Bull Vet Inst Pulawy 51, 547-551, 2007
STUDY ON RAINBOW TROUT NODULAR GILL DISEASE
DETECTED IN POLAND
JERZY ANTYCHOWICZ
Department of Fish Diseases,
National Veterinary Research Institute, 24-100 Pulawy, Poland
[email protected]
Received for publication September 10, 2007
Abstract
Material and Methods
In recent years in Poland a very distinctive form of
proliferative gill disease of rainbow trout, accompanied by the
presence of amoeba-like cells, was observed. The disease
appeared to be the nodular gill disease for the first time
diagnosed in Poland. Amoeba-like cells seemed to be related
to pathological gill lesions, whereas other potential agents such
as bacteria and parasites were excluded. It has been suggested
that those cells are inland free living amoebae.
Three groups of rainbow trout fingerlings
originating from three salmonid farms were delivered to
our laboratory for the purpose of diagnosing the cause of
the severe hypoxia symptoms, which have appeared in
the fish. The clinical symptoms and gross pathological
lesions were described and discussed.
Five fish from each farm with distinctive gill
lesions were collected for microscopic examination. The
fish were killed with anaesthetic overdose and within
12-15 s, the samples of the gills were collected and fixed
in buffered formalin solution for histological
examination. The fresh squash preparations of the gills
were also made for microscopic examinations.
Preparations made from the gills of clinically healthy
rainbow trout fingerlings originating from disease free
farm were used as controls.
The routine histological methods were applied.
The preparations were stained with haematoxylin and
eosin.
Key words: rainbow trout, nodular gill
disease, pathology, Poland.
In recent years in Poland a very distinctive form
of proliferative gill disease of rainbow trout was
observed. The cases of this disease were accompanied
with a high mortality rate. The disease was characterised
by nodular hyperplasia of branchial epithelium
associated with the presence of amoeba-like cells of an
unknown origin. A similar disease was described in
inland rainbow trout fingerlings by Daoust and Ferguson
(1, 2) and by many other authors in marine cultured
salmonids infected with Paramoeba pemaquidensis (6).
In the gills of freshwater fish as well as in sea fish the
lesions were multifocal and uniquely severe in distal
regions of affected filaments. Characteristic proliferative
cell nodules contrasted with other more common forms
of proliferative gill disease found in trout. The aetiology
of inland rainbow trout nodular disease was not
clarified.
Up to now, nodular gill disease was not
reported in Poland. The purpose of this work is to
compare the detected changes in the gills of the rainbow
trout in Poland with those described in others countries
and diagnosed as nodular gill disease. The character of
amoeba-like cells which usually accompany this kind of
proliferative changes in the gills will be discussed.
Results
The majority of fish delivered from the farms
showed the symptoms of severe hypoxia. The gills of
these fish were swollen, non-uniformly coloured, and
their distal regions were protruding from under the gill
covers. Distal regions of the primary gill filaments
showed clubbing. Diffuse whitish discolouration of the
gills was suggesting the presence of diffuse epithelial
hyperplasia, besides the excess of mucous production
was the symptom of gill irritation and hypoxia.
Microscopic examination revealed that focal
massive branchial epithelium hyperplasia had resulted in
fusion of two or more secondary lamellae most often in
apical region of primary filaments, which caused their
clubbing (Figs 1 and 2). The interlamellar spaces were
filled with proliferating cells, and the structure of the
gills was pathologically changed.
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Fig. 1. Clubbing of the distal ends of primary gill filaments of the rainbow
trout fingerling, fresh preparation. 100x.
Fig. 2. Normal primary gill filaments of the rainbow trout fingerling, fresh
preparation. 100x.
Fig. 3. Amoeba-like cells (in the centre) between two proliferative masses
of degenerated gill cells (upper right and lower right). 100x, HE.
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Fig. 4. Amoeba-like cells in the gills of diseased rainbow trout, fresh
preparation. 500x.
Fig. 5. High magnification of isolated amoeba-like cells originating from
the case of nodular gill disease, fresh preparation. 1000x.
Fig. 6. Pathological changes in distal ends of primary gill filaments with
numerous amoeba-like cells, fresh preparation. 100x.
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The proliferation of the gill cells and focal
fusions of secondary lamellae formed nodules. The cells
in the nodules showed anisokaryosis and variable
amount of pale cytoplasm (Fig. 3). Among the cells,
small dark nuclei were also present.
