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Complex gill disorder (CGD): A histopathology workshop report

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A workshop was organised to share findings, experience and knowledge on the histopathology associated with gill diseases in sea water Atlantic salmon. An interesting extended discussion resulted in an agreed criteria for complex gill disorder (CGD)-type histopathology, which we hope will contribute in epidemiological studies, research projects and diagnostic reports.
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172, Bull. Eur. Ass. Fish Pathol., 39(4) 2019
NOTE
Complex gill disorder (CGD): A
histopathology workshop report
P. Noguera1, A.B. Olsen2, J. Hoare3, K.I. Lie4, M.
Marcos-López3, T. T. Poppe5 and H. Rodger6
1 Marine Scotland Science, 375 Victoria Road, AB11 9DB Scotland; 2 Norwegian
Veterinary Institute, Thormøhlens gate 53C, 5006 Bergen, Norway; 3 Fish Vet Group Ltd.,
22 Carsegate Road, IV3 8EX Inverness, Scotland; 4 Fish Vet Group Norge, Hosveien
21 – 23, 0275 Oslo, Norway; Ireland; 5 PHARMAQ Analytiq AS, Harbialléen
2A, N-0213 Oslo, Norway; 6 VAI Consulting, Kinvara, Co. Galway
Abstract
A workshop was organised to share ndings, experience and knowledge on the histopathology
associated with gill diseases in sea water Atlantic salmon. An interesting extended discussion re-
sulted in an agreed criteria for complex gill disorder (CGD)-type histopathology, which we hope
will contribute in epidemiological studies, research projects and diagnostic reports.
The growing issues of gill disease in the salmon
aquaculture industry in Europe have resulted
in a heightened focus on this area for diagnos-
ticians, clinicians and researchers working in
sh health, with the increased use of gross gill
scoring systems, as well as laboratory assays,
including light microscopy and molecular
analysis.
There are a variety of terms being used to
describe gill pathology such as proliferative
gill disease (PGD), proliferative gill inam-
mation (PGI) (Kvellestad et al. 2005), complex
gill disease (CGD) (Herrero et al. 2018) and
amoebic gill disease (AGD) (Rodger 2014). In
order to clarify the histopathology criteria as-
sociated with these terms, a group of diagnostic
sh histopathologists convened for a one-day
workshop at the Scoish Aquaculture Innova-
tion Centre (SAIC), University of Stirling in
May 2019 to share their ndings, experience
and knowledge. As a result of this workshop,
the group was able to agree on criteria for the
above terms to enable their use in epidemiol-
ogy studies, research projects, and diagnostic
reports.
Drs. Patricia Noguera (Marine Scotland Science)
and Hamish Rodger (VAI Consulting) organised
the workshop with the much-appreciated assis-
tance and support of Caroline Grin and Robin
Shields from the SAIC. The diagnosticians who
participated were Drs. Trygve Poppe (Pharmaq
Analytiq, Norway), Mar Marcos-Lopez (Fish Vet
* Corresponding author’s e-mail: Patricia.Noguera@gov.scot
Bull. Eur. Ass. Fish Pathol., 39(4) 2019 173
Group (FVG) UK & Ireland), Kai-Inge Lie (FVG
Norway), Anne Berit Olsen (NVI, Norway),
James Hoare (FVG UK), Patricia Noguera (MSS,
Scotland) and Hamish Rodger (VAI, Ireland).
Drs. Aaron Reeves and Annee Boerlage (SRUC,
Inverness), who are co-ordinating one of the
SAIC & industry funded gill projects, were
also present.
The workshop covered the reporting of dif-
ferent types of histopathology observed in the
various geographical regions associated with
gill disease mainly in sea water salmon, as well
as an extended discussion on the proposed
criteria for complex gill disorder (CGD)- type
histopathology as indicated below.
1. Complex gill disorder (CGD)-type
his topat hology. Signicant non-
specic, proliferative branchitis, which
cannot be aributed to a single known
aetiology (Figure 1.), and is characterised
by:
Moderate to severe hyperplasia and fusion
of the lamellar epithelium, with variable
mucous cell hyperplasia and occasional
lacunae (pseudocysts).
