yellow-pigmented with uneven edges. Agent diagnosis
requires the observation of thin filamentous rods in wet
mounts, with culture on an appropriate medium such as
Anacker and Ordal or Marine Agar, and supported by other
methods such as nested PCR.
6.11 Hafnia alvei
Hafnia alvei is an opportunistic pathogen causing mortality
in juvenile brown and rainbow trout in Europe and cherry
salmon in Japan. Externally, diseased fish show a dark body
surface, abnormal swimming and a swollen abdomen.
Histological changes in the kidney include necrosis,
macrovacuolar degeneration in the liver and loss of the
lymphatic tissue in the spleen, and overall typical signs of
a generalized haemorrhagic septicaemia.
Diagnosis is based upon clinical signs and the isolation
and identification of the bacteria. H. alvei is a Gram negative, facultative anaerobic, rod-shaped bacterium and taxonomically placed in the Enterobacteriaceae. This bacterium
is similar morphologically and serologically to enteric red
mouth disease (see Yersinia ruckeri) but can be
differentiated biochemically.
6.12 Chryseobacterium spp.
The genus Chryseobacterium is widely distributed and can
be recovered from a variety of environments. Although it has
not been considered a relevant pathogen, there has been an
increase in the frequency of clinical cases reported in Chile
and Finland, in which several species or strains of the bacterium have been isolated from rainbow trout and Atlantic
salmon. Diseased fish show skin and muscle ulcerative
lesions on the flank and in the vent or peduncle area.
Histologically, fish exhibit degeneration of kidney
tubules, with oedema in the renal interstitial tissues and
proteinaceous casts within the tubular lumen. Other changes
include a heterophilic cellulitis and myo-degeneration.
Isolates of the bacterium from internal organs are Gram
negative, non-motile rods, catalase negative, and on the
basis of 16S rRNA gene sequence analysis are classified
into species of Chryseobacterium, with the proposed name
of C. piscicola sp. nov., but followed by C. viscerum sp. nov.
The differential diagnosis is Flavobacterium psychrophilum.
6.13 Pasteurella skyensis
A novel Gram negative bacterium, Pasteurella skyensis has
been identified from farmed salmon in Scotland. However,
relevant histopathology and isolation of the bacteria has
been infrequent and currently not considered as a significant
infection for farmed fish.
Histologically, multifocal granulomas are observed in the
kidney, liver and spleen with mild fibrous encapsulation. Most
granulomas contain multinucleate giant cells with an eosinophilic caseous necrotic core (Figs. 6.30 and 6.31). Some functional tissue surrounds the granulomas and some cells show
karyohexis. The ventricle shows proliferative lesions between
the compactum and spongiosum, and some granulomas at the
junction of the two layers are observed with a fibrous pericarditis and loss of staining properties. There is also loss of
structure in the bulbous arteriosus. The gill lamellae may
show old aneurysms, lamellar lifting and basal hyperplasia,
with overgrowth of new epithelial tissue. Giant cells and small
granulomas are also observed. Thickening of the connective
tissue layer around the primary lamellar rods and almost
complete absence of the latter, is noted.
Fig. 6.30 Granulomatous inflammation with associated giant cells in
farmed Atlantic salmon with Pasteurella skyensis. Bar ¼ 100 μm
Fig. 6.31 Granulomatous inflammation with associated giant cells in
farmed Atlantic salmon with Pasteurella skyensis. Bar ¼ 50 μm
86
6 Bacterial Diseases
requires the observation of thin filamentous rods in wet
mounts, with culture on an appropriate medium such as
Anacker and Ordal or Marine Agar, and supported by other
methods such as nested PCR.
6.11 Hafnia alvei
Hafnia alvei is an opportunistic pathogen causing mortality
in juvenile brown and rainbow trout in Europe and cherry
salmon in Japan. Externally, diseased fish show a dark body
surface, abnormal swimming and a swollen abdomen.
Histological changes in the kidney include necrosis,
macrovacuolar degeneration in the liver and loss of the
lymphatic tissue in the spleen, and overall typical signs of
a generalized haemorrhagic septicaemia.
Diagnosis is based upon clinical signs and the isolation
and identification of the bacteria. H. alvei is a Gram negative, facultative anaerobic, rod-shaped bacterium and taxonomically placed in the Enterobacteriaceae. This bacterium
is similar morphologically and serologically to enteric red
mouth disease (see Yersinia ruckeri) but can be
differentiated biochemically.
6.12 Chryseobacterium spp.
The genus Chryseobacterium is widely distributed and can
be recovered from a variety of environments. Although it has
not been considered a relevant pathogen, there has been an
increase in the frequency of clinical cases reported in Chile
and Finland, in which several species or strains of the bacterium have been isolated from rainbow trout and Atlantic
salmon. Diseased fish show skin and muscle ulcerative
lesions on the flank and in the vent or peduncle area.
Histologically, fish exhibit degeneration of kidney
tubules, with oedema in the renal interstitial tissues and
proteinaceous casts within the tubular lumen. Other changes
include a heterophilic cellulitis and myo-degeneration.
Isolates of the bacterium from internal organs are Gram
negative, non-motile rods, catalase negative, and on the
basis of 16S rRNA gene sequence analysis are classified
into species of Chryseobacterium, with the proposed name
of C. piscicola sp. nov., but followed by C. viscerum sp. nov.
The differential diagnosis is Flavobacterium psychrophilum.
6.13 Pasteurella skyensis
A novel Gram negative bacterium, Pasteurella skyensis has
been identified from farmed salmon in Scotland. However,
relevant histopathology and isolation of the bacteria has
been infrequent and currently not considered as a significant
infection for farmed fish.
Histologically, multifocal granulomas are observed in the
kidney, liver and spleen with mild fibrous encapsulation. Most
granulomas contain multinucleate giant cells with an eosinophilic caseous necrotic core (Figs. 6.30 and 6.31). Some functional tissue surrounds the granulomas and some cells show
karyohexis. The ventricle shows proliferative lesions between
the compactum and spongiosum, and some granulomas at the
junction of the two layers are observed with a fibrous pericarditis and loss of staining properties. There is also loss of
structure in the bulbous arteriosus. The gill lamellae may
show old aneurysms, lamellar lifting and basal hyperplasia,
with overgrowth of new epithelial tissue. Giant cells and small
granulomas are also observed. Thickening of the connective
tissue layer around the primary lamellar rods and almost
complete absence of the latter, is noted.
Fig. 6.30 Granulomatous inflammation with associated giant cells in
farmed Atlantic salmon with Pasteurella skyensis. Bar ¼ 100 μm
Fig. 6.31 Granulomatous inflammation with associated giant cells in
farmed Atlantic salmon with Pasteurella skyensis. Bar ¼ 50 μm
86
6 Bacterial Diseases