Bacterial Diseases
6
Abstract
Bacterial infections comprise true obligate and opportunistic facultative pathogens. The
distinction between ‘primary’, ‘facultative’ or ‘opportunistic’ cannot be taken too strictly as
the virulence of the agent will determine its ability to overcome the host defences. Bacterial
diseases are largely represented by Gram negative organisms and affect salmonids in fresh,
sea water or both. A few important diseases of salmon and trout, both in fresh and sea water
are caused by Gram positive bacteria. This chapter covers a wide range of bacterial diseases
representing acute, chronic, systemic or localised infections affecting salmonids in fresh
and sea water.
Keywords
Bacteria Infection Salmon Trout
Bacteria exist everywhere and the majority are capable of
independent existence for varying periods without a ‘host’.
Specific bacteria cause disease, either because they are
‘designed’ to invade a host (true obligate pathogens) or simply because they are in the wrong place at the right time
(opportunistic facultative pathogens). The distinction between
‘primary’, ‘facultative’ or ‘opportunistic’ cannot be taken too
strictly, as the virulence of the agent will determine its ability
to overcome the host defences. These vary between fish stock
and circumstances including stress, water quality or other coexisting infections. When a bacterium is present in a host
and associated with a disease condition, it will normally be
deemed a ‘primary pathogen’, while those agents that can be
found without compromising the fish health but are capable of
inducing disease under certain conditions, will be addressed
as ‘facultative pathogens’. When bacteria survive and multiply in the host tissues without causing clinical disease, fish
harbouring such bacteria are known as ‘asymptomatic
carriers’. The pathology and outcome of a bacterial infection
may vary and dependant on factors linked to the bacterium,
the host and/or the environmental conditions.
Bacteria gain entry to the fish through the gills, gut or via
the skin, and then usually spread throughout the body. As
infections become systemic they can induce acute changes
that externally result in exophthalmia, hyperaemia and
petechial haemorrhage, while internally, ascites, congestion
and haemorrhage can be observed. However, some bacteria
cause chronic infections with tissue proliferation and repairing processes, resulting in typical granulomatous
responses.
Bacterial diseases are mainly represented by Gram negative organisms such as Aeromonas salmonicida, Listonella
anguillarum and Yersinia ruckeri, affecting salmonids in
fresh, sea water or both. Some important diseases however
are caused by Gram positive bacteria, e.g. Renibacterium
salmoninarum in salmon and trout, both in fresh and sea
water. The Chlamydiales and Rickettsiales contain the
genera Chlamydia and Piscirickettsia respectively, and are
obligate intracellular pathogens which multiply within
membrane-bound cytoplasmic vacuoles. Most of these
infections occur in marine or anadromous hosts, but they
have also been reported in freshwater fish. Measures to limit
or control outbreaks in farmed fish include the use of
vaccines and antimicrobial agents respectively.
Taxonomy has progressed dramatically in the last decade
moving from culture-dependent techniques involving phenotype characterization and physiological data, to the application of molecular techniques, 16S rRNA and gene
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_6, # Springer Science+Business Media B.V. 2013
73
6
Abstract
Bacterial infections comprise true obligate and opportunistic facultative pathogens. The
distinction between ‘primary’, ‘facultative’ or ‘opportunistic’ cannot be taken too strictly as
the virulence of the agent will determine its ability to overcome the host defences. Bacterial
diseases are largely represented by Gram negative organisms and affect salmonids in fresh,
sea water or both. A few important diseases of salmon and trout, both in fresh and sea water
are caused by Gram positive bacteria. This chapter covers a wide range of bacterial diseases
representing acute, chronic, systemic or localised infections affecting salmonids in fresh
and sea water.
Keywords
Bacteria Infection Salmon Trout
Bacteria exist everywhere and the majority are capable of
independent existence for varying periods without a ‘host’.
Specific bacteria cause disease, either because they are
‘designed’ to invade a host (true obligate pathogens) or simply because they are in the wrong place at the right time
(opportunistic facultative pathogens). The distinction between
‘primary’, ‘facultative’ or ‘opportunistic’ cannot be taken too
strictly, as the virulence of the agent will determine its ability
to overcome the host defences. These vary between fish stock
and circumstances including stress, water quality or other coexisting infections. When a bacterium is present in a host
and associated with a disease condition, it will normally be
deemed a ‘primary pathogen’, while those agents that can be
found without compromising the fish health but are capable of
inducing disease under certain conditions, will be addressed
as ‘facultative pathogens’. When bacteria survive and multiply in the host tissues without causing clinical disease, fish
harbouring such bacteria are known as ‘asymptomatic
carriers’. The pathology and outcome of a bacterial infection
may vary and dependant on factors linked to the bacterium,
the host and/or the environmental conditions.
Bacteria gain entry to the fish through the gills, gut or via
the skin, and then usually spread throughout the body. As
infections become systemic they can induce acute changes
that externally result in exophthalmia, hyperaemia and
petechial haemorrhage, while internally, ascites, congestion
and haemorrhage can be observed. However, some bacteria
cause chronic infections with tissue proliferation and repairing processes, resulting in typical granulomatous
responses.
Bacterial diseases are mainly represented by Gram negative organisms such as Aeromonas salmonicida, Listonella
anguillarum and Yersinia ruckeri, affecting salmonids in
fresh, sea water or both. Some important diseases however
are caused by Gram positive bacteria, e.g. Renibacterium
salmoninarum in salmon and trout, both in fresh and sea
water. The Chlamydiales and Rickettsiales contain the
genera Chlamydia and Piscirickettsia respectively, and are
obligate intracellular pathogens which multiply within
membrane-bound cytoplasmic vacuoles. Most of these
infections occur in marine or anadromous hosts, but they
have also been reported in freshwater fish. Measures to limit
or control outbreaks in farmed fish include the use of
vaccines and antimicrobial agents respectively.
Taxonomy has progressed dramatically in the last decade
moving from culture-dependent techniques involving phenotype characterization and physiological data, to the application of molecular techniques, 16S rRNA and gene
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_6, # Springer Science+Business Media B.V. 2013
73