Acute, subacute and/or chronic lamellar
inammation (may include either or both
granulocytic to lymphohistiocytic inltra-
tion)
Variable cellular degeneration and necrosis
Variable haemorrhage, hyperaemia and
thrombosis
Variable lament inammation
Variable hypertrophy and hyperplasia of
highly eosinophilic cells (alike chloride
and/ or degenerative epithelial cells)
Rare cartilage proliferation/dysplasia
Low to high numbers of the following agents
or evidence of their presence may be associated
with the above changes (Figure 2.):
Amoebae, epitheliocysts (Branchiomonas-
type, less than 10 μm in diameter and
densely basophilic), Gram-positive mi-
crosporidian spores within degenerate
cells or microvesicles, salmon gill pox
virus (apoptotic cells with nuclear central
clearing of chromatin), other pathogens
(e.g. Ichthyobodo sp., Tenacibaculum spp) or
damaged due harmful planktonic organ-
isms (e.g. jellysh).
This can be compared to the histopathology cri-
teria for other gill disease terms such as amoebic
gill disease and proliferative gill inammation
(PGI), which are outlined below:
2. Amoebic gill disease
his topat hology. (Clark & Nowak
1999, Munday et al. 2001) where there is
presence of all four ndings in the same
gill tissue:
Hyperplasia of gill epithelia
Lamellar fusion
Interlamellar lacunae in hyperplastic tissue
Presence of amoebae (eosinophilic para-
some) adjacent to gill surface (or in lacunae)
3. Proliferative gill inammation.
(Kvellestad et al. 2005) where there is
presence of all four ndings in the same
gill tissue:
Circulatory disturbances (haemorrhage,
thrombi, death of pillar cells)
Epithelial hyperplasia
Inammation in sub-epithelial & epithelial
tissues
Epithelial cell death
174, Bull. Eur. Ass. Fish Pathol., 39(4) 2019
Figure 1. General features of complex gill disorder (CGD)-type histopathology. Signicant, non-specic,
proliferative branchitis characterised by moderate to severe hyperplasia and fusion of the lamellar
epithelium (A, B, C) accompanied by occasional lacunae (pseudoc ysts) (C). Variable mucous cell hyperplasia
(D,E) and hyperplasia and hypertrophy of eosinophilic chloride-like cells along lamellae (E insert and F).
Acute, subacure and/or chronic lamellar and variable lament inammation may include both granulocytic
and lymphohistiocytic inltration (C,D,E,F,G). Variable cellular degeneration and necrosis (E,F,G), blood
congestion, haemorrhage, hyperaemia and t hrombosis (A, D, E, G), a nd rare cart ilage proliferation/dysplasia
(H) can also be observed.
Bull. Eur. Ass. Fish Pathol., 39(4) 2019 175
Figur e 2. Complex gill disorder (CGD)-type histopathology may be seen associated with low to high numbers
of one or more type of agents or evidence of their presence. Examples include amoebae (Neoparamoeba
perura ns) (A,B,C), microsporid ian (Desmozoon lepeopht herii) spores with in degenerate cells or m icrovesicles
(D), epitheliocysts (e.g. Branchiomonas-type, less than 10 μm in diameter and densely basophilic) (E), cell
changes associated with Salmon gill pox virus (ASPV) (apoptotic cells with central clearing of chromatin)
(F), and damaged associated to harmful plankton (e.g. jelly sh) (G, H).
176, Bull. Eur. Ass. Fish Pathol., 39(4) 2019
These can also be compared to the histopa-
thology of specic gill conditions where the
pathology is associated with, or has been dem-
onstrated to be, associated with one pathogen,
agent or factor alone.
Acknowledgements
We thank Drs. Alf Dalum (Pharmaq Analytic,
Norway) and Jorge del Pozo (University of
Edinburgh, UK) who while unable to aend
the meeting in person were also involved in
the post discussion of the nal report.
Funding for travel to the workshop was
made available via SRUC, Inverness (Dr.
