Disease outbreaks have always been one of the major
obstacles to profitable fish farming worldwide and directly
linked to fish survival. A build-up of infectious agents can
occur in susceptible stocks, especially where the general
health status of the population is poor and occurrence of
disease can reduce profit dramatically. The success of health
management in controlling the spread of infectious diseases
can be illustrated by reference to the devastating impact
of the viral disease infectious salmon anaemia (ISA).
The infection was first reported in Norway in 1984 among
sea-farmed Atlantic salmon resulting in significant loses and
economic costs. In 1998 Scottish salmon were reported with
ISA, and in 2007 this virus also impacted on the fast-growing
Chilean aquaculture and resulted in the temporary collapse of
their Atlantic salmon production. Specific control measures
including health certification, segregation of year classes,
fallowing of sites and the disinfection of water effluent
from slaughtering facilities, were introduced in several
countries. The practice of pumping sea water into tanks of
pre-smolts to facilitate smolting was also ceased. These
measures reduced the impact of ISA and other infectious
diseases to a level where outbreaks declined significantly. In
addition, similar policies have resulted in an overall improvement in the health of farmed fish in other countries with the
additional benefit that there has been a marked reduction in
the use of antibiotics to control bacterial infections.
Despite all these actions, losses from all possible causes
during sea water production can reach 20 % and therefore
the health of farmed fish is a major concern as impaired
health or any disease state is not acceptable, neither from a
welfare point of view nor economically sustainable.
Disease conditions are diverse in nature and may elicit a
wide range of responses. The final outcome of individual
infections will depend upon the combination of physiological
and immunological host factors and the virulence properties
of the pathogen. It is also important to bear in mind that under
certain circumstances, fish can act as asymptomatic carriers,
passing the infective agent to susceptible animals. Currently,
techniques involving molecular biology are being used to
support diagnostic work, however, pathological assessment
remains the ‘gold standard’ and pathologists will continue
to play a unique role in diagnosing, understanding and
interpretation of the pathological changes putting into context the results of other laboratory tests.
Common signs of disease include abnormal swimming,
dark skin, inappetence and lethargy. Exophthalmia, distended
abdomen, fin rot, skin ulcers and petechial haemorrhage,
especially at the base of the pectoral and pelvic fins, may
also be encountered. Gross pathology associated with fish
diseases can be frustratingly similar, and difficult for inexperienced personal to distinguish between the different
conditions, as very few are pathognomic. Notably where
overlapping infections may be present, this challenges the
pathologist to differentiate between ‘dying of’ from ‘dying
with’, a given agent.
Non-infectious diseases also raise important ethical
questions, particularly as the affected fish also become
more susceptible to infectious diseases. There is also
increasing evidence that intensively reared fish significantly
alters some aspects of cardiac anatomy and physiology.
Other factors contributing to the outcome of disease in
both wild and farmed fish include stress related factors,
which are recognized to increase the susceptibility of fish
to infectious and non-infectious diseases. Stress in fish production can be summarised as ‘an effect produced by any
environmental or other alteration which requires an adaptation and response by the individual beyond their normal
limits, such that the chance of survival is reduced’.
Continued research and development into the science of
fish health management has been conducted in many
countries alongside the growth of the fish farming industry.
Stocking densities are generally lower, the water quality has
been improved, sites within management areas are often
fallowed and a policy of year-class segregation is applied.
Statutory health surveillance and restrictions on the movement of fish with certain categories of infectious agents have
also contributed to the containment of disease outbreaks.
Considerable effort has also been made to reduce the impact
of infectious conditions such as vibriosis, furunculosis,
enteric redmouth, infectious pancreatic necrosis and pancreas disease through the introduction of effective vaccines.
Overall an improved understanding of diseases to which
salmonids are vulnerable have resulted in a decrease in the
incidence of disease among farmed fish.
In 1996 we published ‘A Colour Atlas of Salmonid
diseases’ and now completely revised, this new edition
represents our current knowledge and the significant
advances in the field of fish pathology that have occurred
during this time. For instance, several conditions of unknown
aetiology have been confirmed as viral diseases, for example
cardiomyopathy syndrome (CMS) and heart and skeletal
muscle inflammation (HSMI). Furthermore, new diseases or
different manifestations of disease have been highlighted,
while the significance of other diseases is fading.
We have set our main objective for this book as ‘informative’ so that the reader can familiarise themselves with
the wide range of conditions that we consider as ‘abnormal’
among wild and farmed salmonids. This is supported
through the inclusion of a chapter covering functional anatomy, namely the normal histological structure and function
of all major organs. Furthermore, a necropsy guide and the
recognition of tissue abnormalities as prerequisites to disease diagnosis, the use of appropriate terminology covering
cell injury to tissue and organs, as well as disturbances in
circulation, inflammation and healing process, have been
included. We have chosen to classify diseases according to
1 Introduction
3
obstacles to profitable fish farming worldwide and directly
linked to fish survival. A build-up of infectious agents can
occur in susceptible stocks, especially where the general
health status of the population is poor and occurrence of
disease can reduce profit dramatically. The success of health
management in controlling the spread of infectious diseases
can be illustrated by reference to the devastating impact
of the viral disease infectious salmon anaemia (ISA).
