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Necropsy Examination of Fish
Tropical Fish Medicine
0195--5616/88 $0.00
+
.20
Necropsy Examination of Fish Renate Reimschuessel, VMD, * Eric B. May, PhD, t Richard 0. Bennett, PhD,t and Michael M. Lipsky, PhD§
Necropsy examination of moribund or dead specimens is an essential step in diagnosing fish disease. The general lack of pathology services that work with fish makes it especially important that the clinician know the basics of fish necropsy analysis. A careful external and internal examination must, however, follow a detailed history and clinical assessment. This should include the number of fish affected, the total number of mortalities, the number of mortalities per day, the color, behavior, and age of the affected fish, and any current or prophylactic treatments. This information may determine what type of samples to take (bacterial cultures, tissues for viral or parasite isolation, or tissues critical for histologic or toxicologic evaluation). Interpretation of any necropsy findings depends on an understanding of the entire case, not just the physical lesions and laboratory analyses. This article briefly discusses basic anatomy and necropsy procedures that are referred to frequently through~ie~lprovides a step-by-step outline of the systems t9 ~valuate and sample for histology.
ANATOMY It is beyond the scope of this article to present a detailed description of the anatomy of every type of fish. Figures 1 and 2 show the basic external and internal anatomy of the fusiform fish. External variations in shape may *Research Associate, Department of Pathology, University of Maryland School of Medicine; Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; and Pathologist, National Aquarium in Baltimore, Inc. tAssistant Professor, Department of Pathology, University of Maryland School of Medicine; Coordinator, Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; and Pathologist, National Aquarium in Baltimore, Inc. tAssistant Professor, Department of Pathology, University of Maryland School of Medicine; Operations Manager, Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; Pathologist, National Aquarium in Baltimore, Inc. §Associate Professor, Department of Pathology, University of Maryland School of Medicine; Director, Environmental Pathology Laboratory. Experimental Pathologist. Veterinary Clinics of North Anwrica: Small Animal Practice-Vol. 18, No. 2, March 1988
427
428
RENATE REIMSCHUESSEL ET AL dorsal fin caudal fin lateral line
nares
caudal operculum
anal
pelvic fin
Figure l. External anatomy.
be marked. The external anatomy of the eel with its serpentine shape is quite different from that of flattened fish like the angelfish. Fins vary in shape and size with different species of fish. Some have an extra adipose fin located between the dorsal and caudal fins. Others are missing some fins entirely. The internal anatomy also varies with the shape, age, and species of the specimen. 1- 3 Although the organs retain their positions with respect to each other, they may be pushed craniad, as in the flat fish, or surrounded by thick fibrous connective tissue, as in the eel. Some organs may be hard to locate. The pseudobranch, a gill-like structure present in some fish, is located on the medial-cranial surface of the operculum. The heart is found between and just caudal to the gills. The spleen is a flat triangular dc!rk-red organ located adjacent to the left dorsal aspect of the stomach. In some species, the spleen is embedded in fat and cannot be easily seen on gross examination. Unlike the mammal, the white pulp of a fish spleen is not prominent unless it is active. The kidneys of fish are retroperitoneal, lying next to the spinal column and dorsal to the swimbladder. They are dark reddish-brown and may be distinctly lobed or intimately embedded along the vertebrae. The cranial kidney may be separate or fused with the caudal kidney. A urinary "bladder" formed by fusion of the mesonephric ducts may be present near the anus. The gonads are located in the caudal abdomen and may not be grossly visible in sexually inactive animals. The swimbladder, if present, is a white hollow fibrous organ located dorsally near the spinal column. It is usually easily identified if it has not been ruptured. The number of chambers can vary with the species, as can the prominence of the red gas-forming organ. The digestive tract is also easily located. Variations in the length and shape of the intestines occur in different species. Some fish have multiple pyloric cecae, others may have a spiral intestine, and others may have a short, relatively straight gut. The gallbladder is located between the liver lobes and may be quite distended if the fish has not been eating. The color of the bile may vary from dark green to straw-colored yellow. The liver is
429
NECROPSY EXAMINATION OF FISH
arches
urinary bladder intestine
ovary gall bladder
spleen
stomach
pyloric cecae
Figure 2. Internal anatomy.
