using small curved scissors or a scalpel until sufficient tissue
is available to grip with forceps. Pull the eye forwards in
order to expose the associated muscles and optic nerve and
then cut free the entire eye ball. Briefly examine to note
presence of haemorrhage or exudate in the anterior chamber
or visible parasites. Some laboratories use Carnoy’s instead
of formalin for eye fixation. Where the main interest is
parasitological examination this will usually require fixation
in 70 % alcohol. It is advisable to make an incision on the
eye ball to allow proper fixation.
The mouth and the oral cavity should be examined recording the presence of any possible petechial haemorrhage,
vesicles, parasites or abnormality associated with these
structures.
Finally, the cranium needs to be opened in order to expose
and examine the brain, an organ relatively ‘protected’ from
contamination within the cranium. The brain is not routinely
sampled for histological assessment but when required, it can
be performed either at this point of the external examination,
or at the end of the necropsy, after the internal examination is
finished and provided dissection has not been extensively
delayed. In salmonid fish up to ~500 g the cranial structures
are sufficiently soft that they can be cut with a sharp scalpel.
Holding the head firmly with forceps introduced into the oral
cavity, cut open the top of the head with a single decisive
movement in a horizontal plane across the head just above the
eye level. Continue the cut along the top edge of the opercula
and upwards into the dorsal musculature. The severed top of
the cranium will contain the brain, already separated from the
ten pairs of nerves below and the connection to the medulla
oblongata. Carefully remove and place in the fixative. Alternatively, the ‘box’ of cartilage surrounding the brain can be
fixed in situ for 24 h before doing the delicate work of
removing it from the cranium. Occasionally, when the cut is
performed too high, half the brain remains in the lower
portion, nevertheless the tissue is sufficiently exposed as to
become readily accessible. When larger fish are examined the
opening of the cranium requires a sharp strong knife to perform the same cut. A record of blood staining or other
discolouration of the cerebrospinal fluid or the organ itself
should be recorded.
3.3.2 Internal Examination
To access the internal organs the body cavity is opened.
There are several ways to approach dissection, however,
the choice must prevent or reduce the likelihood of the
process
introducing
artefacts, damaging tissues,
compressing, cutting, moving or displacing organs as well
as avoiding the risk of contamination.
The instruction provided in this section is a guide for easy
internal examination with the naked eye of fish from
~>15 cm and above. Smaller fish can also be dissected,
however, the tools required should be adapted and working
under the dissecting microscope should be considered.
One of the most common approaches is to tilt the fish so the
belly is facing upwards and, using a scalpel, carefully cut a
small incision through the skin and underlying tissues either
~1–2 cm in front of the vent, or in the isthmus, the fleshy part
between the opercula beneath the head. The incision is to allow
the blunt end of the scissors to be introduced to cut open along
the mid-ventral line of the belly sectioning the harder
structures encountered at the pectoral and pelvic girdles. The
cut should not start with the scissors within the vent as this will
damage the hind gut and contaminate the rest of the organs; for
the same reason if the cut starts at the isthmus approaching
from the opposite direction, it should not go any further than
~1–2 cm in front of the vent. As experience increases all the
procedures can be performed using a scalpel.
With this single cut it is possible to start the internal
examination and the sampling by lifting the flap with forceps
but without exposing the cavity, a practice deemed to contribute to the protection of the internal organs from contamination. Generally exposure of the entire body cavity is
required for assessment and allows access to organs for
sampling. A second cut to dissect and remove the body
wall starts at the caudal end of the first, moving upwards
and slightly backwards to reach a level just below the lateral
line. From there, the cut turns towards the head almost
horizontally just below the lateral line reaching the opercula.
Lifting the sectioned flap with forceps will help to guide the
last cut behind the opercula, downwards to the isthmus
region to completely dissect free the body flank (fillet).
The cranial boundary of the body cavity is defined by the
septum transversum, separating the peritoneal and the pericardial cavities. If the heart needs to be accessed at the same
time, continue anteriorly with the ventral cut slightly further
to expose the pericardial cavity and heart, just cranial to
the septum transversum. Notes on the general appearance
of the body cavity may include references to the extent of the
body fat, tissue growth or colour changes, swelling, ascites,
adhesions and absence of encysted parasites. For tissue
sampling and depending of the fish size, whole organs may
be fixed from small individuals (e.g. heart or the entire
gastrointestinal tract); conversely from larger fish portions
of ~1 cm
3 of each organ should be removed and fixed.
The heart is dissected by lifting the cranial part of the
bulbous arteriosus and cutting the connection to the ventral
aorta and by holding this end, pulling gently to enable the heart
to be moved sufficiently forward to expose and cut free the
sinus venosus, connecting with the cardinal veins and hepatic
sinus. The heart can show lesions involving the myocardium
and associated blood vessels, including blood clots filling the
pericardial cavity (haemopericardium) and occasionally,
parasites can also be found on or around the heart. For
improved fixation of large hearts it may be necessary to divide
the organ longitudinally before placing into the fixative.
