6.14 Francisella noatunensis subsp.
noatunensis
Francisellosis was initially reported as a significant threat to
the Norwegian cod farming industry. The disease develops
as a systemic, chronic, granulomatous infection with high
morbidity and resulting in varying degrees of mortality but
cumulative losses ranging from 5 to 20 % have been
reported. In 2006, francisellosis was also reported among
Atlantic salmon parr held in freshwater cages in Lake
Llanquihue, Chile but the disease develops in a similar
fashion independent of host and regions.
Francisella noatunensis is a facultative intracellular
Gram negative bacterium refractive to culture on standard
laboratory media and recently reported that it requires cysteine enriched media. A PCR test has been developed and
the 16S ribosomal RNA gene sequence show the Chilean
isolate to be 98–100 % identical to F. noatunensis subsp.
noatunensis from cod. Further studies are required to establish the risks to farmed salmonids.
6.15 Renibacterium salmoninarum
Renibacterium salmoninarum is the aetiological agent of
bacterial kidney disease (BKD), a serious, usually chronic,
condition of wild and farmed salmonids. The first record of
BKD occurred in wild Atlantic salmon in Scotland during
the 1930s but is now reported worldwide, namely from all
major areas where these fish are farmed. Outbreaks occur in
freshwater as well as in the marine environment. The disease
is transmitted horizontally by cohabitation, and vertically via
the eggs directly from ovarian tissue prior to ovulation, with
the highest mortality recorded as temperatures reach 12
C.
Clinical observations and external lesions are variable,
however loss of balance, darkening and mottled appearance
of the skin, distended abdomen, exophthalmia, petechiae and
haemorrhaging around the base of the pectoral fins and the
lateral line, are described. Superficial blisters with vesicle
formation, ulceration and abscesses may develop in the
tegument.
At necropsy, the gills and internal organs are pale giving
an indication of anaemia. The most obvious internal lesion is
a swollen kidney that may show greyish-white nodular
lesions. Similar nodules also occur in heart, liver and spleen
(Figs. 6.32, 6.33, 6.34 and 6.35). Petechial haemorrhage of
the muscle, the peritoneum and ascites is reported. The
pyloric caeca are similarly pale with a ‘fat-like’ appearance.
A diffuse white membranous layer (pseudomembrane) covering internal organs is described in affected fish (see
Fig. 6.34). A yellow, viscous fluid occurs in the intestine
and may contain blood.
Histologically, a chronic proliferating granulomatous
response affects the haematopoietic tissues. Multiple and
often large necrotic areas and granulomas with a central
caseous zone are bounded by epithelioid and other
infiltrating lymphoid cells (Figs. 6.36, 6.37 and 6.38). During the early stages of infection R. salmoninarum may
aggregate at the surface of the spleen and around pancreatic
tissues, followed by widespread necrosis with pyknotic cells
within the ellipsoids. A fibrous capsule may form around the
spreading necrotic lesion that traps bacteria as well as
phagocytes (Fig. 6.39). R. salmoninarum is recorded intraand extracellularly and within the kidney, focal necrosis and
glomerular oedema is observed, as well as a granulomatous
inflammatory reaction and membranous glomerulopathy due
to deposition of immune complexes. In cases where the
bacteria are killed by host cells, such granulomas gradually
resolve. R. salmoninarum can also be found in the gill
filaments (Fig. 6.40).
A granulomatous peritonitis occurs in the pancreatic area
with numerous bacteria accompanied by focal infiltration of
leucocytes (Fig. 6.41). Small foci containing phagocytised
R. salmoninarum can develop within the liver parenchyma
which coalesce and become the centre of an inflammatory reaction. Deposits of fibrin and collagen accumulate around the swim
bladder and intestine, with some hypertrophy and numerous
phagocytic cells containing bacteria. A diptheric epicarditis
comprising thin layers of fibrin, collagen and macrophages
containing R. salmoninarum results in restrictive pericarditis.
In Canada, a seasonal ‘spawning rash’ has been reported in
mature rainbow trout, where a pustulous dermatitis may cover
large areas of the skin with many small blisters or raised
haemorrhagic nodules within the epidermis. Granulomatous
tissue invades adjacent scale pockets and extends longitudinally along the fibrous tissue layer of the dermis.