usually a reddish-tan. It can show marked variations in color, becoming quite yellow in fish raised on some artificial diets. The brain is located in the skull, caudal and dorsal to the eyes. It is normally white and fairly firm. PREPARATION When possible, use a live fish exhibiting signs of disease for a necropsy. Dead fish to be submitted for necropsy should be wrapped in moist paper to prevent drying and refrigerated or placed on ice. Under no circumstances should dead fish be kept floating in water or frozen. Freezing renders the tissues unsuitable for many diagnostic laboratory procedures. If alive, the fish may be narcotized prior to sacrifice by severing the spinal cord caudal to the brain case (see the article "Anesthesia in Fish"). Use the caudal margin of the operculum as a guide for the location of the incision. Select instruments appropriate for the size of the fish. Usually a scalpel, a pair of forceps, and two pairs of scissors (one pair of heavy dissecting scissors and one pair of fine tissue scissors) are all that is needed. For very small fish, a dissecting microscope is very useful. Have culture materials and fixatives at hand. For histology, fixatives such as Bouin' s or 10 per cent neutral buffered formalin are routinely used. Find out from your diagnostic laboratory how specimens for virology, parasitology, microbiology, and histology should be prepared and submitted. NECROPSY Each necropsy should be done in a systematic and consistent sequence. Table 1 presents a step-by-step outline of the protocol. The prosector
430
RENATE REIMSCHUESSEL ET AL
Table 1. Outline of Necropsy Procedures EXTERNAL EXAMINATION
4. 5. 6. 7.
l. General condition
2. Fins 3. Skin scraping
Weight and length Eyes and nares Oral cavity Anus
INTERNAL EXAMINATION
l. Disinfect
2. Remove the operculum with the pseudobranch 3. Remove the eye 4. Remove the second and third gill arches 5. Open the body cavities 6. Remove the heart 7. Remove the abdominal body block 8. Dissect out the liver 9. Dissect out the spleen
10. Remove the gonads 11. Remove the swimbladder 12. Remove a cranial and caudal wedge of kidney, vertebrae, spinal cord, and muscle 13. Sample the skin, including lateral line and nares 14. Remove the brain 15. Open and sample the stomach and intestines
should follow this sequence to ensure that all organs have been examined and that appropriate samples have been taken. External Examination
General Condition. Examine all external surfaces, noting loss of scales, ulcers, areas of discoloration, or other abnormalities. The location of any lesion is important and must be noted. Look at the body form and palpate it to determine if there is any change suggestive of muscle atrophy, ascites, or skeletal deformity. Fins. Examine all fins for fraying, erosion, necrosis, or small discolored spots that may be parasites. Skin Scraping. Prepare a skin scraping with a clean scalpel, evaluating both normal skin and the margms of any lesions. Because time and subsequent procedures will reduce the quality of the preparations, this should be done during examination of external surfaces. Examine the fresh scraping for bacteria, fungi, or parasites, as you would a biopsy specimen (see the article "Biopsy and Rapid Postmortem Techniques for Diagnosing Diseases of Fish"). Because parasites often leave the host shortly after death and bacteria proliferate, the time of death is important. Using sterile technique, take microbiological cultures from ulcerative lesions (see the article "Bacterial Culture and Evaluation of Diseases of Fish"). Note any excess mucus. Weight and Length. Record the weight. Commonly encountered fish may range from less than 1.0 gm to a few kilograms. Length is taken from either the snout to the tail fork (fork length) or from the snout to the caudal fin margin (total length). Pick one length and be consistent. Fork length is less susceptible to error caused by eroded fins. Eyes and Nares. Examine the eyes for corneal opacity or exophthalmia. Also examine the anterior chamber for the presence of blood or exudates. The nares can be examined by pressing behind the external orifices. Note if any exudates are expressed. Oral Cavity. Open the mouth and view the oral cavity. Note any raised or discolored areas or macroscopic parasites.