30
3 Post-mortem Examination and the Recognition of Tissue Abnormalities
is available to grip with forceps. Pull the eye forwards in
order to expose the associated muscles and optic nerve and
then cut free the entire eye ball. Briefly examine to note
presence of haemorrhage or exudate in the anterior chamber
or visible parasites. Some laboratories use Carnoy’s instead
of formalin for eye fixation. Where the main interest is
parasitological examination this will usually require fixation
in 70 % alcohol. It is advisable to make an incision on the
eye ball to allow proper fixation.
The mouth and the oral cavity should be examined recording the presence of any possible petechial haemorrhage,
vesicles, parasites or abnormality associated with these
structures.
Finally, the cranium needs to be opened in order to expose
and examine the brain, an organ relatively ‘protected’ from
contamination within the cranium. The brain is not routinely
sampled for histological assessment but when required, it can
be performed either at this point of the external examination,
or at the end of the necropsy, after the internal examination is
finished and provided dissection has not been extensively
delayed. In salmonid fish up to ~500 g the cranial structures
are sufficiently soft that they can be cut with a sharp scalpel.
Holding the head firmly with forceps introduced into the oral
cavity, cut open the top of the head with a single decisive
movement in a horizontal plane across the head just above the
eye level. Continue the cut along the top edge of the opercula
and upwards into the dorsal musculature. The severed top of
the cranium will contain the brain, already separated from the
ten pairs of nerves below and the connection to the medulla
oblongata. Carefully remove and place in the fixative. Alternatively, the ‘box’ of cartilage surrounding the brain can be
fixed in situ for 24 h before doing the delicate work of
removing it from the cranium. Occasionally, when the cut is
performed too high, half the brain remains in the lower
portion, nevertheless the tissue is sufficiently exposed as to
become readily accessible. When larger fish are examined the
opening of the cranium requires a sharp strong knife to perform the same cut. A record of blood staining or other
discolouration of the cerebrospinal fluid or the organ itself
should be recorded.
3.3.2 Internal Examination
To access the internal organs the body cavity is opened.
There are several ways to approach dissection, however,
the choice must prevent or reduce the likelihood of the
process
introducing
artefacts, damaging tissues,
compressing, cutting, moving or displacing organs as well
as avoiding the risk of contamination.
The instruction provided in this section is a guide for easy
internal examination with the naked eye of fish from
~>15 cm and above. Smaller fish can also be dissected,
however, the tools required should be adapted and working
under the dissecting microscope should be considered.
One of the most common approaches is to tilt the fish so the
belly is facing upwards and, using a scalpel, carefully cut a
small incision through the skin and underlying tissues either
~1–2 cm in front of the vent, or in the isthmus, the fleshy part
between the opercula beneath the head. The incision is to allow
the blunt end of the scissors to be introduced to cut open along
the mid-ventral line of the belly sectioning the harder
structures encountered at the pectoral and pelvic girdles. The
cut should not start with the scissors within the vent as this will
damage the hind gut and contaminate the rest of the organs; for
the same reason if the cut starts at the isthmus approaching
from the opposite direction, it should not go any further than
~1–2 cm in front of the vent. As experience increases all the
procedures can be performed using a scalpel.
With this single cut it is possible to start the internal
examination and the sampling by lifting the flap with forceps
but without exposing the cavity, a practice deemed to contribute to the protection of the internal organs from contamination. Generally exposure of the entire body cavity is
required for assessment and allows access to organs for
sampling. A second cut to dissect and remove the body
wall starts at the caudal end of the first, moving upwards
and slightly backwards to reach a level just below the lateral
line. From there, the cut turns towards the head almost
horizontally just below the lateral line reaching the opercula.
Lifting the sectioned flap with forceps will help to guide the
last cut behind the opercula, downwards to the isthmus
region to completely dissect free the body flank (fillet).
The cranial boundary of the body cavity is defined by the
septum transversum, separating the peritoneal and the pericardial cavities. If the heart needs to be accessed at the same
time, continue anteriorly with the ventral cut slightly further
to expose the pericardial cavity and heart, just cranial to
the septum transversum. Notes on the general appearance
of the body cavity may include references to the extent of the
body fat, tissue growth or colour changes, swelling, ascites,
adhesions and absence of encysted parasites. For tissue
sampling and depending of the fish size, whole organs may
be fixed from small individuals (e.g. heart or the entire
gastrointestinal tract); conversely from larger fish portions
of ~1 cm
3 of each organ should be removed and fixed.
The heart is dissected by lifting the cranial part of the
bulbous arteriosus and cutting the connection to the ventral
aorta and by holding this end, pulling gently to enable the heart
to be moved sufficiently forward to expose and cut free the
sinus venosus, connecting with the cardinal veins and hepatic
sinus. The heart can show lesions involving the myocardium
and associated blood vessels, including blood clots filling the
pericardial cavity (haemopericardium) and occasionally,
parasites can also be found on or around the heart. For
improved fixation of large hearts it may be necessary to divide
the organ longitudinally before placing into the fixative.
30
3 Post-mortem Examination and the Recognition of Tissue Abnormalities