In fresh squash preparations as well as in
histological preparations unidentified amoeba-like cells
were seen (Figs 3, 4, and 5). They appeared as
hyperplastic masses focally attached to the surface of the
gill in single, double or triple layers (Fig. 6). In fresh
preparations, the amoeba-like cells were round with one
or two nuclei-like bodies. Their vacuolated plasma
contained numerous small refractive granules. In
histological preparations these cells were angular and
distinctly darker than the proliferative gill cells (Fig. 3).
The size of these cells was between 15µm and 25µm.
The amoeba-like cells had slightly basophilic cytoplasm,
whereas the gill cells were eosinophilic. In some places,
the congregation of the amoeba-like cells detached from
the gill tissue, probably due to the process of tissue
conservation (Fig. 3).
Discussion
The three cases of proliferative gill disease
presented in rainbow trout fingerlings in Poland are
identical to the cases nodular gill diseases described by
other authors. The lesions were considered severe
enough to have contributed to clinical symptoms, which
were observed in fish and could cause mortality. The
proliferating process destroyed gill structure, obliterated
spaces between the secondary filaments, and impaired
gas and ion exchange between environment and blood.
The gill lesions presented in this paper in some
aspect differ from the epithelial hypertrophy and
hyperplasia, which usually appear in fish in acute or
chronic intoxication by pollutants or are associated with
various microorganisms. According to Daoust and
Ferguson (1, 2), in all those cases lesions appeared to be
diffused along primary filaments, whereas in the cases
of nodular gill disease they appear only in some regions
of the gill being especially severe in distal parts of the
affected primary filaments.
Toxic lesions and the pathological changes
caused by hypoxia or stress are characterised by the
oedema of individual secondary gill filaments as well as
detachment of epidermal layer and, in some cases, also
by haemorrhages. In ammonia intoxication, the
proliferation of eosinophilic granular cells is the usual
symptom.
In the studied case of the gill disease, none of
the above mentioned characteristics was detected and
neither filamentous bacteria nor parasites were present.
The identity of the so-called amoeba-like cells was not
yet determined and is still a matter of speculation. It has
been suggested that these cells are inland free living
amoebae, similar to saltwater amoebae causing the
condition known as amoebic gill disease (AGD) in
marine cultured salmonids in Tasmania, and
increasingly in other fish species of different geographic
areas (3, 7, 8). Karlsson–Norrgren (4, 5) suggested they
could be proliferating chloride cells, which migrate and
accumulate on the surface of other proliferating gill
cells. The migration of chloride cells could be induced
by aluminium poisoning.
According to Daoust and Ferguson (1, 2), a
marked epithelial hyperplasia accompanied usually by
the fusion of adjacent lamellae and obliteration of the
interlamellar space is connected with free living amoeba
infection. This hypothesis is not confirmed and the
significance of the large cells detected in nodular gill
disease (called also A cells) is not clear. The varied size
and the lack of degenerative changes in these cells as
well as the absence of bacterial decomposition could
suggest that they are live cells and the possibility that
they are amoebae cannot be discounted. These cells are
distributed focally on the mass of proliferating gill cells
but the congregations of these amoeba-like cells are
distinctly marked. Sometimes, in histological
preparations, they are seen detached from proliferating
gill cells. These phenomena suggest that they are
possibly not transformed gill cells (Fig. 3). Moreover,
inland fish chloride cells are only occasionally seen
among lamellar epithelial cells, so they probably could
not achieve such a great number even due to the
proliferation. The cytoplasm of amoeba-like cells
appeared to be slightly basophilic, whereas the
cytoplasm of chloride cells is acidophilic.
According to Taylor (9), free living amoebae
are capable of parasitizing freshwater fish. Some genera
appear to be casual facultative dwellers that become
entrapped in mucus of the gills where they feed on
bacteria. Under some conditions, fish can become
heavily infected by amoeba and then it results in fish
mortality.
In the cases detected in Poland, the amoeba-like
cells seemed to be related to pathological lesions in the
gill, whereas other potential agents such as bacteria or
parasites were excluded. The investigations of the
aetiology of nodular gill disease are important and
should be continued with special emphasis on the study
of eventual infectivity and pathogenicity of the amoebalike cells. Account should be taken of the fact that
nodular gill disease could be dangerous for trout and is a
new emerging disease for the first time reported in
Poland.
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