Viral Diseases
5
Abstract
Some viral diseases are prevented through vaccination but for many no vaccines are
available hence infection is a major concern for farmed fish. Among salmonids, RNA
viruses have the greatest ecological and socio-economic impact. Acute infections may lead
to host death, or induce a more chronic condition. Stress including overcrowding, sexual
maturation and handling may also reactivate a latent infection resulting in clinical disease.
This chapter covers a range of viral diseases in wild and farmed salmonids.
Keywords
Fish virus Salmon Trout
Infectious viral diseases continue to be a major concern in
wild and farmed fish and responsible of important losses,
particularly under farming conditions. These infections can
manifest as a septicaemia-like disease with a characteristic
acute presentation, but some viral agents may induce neoplasia or a more chronic condition. Overall, the
consequences of viral infections depend on the outcome of
the interaction between a number of factors both from the
virus and the host. Acute infections may lead to host death,
recovery with no long term effects, or develop into a chronic
infection. The later can remain subclinical for life, have a
long silent period before it manifests or can have periods of
reactivation with relapses and exacerbation causing acute
disease. Different types of stress including overcrowding,
sexual maturation and handling may reactivate a latent
infection resulting in clinical disease. Avoidance of viral
infection is difficult, unless spring or disinfected water is
used. This is possible in the early developmental fresh water
stage, but very costly or impractical when farming in open
environments e.g. sea water cage culture, or when high
volumes are required for growers in land based systems.
Among salmonids, RNA viruses have the greatest ecological and socio-economic impact. The occurrence of viral
diseases has seen an increment over the last 10 years and it is
likely that more will be identified in the future. The most
important conditions and host species are listed in Table 5.1.
5.1
Infectious Pancreatic Necrosis
Infectious pancreatic necrosis virus (IPNV) is the
aetiological agent of the highly contagious and acute
catarrhal enteritis ‘infectious pancreatic necrosis’, primarily
affecting cultured fish including rainbow trout and Atlantic
salmon worldwide. The virus has a widespread distribution
in many wild fish species but there is little evidence of
transmission to farmed stock.
Significant mortalities can occur from clinical outbreaks
in fry, with a relative reduced mortality in older fish. However, outbreaks in post-smolts in sea water are common and
fish which survive infection can become carriers and continue to shed infective material into the water. Carrier fish
are a particular hazard if used as broodstock since the virus
has a germ-line associated vertical transmission via ova or
milt, which occur both extra- or intra-ovum. Surface extraovum infection of the gametes can be managed through
proper biosecurity procedures and disinfection, but the
intra-ovum transmission imposes an important risk and can
only be controlled through rigorous testing of the
broodstock.
Salmonids may show clinical signs with high mortality or
become asymptomatic carriers, thus establishing covert
infections. Clinical signs include moribund fish which are
darker in appearance, slightly emaciated and lethargic.
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_5, # Springer Science+Business Media B.V. 2013
53
5
Abstract
Some viral diseases are prevented through vaccination but for many no vaccines are
available hence infection is a major concern for farmed fish. Among salmonids, RNA
viruses have the greatest ecological and socio-economic impact. Acute infections may lead
to host death, or induce a more chronic condition. Stress including overcrowding, sexual
maturation and handling may also reactivate a latent infection resulting in clinical disease.
This chapter covers a range of viral diseases in wild and farmed salmonids.
Keywords
Fish virus Salmon Trout
Infectious viral diseases continue to be a major concern in
wild and farmed fish and responsible of important losses,
particularly under farming conditions. These infections can
manifest as a septicaemia-like disease with a characteristic
acute presentation, but some viral agents may induce neoplasia or a more chronic condition. Overall, the
consequences of viral infections depend on the outcome of
the interaction between a number of factors both from the
virus and the host. Acute infections may lead to host death,
recovery with no long term effects, or develop into a chronic
infection. The later can remain subclinical for life, have a
long silent period before it manifests or can have periods of
reactivation with relapses and exacerbation causing acute
disease. Different types of stress including overcrowding,
sexual maturation and handling may reactivate a latent
infection resulting in clinical disease. Avoidance of viral
infection is difficult, unless spring or disinfected water is
used. This is possible in the early developmental fresh water
stage, but very costly or impractical when farming in open
environments e.g. sea water cage culture, or when high
volumes are required for growers in land based systems.
Among salmonids, RNA viruses have the greatest ecological and socio-economic impact. The occurrence of viral
diseases has seen an increment over the last 10 years and it is
likely that more will be identified in the future. The most
important conditions and host species are listed in Table 5.1.
5.1
Infectious Pancreatic Necrosis
Infectious pancreatic necrosis virus (IPNV) is the
aetiological agent of the highly contagious and acute
catarrhal enteritis ‘infectious pancreatic necrosis’, primarily
affecting cultured fish including rainbow trout and Atlantic
salmon worldwide. The virus has a widespread distribution
in many wild fish species but there is little evidence of
transmission to farmed stock.
Significant mortalities can occur from clinical outbreaks
in fry, with a relative reduced mortality in older fish. However, outbreaks in post-smolts in sea water are common and
fish which survive infection can become carriers and continue to shed infective material into the water. Carrier fish
are a particular hazard if used as broodstock since the virus
has a germ-line associated vertical transmission via ova or
milt, which occur both extra- or intra-ovum. Surface extraovum infection of the gametes can be managed through
proper biosecurity procedures and disinfection, but the
intra-ovum transmission imposes an important risk and can
only be controlled through rigorous testing of the
broodstock.
Salmonids may show clinical signs with high mortality or
become asymptomatic carriers, thus establishing covert
infections. Clinical signs include moribund fish which are
darker in appearance, slightly emaciated and lethargic.
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_5, # Springer Science+Business Media B.V. 2013
53