Where R. salmoninarum occurs within the central nervous
system, haematogenous spread to the meninges has been
Fig. 6.32 Granulomas in kidney of Atlantic salmon with bacterial
kidney disease
6.15 Renibacterium salmoninarum
87
noatunensis
Francisellosis was initially reported as a significant threat to
the Norwegian cod farming industry. The disease develops
as a systemic, chronic, granulomatous infection with high
morbidity and resulting in varying degrees of mortality but
cumulative losses ranging from 5 to 20 % have been
reported. In 2006, francisellosis was also reported among
Atlantic salmon parr held in freshwater cages in Lake
Llanquihue, Chile but the disease develops in a similar
fashion independent of host and regions.
Francisella noatunensis is a facultative intracellular
Gram negative bacterium refractive to culture on standard
laboratory media and recently reported that it requires cysteine enriched media. A PCR test has been developed and
the 16S ribosomal RNA gene sequence show the Chilean
isolate to be 98–100 % identical to F. noatunensis subsp.
noatunensis from cod. Further studies are required to establish the risks to farmed salmonids.
6.15 Renibacterium salmoninarum
Renibacterium salmoninarum is the aetiological agent of
bacterial kidney disease (BKD), a serious, usually chronic,
condition of wild and farmed salmonids. The first record of
BKD occurred in wild Atlantic salmon in Scotland during
the 1930s but is now reported worldwide, namely from all
major areas where these fish are farmed. Outbreaks occur in
freshwater as well as in the marine environment. The disease
is transmitted horizontally by cohabitation, and vertically via
the eggs directly from ovarian tissue prior to ovulation, with
the highest mortality recorded as temperatures reach 12
C.
Clinical observations and external lesions are variable,
however loss of balance, darkening and mottled appearance
of the skin, distended abdomen, exophthalmia, petechiae and
haemorrhaging around the base of the pectoral fins and the
lateral line, are described. Superficial blisters with vesicle
formation, ulceration and abscesses may develop in the
tegument.
At necropsy, the gills and internal organs are pale giving
an indication of anaemia. The most obvious internal lesion is
a swollen kidney that may show greyish-white nodular
lesions. Similar nodules also occur in heart, liver and spleen
(Figs. 6.32, 6.33, 6.34 and 6.35). Petechial haemorrhage of
the muscle, the peritoneum and ascites is reported. The
pyloric caeca are similarly pale with a ‘fat-like’ appearance.
A diffuse white membranous layer (pseudomembrane) covering internal organs is described in affected fish (see
Fig. 6.34). A yellow, viscous fluid occurs in the intestine
and may contain blood.
Histologically, a chronic proliferating granulomatous
response affects the haematopoietic tissues. Multiple and
often large necrotic areas and granulomas with a central
caseous zone are bounded by epithelioid and other
infiltrating lymphoid cells (Figs. 6.36, 6.37 and 6.38). During the early stages of infection R. salmoninarum may
aggregate at the surface of the spleen and around pancreatic
tissues, followed by widespread necrosis with pyknotic cells
within the ellipsoids. A fibrous capsule may form around the
spreading necrotic lesion that traps bacteria as well as
phagocytes (Fig. 6.39). R. salmoninarum is recorded intraand extracellularly and within the kidney, focal necrosis and
glomerular oedema is observed, as well as a granulomatous
inflammatory reaction and membranous glomerulopathy due
to deposition of immune complexes. In cases where the
bacteria are killed by host cells, such granulomas gradually
resolve. R. salmoninarum can also be found in the gill
filaments (Fig. 6.40).
A granulomatous peritonitis occurs in the pancreatic area
with numerous bacteria accompanied by focal infiltration of
leucocytes (Fig. 6.41). Small foci containing phagocytised
R. salmoninarum can develop within the liver parenchyma
which coalesce and become the centre of an inflammatory reaction. Deposits of fibrin and collagen accumulate around the swim
bladder and intestine, with some hypertrophy and numerous
phagocytic cells containing bacteria. A diptheric epicarditis
comprising thin layers of fibrin, collagen and macrophages
containing R. salmoninarum results in restrictive pericarditis.
In Canada, a seasonal ‘spawning rash’ has been reported in
mature rainbow trout, where a pustulous dermatitis may cover
large areas of the skin with many small blisters or raised
haemorrhagic nodules within the epidermis. Granulomatous
tissue invades adjacent scale pockets and extends longitudinally along the fibrous tissue layer of the dermis.
Where R. salmoninarum occurs within the central nervous
system, haematogenous spread to the meninges has been
Fig. 6.32 Granulomas in kidney of Atlantic salmon with bacterial
kidney disease
6.15 Renibacterium salmoninarum
87