NECROPSY EXAMINATION OF FISH
431
Anus. Examine the anus for any swelling, redness, or ulcerations. If fecal material protrudes, examine it at this time. Internal Examination Disinfecting. After the external examination, disinfect the surface of the fish with 70 per cent alcohol. Place the fish in left lateral recumbency on a nonslip surface such as wood or a disposable drape. An approach through the right side gives better access to the spleen. 4 Removing the Operculum with the Pseudobranch. The pseudobranch should be included with tissues submitted for histology. Removing the Eye. The eye should be included with tissues submitted for histology. Examine the orbit for exudates or hemorrhage. Removing the Second and Third GiU Arches. The gills should be bright red in fresh specimens. Animals submitted for necropsy longer than 8 hours after death often have very pale or white gills and are unsuitable for histology because of autolysis. Look for any abrasions, hemorrhages, excess mucus, or parasites. Examine a wet mount of gill microscopically for parasites. Handle gills gently, grasping and cutting them at each end of the cartilaginous arch. A section of the second and third arch usually provides an adequate sample for histopathologic examination. Opening the Body Cavities. Using sterile technique, make an incision starting at the operculum. Cut through the pelvic girdle, and extend the incision dorsally to the spine. Cut through the pectoral girdle. Continue the incision caudally along the spine, then follow the curve of the abdominal cavity in a ventrocaudal direction to the anus. Finally, extend the incision from the operculum along the ventral midline to the anus, and remove the body wall (see Fig. 1). The contents of the pericardia! cavity. and the abdomen are now exposed. Culture any B.uid in the body cavities and note its color and consistency. Next, examine the organs in situ. Note the size, color, and consistency of each organ. Note how much adipose tissue is located between the organs. Before disturbing any of the organs, culture any tissue that appears abnormal. Organs can then be removed for closer examination, and specimens or impression smears taken for laboratory evaluation. With very small fish, it may be best to place the entire fish in fixative while the organs are still in situ. Be sure to open the skull dorsal to the operculum to allow fixative to penetrate the brain. Removing the Heart. Grasp the cranial border of the bulbus arteriosus, pull caudally gently, and sever the connection to the ventral aorta. The heart can now be pulled far enough out of the body to expose and sever the sinus venosus. Examine the heart for any raised or discolored lesions. Removing the Abdominal Body Block. Remove the abdominal body block by first transecting the esophagus. Grasp the esophagus and lift it out of the body while gently pulling caudally. Transect the intestine at the anus. The stomach, intestines, liver, spleen, and possibly the swimbladder and gonads will be removed by this procedure. Dissecting Out the Liver. Free the liver from the surrounding tissue. Examine the edges, which should be sharp. Make several slices into the parenchyma, and examine the color and texture of the cut surface. Note whether or not the gallbladder is distended.
432
RENATE REIMSCHUESSEL ET AL
Dissecting Out the Spleen. As with the liver, examine the edges and a cut surface. If the spleen is swollen and round, it should be cultured. Removing the Gonads. If the gonads are visible, note their size, color, and consistency. Make several incisions into them, and evaluate the cut surfaces. Removing the Swimbladder. If the swimbladder is present, gently open it, watching for any exudates or hemorrhagic areas. Be ready to take cultures if this is the case.
