achieved through tissue culture using a variety of salmonid and
non-salmonid cell lines including CHSE-214 (Chinook salmon
embryo), BF-2 (bluegill fry) RTG-2 (rainbow trout gonad), a
neutralisation test, or a reverse transcription-polymerase chain
reaction (RT-PCR) and sequence analysis. In addition, the
detection in adherent leucocytes using immunofluorescence
labelled specific antibodies is useful (Fig. 5.8). Infectious pancreatic necrosis virus is a bisegmented double-stranded RNA
virus of the family Birnaviridae and at least nine serotypes exist.
Commercially, both oral and injectable vaccines are available.
5.2
Infectious Salmon Anaemia
Infectious salmon anaemia (ISA) is a highly infectious viral
disease which has only been observed naturally in Atlantic
salmon. ISA was first diagnosed in Norwegian aquaculture
in the mid 1980s, but also reported from farmed fish in
Canada, Faroe Islands, Scotland, USA, Ireland and Chile.
Outbreaks of ISA occur in sea water or in hatcheries where
sea water is mixed with fresh water for pH or adjustment to
enhance smoltification. Under experimental conditions, searun brown trout can harbour and transmit the virus. Most
new outbreaks have occurred in connection with rapid
changes in temperature during the spring and to some extent
in the autumn. Importantly, salmon stocks show large variation in their susceptibility to the virus.
Diseased fish are lethargic and listless and tend to sink to
the bottom of the cages. During the acute stage, mortality is
usually high but in the pre-acute stages mortality may not
be observed. Externally, fish show abdominal distension,
haemorrhagic spots surrounding the lens, pale gills and
petechiae. Occasionally, some fish are observed with hyperactive behaviour and presumably nervous movements, spinning
around the longitudinal axis. Internally, ascites, oedema of the
swim bladder, splenomegaly and a homogeneous dark liver
are common (Fig. 5.9).
The virus is described as non-cytolytic endotheliotropic
and infection of the endothelium lining the circulatory system is observed without vasculitis and with virus absent
from necrotic parenchymal cells, e.g. in liver, heart or kidney. The pattern of virus attachment mirrors the distribution
of infection, showing that the virus receptor is important for
cell tropism as well as for adsorption to erythrocytes. During
the final stages, a confluent haemorrhagic focus in the kidney
is characteristic, but more frequent in North American stock.
The liver lesions in affected salmon from Norway and
Scotland show dilation of the hepatic sinusoids and zonal
haemorrhagic necrosis with a bridge-like pattern that leaves
the hepatic tissue surrounding small and medium-sized veins
viable (Fig. 5.10 and 5.11). There is pronounced congestion,
often combined with extensive haemorrhage in the lamina
propria of the foregut and congestion of the splenic parenchyma with occasional erythrophagocytosis (see Fig. 4.11).
Fig. 5.7 Liver of Atlantic salmon infected with infectious pancreatic
necrosis. Pyknotic nuclei (block arrows) with loss of cell integrity are
found concurrently with apoptosis (arrows) and necrosis. Bar ¼ 20 μm
Fig. 5.8 Farmed Atlantic salmon with infectious pancreatic necrosis.
Strong positive immunohistochemical reaction in post-vaccination
granulomatous tissue in pancreatic area
Fig. 5.9 Characteristic dark liver in Atlantic salmon with infectious
salmon anaemia
56
5 Viral Diseases
non-salmonid cell lines including CHSE-214 (Chinook salmon
embryo), BF-2 (bluegill fry) RTG-2 (rainbow trout gonad), a
neutralisation test, or a reverse transcription-polymerase chain
reaction (RT-PCR) and sequence analysis. In addition, the
detection in adherent leucocytes using immunofluorescence
labelled specific antibodies is useful (Fig. 5.8). Infectious pancreatic necrosis virus is a bisegmented double-stranded RNA
virus of the family Birnaviridae and at least nine serotypes exist.
Commercially, both oral and injectable vaccines are available.
5.2
Infectious Salmon Anaemia
Infectious salmon anaemia (ISA) is a highly infectious viral
disease which has only been observed naturally in Atlantic
salmon. ISA was first diagnosed in Norwegian aquaculture
in the mid 1980s, but also reported from farmed fish in
Canada, Faroe Islands, Scotland, USA, Ireland and Chile.
Outbreaks of ISA occur in sea water or in hatcheries where
sea water is mixed with fresh water for pH or adjustment to
enhance smoltification. Under experimental conditions, searun brown trout can harbour and transmit the virus. Most
new outbreaks have occurred in connection with rapid
changes in temperature during the spring and to some extent
in the autumn. Importantly, salmon stocks show large variation in their susceptibility to the virus.
Diseased fish are lethargic and listless and tend to sink to
the bottom of the cages. During the acute stage, mortality is
usually high but in the pre-acute stages mortality may not
be observed. Externally, fish show abdominal distension,
haemorrhagic spots surrounding the lens, pale gills and
petechiae. Occasionally, some fish are observed with hyperactive behaviour and presumably nervous movements, spinning
around the longitudinal axis. Internally, ascites, oedema of the
swim bladder, splenomegaly and a homogeneous dark liver
are common (Fig. 5.9).
The virus is described as non-cytolytic endotheliotropic
and infection of the endothelium lining the circulatory system is observed without vasculitis and with virus absent
from necrotic parenchymal cells, e.g. in liver, heart or kidney. The pattern of virus attachment mirrors the distribution
of infection, showing that the virus receptor is important for
cell tropism as well as for adsorption to erythrocytes. During
the final stages, a confluent haemorrhagic focus in the kidney
is characteristic, but more frequent in North American stock.
The liver lesions in affected salmon from Norway and
Scotland show dilation of the hepatic sinusoids and zonal
haemorrhagic necrosis with a bridge-like pattern that leaves
the hepatic tissue surrounding small and medium-sized veins
viable (Fig. 5.10 and 5.11). There is pronounced congestion,
often combined with extensive haemorrhage in the lamina
propria of the foregut and congestion of the splenic parenchyma with occasional erythrophagocytosis (see Fig. 4.11).
Fig. 5.7 Liver of Atlantic salmon infected with infectious pancreatic
necrosis. Pyknotic nuclei (block arrows) with loss of cell integrity are
found concurrently with apoptosis (arrows) and necrosis. Bar ¼ 20 μm
Fig. 5.8 Farmed Atlantic salmon with infectious pancreatic necrosis.
Strong positive immunohistochemical reaction in post-vaccination
granulomatous tissue in pancreatic area
Fig. 5.9 Characteristic dark liver in Atlantic salmon with infectious
salmon anaemia
56
5 Viral Diseases