Aaron Reeves) and the University of Aber-
deen (Prof. Sam Martin) as part of SAIC &
industry funded projects on gill health. SAIC
is also gratefully acknowledged for support
for accommodation and meeting organisation.
References
Clark A and Nowak BF (1999). Field
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Herrero A, Thompson KD, Ashby A, Rodger
HD and Dalgleish MP (2018). Complex gill
disease: an emerging syndrome in farmed
Atlantic salmon (Salmo salar L.). Journal of
Comparative Pathology 163, 23 – 28.
Kvellestad A, Falk K, Nygaard SMR,
Flesjå K and Holm JA (2005). Atlantic
salmon paramyxovirus (ASPV)
infection contributes to proliferative gill
inammation (PGI) in seawater-reared
Salmo salar. Diseases of Aquatic Organisms
67, 47 – 54.
Munday BL, Zilberg D and Findlay V (2001).
Gill disease of marine sh caused by
infection with Neoparamoeba pemaquidensis.
Journal of Fish Diseases 24, 497 - 507.
Rodger HD (2014). Amoebic gill disease in
farmed salmon (Salmo salar) in Europe.
Fish Veterinary Journal 14, 16 - 26.
... .. (the types of specific gill diseases)'. When principal pathological changes are non-specific, either in combination with, or in the absence of, one or more of the seven distinctive types (including AGD), the type of gill disease is referred to as 'complex gill disease or disorder (CGD)' (Noguera et al. 2019). The terms CGD and multifactorial gill disease are often used interchangeably and are overlapping. ...
... The different terms that have been used to describe marine gill disease have led to confusion and make it difficult to compare between studies and areas. CGD as currently used, includes most other pathologies (Herrero et al. 2018;Noguera et al. 2019), but its boundaries are not well defined. A clear case definition would allow for a systematic estimation of prevalences across the salmon industry in different areas and countries and could aid epidemiological studies such as risk-factor analyses. ...
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Gill disorders have become a significant problem during the marine phase of farming Atlantic salmon (Salmo salar L.). The term complex gill disease (CGD) includes a wide range of clinical gill disease presentations generally occurring from the end of summer to early winter on marine Atlantic salmon farms. The gross and histological lesions observed are the resultant culmination of exposure to a mixture of environmental insults, pathogenic organisms and farm management practices. None of the three principal agents purportedly associated with CGD (Desmozoon lepeophtherii, salmon gill poxvirus or Candidatus Branchiomonas cysticola) have been cultured successfully in-vitro, so individual in-vivo challenge studies to identify their pathogenesis have not been possible. Studies of cohabitation of single pathogen-infected fish with naïve fish, and epidemiological investigations are required urgently to elucidate the roles of these pathogens and other factors in CGD.
Article
Amoebic gill disease (AGD) is the most serious health problem in Atlantic salmon cultured in Tasmania. Our field investigation examined prevalence of AGD during 2 years, every year for up to 7 months after transfer to sea water. The relationship between environmental factors and AGD prevalence was determined. Additionally, effects of adding levamisole to freshwater baths were investigated in a field trial. AGD was recorded on all farms, except for farm A, which did not move salmon from a brackish site to a full-salinity site during the study. The prevalence showed a bimodal distribution with the first larger peak in summer (usually in January) and the second smaller peak in autumn (between March and May). During both years the prevalence of AGD was significantly greater in January than any other month. Sampling month and the interaction between farm and month had a statistically significant effect on AGD prevalence. AGD was recorded at a minimum temperature of 10.6 °C and minimum salinity of 7.2 ppt. There was a positive relationship between the time since the freshwater bath and the prevalence of AGD for the first 30 days after the bath, with a dramatic increase in the AGD prevalence about 3 weeks after the bath. After 30 days, there was no statistically significant relationship between AGD prevalence and days since the last bath, except for the second bath. The addition of levamisole to the freshwater bath did not significantly increase the time between treatments. The relationship between diagnosis on the basis of gross signs and histological diagnosis was significant, however, the gross diagnosis was unreliable within the lower range, with 31.8% false negatives and 15.9% false positives and kappa value of 0.2742.