Removing a Cranial and Caudal Wedge of Kidney, Vertebra, Spinal Cord, and Muscle. Return to the body and examine the kidneys. If samples
are being taken for histology, be sure to include tissues from the cranial and caudal portions. Sections of tissue containing kidney, spinal cord, and muscle can be obtairwd by cutting a wedge from the abdomen through the dorsal body of the fish. If septicemia is suspected, culture the kidney. Bacteria tend to localize within the kidney. 5• 6 Sampling the Skin, Including Lateral Line and Nares. Take samples of skin for histologic analysis. The sections should include normal skin at the margins of lesions. Submit samples of the lateral line and the nares. Removing the Brain. The brain should be examined by opening the skull just dorsal to the eyes. If it is difficult to find the brain, remove both eyes and follow the optic nerves caudad. Remove enough of the bone to fully expose the brain. Note any hemorrhage or discoloration of the cerebrospinal fluid or the brain itself. Transect the spinal cord and gently tease the brain away from its neural connections. Brain undergoes autolysis and liquefaction rapidly; therefore, if any histologic work is to be done, it is important that the specimen be fresh. Opening and Sampling the Stomach and Intestines. Examine the stomach and intestinal tract. Note any raised or hemorrhagic areas on the serosal surface. Cut open the lumen, and examine the contents of the entire tract. Note any irregularities in the mucosal surface. Scrapings can be evaluated for microscopic parasites (see the article "Parasites Associated with Ornamental Fish"),
CONCLUSION It is important to maintain an organized and systematic approach to the necropsy. To develop good necropsy technique, one must first perform several practice necropsies. Follow the procedure outlined here step-bystep so that every organ has been examined and sampled. A large part of this article has been devoted to procuring samples for laboratory analysis and histologic examination. Indeed, laboratory results and histology often provide a wealth of information. However, to determine which findings are responsible for the mortalities, the entire case history and gross necropsy findings must be reviewed. Careful observations made during the necropsy examination provide valuable information immediately, as well as later on in the interpretation of laboratory data. Adhering to the protocol outlined in this article will reward the clinician's labors.
NECROPSY EXAMINATION OF FISH
433
ACKNOWLEDGMENTS The authors would like to express their gratitude to Andrew Kane, Pakika de MenezesFerreira, and Ann Muhvich for their support.
REFERENCES l. Amlacher E: Textbook of Fish Diseases. Neptune, New Jersey, TFH Publications, 1970, pp 17-52 2. Groman DB: Histology of the striped bass. Bethesda, Maryland, American Fisheries Society Monograph no. 3, 1982 3. Lagler KF, Bardach JE, Miller RR, May Passino DR: Ichthyology. New York, John Wiley & Sons, 1977 4. May EB: Workshop, "The Aquatic Animal," held at the National Aquarium in Baltimore on January 16-17; 1985 5. Roberts RJ: Fish Pathology. London, Bailliere Tindall, 1978 6. Sindermann CJ, Ziskowski JJ, Anderson VT Jr: A guide for the recognition of some disease conditions and abnormalities in marine fish. National Marine Fisheries Service, U.S. Department of Commerce, Technical Series Report no. 14, 1978
Department of Pathology School of Medicine University of Maryland Baltimore, Maryland 21201
0195--5616/88 $0.00
+
.20
Necropsy Examination of Fish Renate Reimschuessel, VMD, * Eric B. May, PhD, t Richard 0. Bennett, PhD,t and Michael M. Lipsky, PhD§
Necropsy examination of moribund or dead specimens is an essential step in diagnosing fish disease. The general lack of pathology services that work with fish makes it especially important that the clinician know the basics of fish necropsy analysis. A careful external and internal examination must, however, follow a detailed history and clinical assessment. This should include the number of fish affected, the total number of mortalities, the number of mortalities per day, the color, behavior, and age of the affected fish, and any current or prophylactic treatments. This information may determine what type of samples to take (bacterial cultures, tissues for viral or parasite isolation, or tissues critical for histologic or toxicologic evaluation). Interpretation of any necropsy findings depends on an understanding of the entire case, not just the physical lesions and laboratory analyses. This article briefly discusses basic anatomy and necropsy procedures that are referred to frequently through~ie~lprovides a step-by-step outline of the systems t9 ~valuate and sample for histology.
