopathology) for (e) autumn‐transferred fish and (f) spring‐transferred fish. The mean daily sea temperature across all study sites is shown in each figure

FIGURE 4

Lesions and pathogens observed. (a) Bacteria, haemorrhage, necrosis and lamellar epithelial hyperplasia. Note basophilic granular material (bacteria) partially embedded in eosinophilic material (fibrin) expanding filament vessels (*) and lamellar sinusoids (arrowheads). Pasteurella spp.– infection in fish group C1. Bar 300 μm. (b) Foreign material, possibly plant material (*), caught between filaments. Inflammation and haemorrhage are seen in the filament and there are necrosis (loss) of surrounding lamella (arrowheads). Fish group F2. Bar 300 μm. (c) Necrosis of lamella with loss of normal tissue structures and large amounts of filamentous bacteria, likely Tenacibaculum sp., in the necrotic tissue. Inflammatory cells and haemorrhage are seen in the filament. Fish group C1. Bar 60 μm. (d) Focal proliferation of filament cartilage—possibly callus formation caused by previous trauma. Recorded as a deformity. Bar 400 μm. (e) Small crustacean between lamellae. Fish group A1. Bar 80 μm. (f) Metacercaria surrounded by a thin fibrous capsule in the filament. Fish group A2. Bar 50 μm

FIGURE 5

Median reverse Ct‐values per fish group throughout the sea phase for autumn‐transferred (S0) and spring‐transferred (S1) fish groups. (a) SGPV (S0), (b) SGPV (S1), (c) Ca. B. cysticola (S0), (d) Ca. B. cysticola (S1), (e) D. lepeophtherii (S0), (f) D. lepeophtherii (S1), (g) N. perurans (S0), (h) N. perurans (S1)

FIGURE 6

Prevalence of N. perurans, AGD and percentage of fish with >5% of gill tissue with lamellar epithelial hyperplasia and hyperplasia and inflammation over time at sea per fish group. (a) N. perurans (S0), (b) N. perurans (S1), (c) AGD (S0), (d) AGD (S1), (e) hyperplasia (S0), (f) hyperplasia (S1). Non‐medicinal and medicinal sea lice treatment events that could impact the development of AGD and N. perurans prevalence are represented as x for fish groups A1‐H1 and o for fish groups A2‐H2

FIGURE 7

Relationship between pathogens observed in tissue sections and Ct‐levels detected by RT‐qPCR per fish group. Scatterplots show the percentage of fish per sampling point and fish group (n = 139) where a) amoeba and b) intracellular bacteria (epitheliocysts) were observed in the tissue sections against median reverse Ct‐values for N. perurans and Ca.B. cysticola, respectively. Fish groups with a median reverse Ct of 0 are excluded for clarity leaving (a) n = 28 and (b) n = 107 fish groups. [Correction added on 12 July 2022, after first online publication: figure 7(a) has been updated in this version]

FIGURE 8

The extent of lamellar epithelial hyperplasia at different loads of pathogens detected in the gill tissue. Box and whisker plots show percent lesions (n = 3885) grouped by ordinal RT‐qPCR‐results for (a) N. perurans, (b) Ca. B. cysticola. Outliers are excluded for clarity