The infection was first reported in Norway in 1984 among
sea-farmed Atlantic salmon resulting in significant loses and
economic costs. In 1998 Scottish salmon were reported with
ISA, and in 2007 this virus also impacted on the fast-growing
Chilean aquaculture and resulted in the temporary collapse of
their Atlantic salmon production. Specific control measures
including health certification, segregation of year classes,
fallowing of sites and the disinfection of water effluent
from slaughtering facilities, were introduced in several
countries. The practice of pumping sea water into tanks of
pre-smolts to facilitate smolting was also ceased. These
measures reduced the impact of ISA and other infectious
diseases to a level where outbreaks declined significantly. In
addition, similar policies have resulted in an overall improvement in the health of farmed fish in other countries with the
additional benefit that there has been a marked reduction in
the use of antibiotics to control bacterial infections.
Despite all these actions, losses from all possible causes
during sea water production can reach 20 % and therefore
the health of farmed fish is a major concern as impaired
health or any disease state is not acceptable, neither from a
welfare point of view nor economically sustainable.
Disease conditions are diverse in nature and may elicit a
wide range of responses. The final outcome of individual
infections will depend upon the combination of physiological
and immunological host factors and the virulence properties
of the pathogen. It is also important to bear in mind that under
certain circumstances, fish can act as asymptomatic carriers,
passing the infective agent to susceptible animals. Currently,
techniques involving molecular biology are being used to
support diagnostic work, however, pathological assessment
remains the ‘gold standard’ and pathologists will continue
to play a unique role in diagnosing, understanding and
interpretation of the pathological changes putting into context the results of other laboratory tests.
Common signs of disease include abnormal swimming,
dark skin, inappetence and lethargy. Exophthalmia, distended
abdomen, fin rot, skin ulcers and petechial haemorrhage,
especially at the base of the pectoral and pelvic fins, may
also be encountered. Gross pathology associated with fish
diseases can be frustratingly similar, and difficult for inexperienced personal to distinguish between the different
conditions, as very few are pathognomic. Notably where
overlapping infections may be present, this challenges the
pathologist to differentiate between ‘dying of’ from ‘dying
with’, a given agent.
Non-infectious diseases also raise important ethical
questions, particularly as the affected fish also become
more susceptible to infectious diseases. There is also
increasing evidence that intensively reared fish significantly
alters some aspects of cardiac anatomy and physiology.
Other factors contributing to the outcome of disease in
both wild and farmed fish include stress related factors,
which are recognized to increase the susceptibility of fish
to infectious and non-infectious diseases. Stress in fish production can be summarised as ‘an effect produced by any
environmental or other alteration which requires an adaptation and response by the individual beyond their normal
limits, such that the chance of survival is reduced’.
Continued research and development into the science of
fish health management has been conducted in many
countries alongside the growth of the fish farming industry.
Stocking densities are generally lower, the water quality has
been improved, sites within management areas are often
fallowed and a policy of year-class segregation is applied.
Statutory health surveillance and restrictions on the movement of fish with certain categories of infectious agents have
also contributed to the containment of disease outbreaks.
Considerable effort has also been made to reduce the impact
of infectious conditions such as vibriosis, furunculosis,
enteric redmouth, infectious pancreatic necrosis and pancreas disease through the introduction of effective vaccines.
Overall an improved understanding of diseases to which
salmonids are vulnerable have resulted in a decrease in the
incidence of disease among farmed fish.
In 1996 we published ‘A Colour Atlas of Salmonid
diseases’ and now completely revised, this new edition
represents our current knowledge and the significant
advances in the field of fish pathology that have occurred
during this time. For instance, several conditions of unknown
aetiology have been confirmed as viral diseases, for example
cardiomyopathy syndrome (CMS) and heart and skeletal
muscle inflammation (HSMI). Furthermore, new diseases or
different manifestations of disease have been highlighted,
while the significance of other diseases is fading.
We have set our main objective for this book as ‘informative’ so that the reader can familiarise themselves with
the wide range of conditions that we consider as ‘abnormal’
among wild and farmed salmonids. This is supported
through the inclusion of a chapter covering functional anatomy, namely the normal histological structure and function
of all major organs. Furthermore, a necropsy guide and the
recognition of tissue abnormalities as prerequisites to disease diagnosis, the use of appropriate terminology covering
cell injury to tissue and organs, as well as disturbances in
circulation, inflammation and healing process, have been
included. We have chosen to classify diseases according to
1 Introduction
3