ANATOMY It is beyond the scope of this article to present a detailed description of the anatomy of every type of fish. Figures 1 and 2 show the basic external and internal anatomy of the fusiform fish. External variations in shape may *Research Associate, Department of Pathology, University of Maryland School of Medicine; Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; and Pathologist, National Aquarium in Baltimore, Inc. tAssistant Professor, Department of Pathology, University of Maryland School of Medicine; Coordinator, Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; and Pathologist, National Aquarium in Baltimore, Inc. tAssistant Professor, Department of Pathology, University of Maryland School of Medicine; Operations Manager, Aquatic Toxicology and Pathobiology Division, Environmental Pathology Laboratory; Pathologist, National Aquarium in Baltimore, Inc. §Associate Professor, Department of Pathology, University of Maryland School of Medicine; Director, Environmental Pathology Laboratory. Experimental Pathologist. Veterinary Clinics of North Anwrica: Small Animal Practice-Vol. 18, No. 2, March 1988
427
428
RENATE REIMSCHUESSEL ET AL dorsal fin caudal fin lateral line
nares
caudal operculum
anal
pelvic fin
Figure l. External anatomy.
be marked. The external anatomy of the eel with its serpentine shape is quite different from that of flattened fish like the angelfish. Fins vary in shape and size with different species of fish. Some have an extra adipose fin located between the dorsal and caudal fins. Others are missing some fins entirely. The internal anatomy also varies with the shape, age, and species of the specimen. 1- 3 Although the organs retain their positions with respect to each other, they may be pushed craniad, as in the flat fish, or surrounded by thick fibrous connective tissue, as in the eel. Some organs may be hard to locate. The pseudobranch, a gill-like structure present in some fish, is located on the medial-cranial surface of the operculum. The heart is found between and just caudal to the gills. The spleen is a flat triangular dc!rk-red organ located adjacent to the left dorsal aspect of the stomach. In some species, the spleen is embedded in fat and cannot be easily seen on gross examination. Unlike the mammal, the white pulp of a fish spleen is not prominent unless it is active. The kidneys of fish are retroperitoneal, lying next to the spinal column and dorsal to the swimbladder. They are dark reddish-brown and may be distinctly lobed or intimately embedded along the vertebrae. The cranial kidney may be separate or fused with the caudal kidney. A urinary "bladder" formed by fusion of the mesonephric ducts may be present near the anus. The gonads are located in the caudal abdomen and may not be grossly visible in sexually inactive animals. The swimbladder, if present, is a white hollow fibrous organ located dorsally near the spinal column. It is usually easily identified if it has not been ruptured. The number of chambers can vary with the species, as can the prominence of the red gas-forming organ. The digestive tract is also easily located. Variations in the length and shape of the intestines occur in different species. Some fish have multiple pyloric cecae, others may have a spiral intestine, and others may have a short, relatively straight gut. The gallbladder is located between the liver lobes and may be quite distended if the fish has not been eating. The color of the bile may vary from dark green to straw-colored yellow. The liver is
429
NECROPSY EXAMINATION OF FISH
arches
urinary bladder intestine
ovary gall bladder
spleen
stomach
pyloric cecae
Figure 2. Internal anatomy.
usually a reddish-tan. It can show marked variations in color, becoming quite yellow in fish raised on some artificial diets. The brain is located in the skull, caudal and dorsal to the eyes. It is normally white and fairly firm. PREPARATION When possible, use a live fish exhibiting signs of disease for a necropsy. Dead fish to be submitted for necropsy should be wrapped in moist paper to prevent drying and refrigerated or placed on ice. Under no circumstances should dead fish be kept floating in water or frozen. Freezing renders the tissues unsuitable for many diagnostic laboratory procedures. If alive, the fish may be narcotized prior to sacrifice by severing the spinal cord caudal to the brain case (see the article "Anesthesia in Fish"). Use the caudal margin of the operculum as a guide for the location of the incision. Select instruments appropriate for the size of the fish. Usually a scalpel, a pair of forceps, and two pairs of scissors (one pair of heavy dissecting scissors and one pair of fine tissue scissors) are all that is needed. For very small fish, a dissecting microscope is very useful. Have culture materials and fixatives at hand. For histology, fixatives such as Bouin' s or 10 per cent neutral buffered formalin are routinely used. Find out from your diagnostic laboratory how specimens for virology, parasitology, microbiology, and histology should be prepared and submitted. NECROPSY Each necropsy should be done in a systematic and consistent sequence. Table 1 presents a step-by-step outline of the protocol. The prosector
430
RENATE REIMSCHUESSEL ET AL
Table 1. Outline of Necropsy Procedures EXTERNAL EXAMINATION
4. 5. 6. 7.
l. General condition
2. Fins 3. Skin scraping
Weight and length Eyes and nares Oral cavity Anus
INTERNAL EXAMINATION
l. Disinfect
2. Remove the operculum with the pseudobranch 3. Remove the eye 4. Remove the second and third gill arches 5. Open the body cavities 6. Remove the heart 7. Remove the abdominal body block 8. Dissect out the liver 9. Dissect out the spleen
10. Remove the gonads 11. Remove the swimbladder 12. Remove a cranial and caudal wedge of kidney, vertebrae, spinal cord, and muscle 13. Sample the skin, including lateral line and nares 14. Remove the brain 15. Open and sample the stomach and intestines
should follow this sequence to ensure that all organs have been examined and that appropriate samples have been taken. External Examination
General Condition. Examine all external surfaces, noting loss of scales, ulcers, areas of discoloration, or other abnormalities. The location of any lesion is important and must be noted. Look at the body form and palpate it to determine if there is any change suggestive of muscle atrophy, ascites, or skeletal deformity. Fins. Examine all fins for fraying, erosion, necrosis, or small discolored spots that may be parasites. Skin Scraping. Prepare a skin scraping with a clean scalpel, evaluating both normal skin and the margms of any lesions. Because time and subsequent procedures will reduce the quality of the preparations, this should be done during examination of external surfaces. Examine the fresh scraping for bacteria, fungi, or parasites, as you would a biopsy specimen (see the article "Biopsy and Rapid Postmortem Techniques for Diagnosing Diseases of Fish"). Because parasites often leave the host shortly after death and bacteria proliferate, the time of death is important. Using sterile technique, take microbiological cultures from ulcerative lesions (see the article "Bacterial Culture and Evaluation of Diseases of Fish"). Note any excess mucus. Weight and Length. Record the weight. Commonly encountered fish may range from less than 1.0 gm to a few kilograms. Length is taken from either the snout to the tail fork (fork length) or from the snout to the caudal fin margin (total length). Pick one length and be consistent. Fork length is less susceptible to error caused by eroded fins. Eyes and Nares. Examine the eyes for corneal opacity or exophthalmia. Also examine the anterior chamber for the presence of blood or exudates. The nares can be examined by pressing behind the external orifices. Note if any exudates are expressed. Oral Cavity. Open the mouth and view the oral cavity. Note any raised or discolored areas or macroscopic parasites.
NECROPSY EXAMINATION OF FISH
431
Anus. Examine the anus for any swelling, redness, or ulcerations. If fecal material protrudes, examine it at this time. Internal Examination Disinfecting. After the external examination, disinfect the surface of the fish with 70 per cent alcohol. Place the fish in left lateral recumbency on a nonslip surface such as wood or a disposable drape. An approach through the right side gives better access to the spleen. 4 Removing the Operculum with the Pseudobranch. The pseudobranch should be included with tissues submitted for histology. Removing the Eye. The eye should be included with tissues submitted for histology. Examine the orbit for exudates or hemorrhage. Removing the Second and Third GiU Arches. The gills should be bright red in fresh specimens. Animals submitted for necropsy longer than 8 hours after death often have very pale or white gills and are unsuitable for histology because of autolysis. Look for any abrasions, hemorrhages, excess mucus, or parasites. Examine a wet mount of gill microscopically for parasites. Handle gills gently, grasping and cutting them at each end of the cartilaginous arch. A section of the second and third arch usually provides an adequate sample for histopathologic examination. Opening the Body Cavities. Using sterile technique, make an incision starting at the operculum. Cut through the pelvic girdle, and extend the incision dorsally to the spine. Cut through the pectoral girdle. Continue the incision caudally along the spine, then follow the curve of the abdominal cavity in a ventrocaudal direction to the anus. Finally, extend the incision from the operculum along the ventral midline to the anus, and remove the body wall (see Fig. 1). The contents of the pericardia! cavity. and the abdomen are now exposed. Culture any B.uid in the body cavities and note its color and consistency. Next, examine the organs in situ. Note the size, color, and consistency of each organ. Note how much adipose tissue is located between the organs. Before disturbing any of the organs, culture any tissue that appears abnormal. Organs can then be removed for closer examination, and specimens or impression smears taken for laboratory evaluation. With very small fish, it may be best to place the entire fish in fixative while the organs are still in situ. Be sure to open the skull dorsal to the operculum to allow fixative to penetrate the brain. Removing the Heart. Grasp the cranial border of the bulbus arteriosus, pull caudally gently, and sever the connection to the ventral aorta. The heart can now be pulled far enough out of the body to expose and sever the sinus venosus. Examine the heart for any raised or discolored lesions. Removing the Abdominal Body Block. Remove the abdominal body block by first transecting the esophagus. Grasp the esophagus and lift it out of the body while gently pulling caudally. Transect the intestine at the anus. The stomach, intestines, liver, spleen, and possibly the swimbladder and gonads will be removed by this procedure. Dissecting Out the Liver. Free the liver from the surrounding tissue. Examine the edges, which should be sharp. Make several slices into the parenchyma, and examine the color and texture of the cut surface. Note whether or not the gallbladder is distended.
432
RENATE REIMSCHUESSEL ET AL
Dissecting Out the Spleen. As with the liver, examine the edges and a cut surface. If the spleen is swollen and round, it should be cultured. Removing the Gonads. If the gonads are visible, note their size, color, and consistency. Make several incisions into them, and evaluate the cut surfaces. Removing the Swimbladder. If the swimbladder is present, gently open it, watching for any exudates or hemorrhagic areas. Be ready to take cultures if this is the case.
Removing a Cranial and Caudal Wedge of Kidney, Vertebra, Spinal Cord, and Muscle. Return to the body and examine the kidneys. If samples
are being taken for histology, be sure to include tissues from the cranial and caudal portions. Sections of tissue containing kidney, spinal cord, and muscle can be obtairwd by cutting a wedge from the abdomen through the dorsal body of the fish. If septicemia is suspected, culture the kidney. Bacteria tend to localize within the kidney. 5• 6 Sampling the Skin, Including Lateral Line and Nares. Take samples of skin for histologic analysis. The sections should include normal skin at the margins of lesions. Submit samples of the lateral line and the nares. Removing the Brain. The brain should be examined by opening the skull just dorsal to the eyes. If it is difficult to find the brain, remove both eyes and follow the optic nerves caudad. Remove enough of the bone to fully expose the brain. Note any hemorrhage or discoloration of the cerebrospinal fluid or the brain itself. Transect the spinal cord and gently tease the brain away from its neural connections. Brain undergoes autolysis and liquefaction rapidly; therefore, if any histologic work is to be done, it is important that the specimen be fresh. Opening and Sampling the Stomach and Intestines. Examine the stomach and intestinal tract. Note any raised or hemorrhagic areas on the serosal surface. Cut open the lumen, and examine the contents of the entire tract. Note any irregularities in the mucosal surface. Scrapings can be evaluated for microscopic parasites (see the article "Parasites Associated with Ornamental Fish"),
CONCLUSION It is important to maintain an organized and systematic approach to the necropsy. To develop good necropsy technique, one must first perform several practice necropsies. Follow the procedure outlined here step-bystep so that every organ has been examined and sampled. A large part of this article has been devoted to procuring samples for laboratory analysis and histologic examination. Indeed, laboratory results and histology often provide a wealth of information. However, to determine which findings are responsible for the mortalities, the entire case history and gross necropsy findings must be reviewed. Careful observations made during the necropsy examination provide valuable information immediately, as well as later on in the interpretation of laboratory data. Adhering to the protocol outlined in this article will reward the clinician's labors.
NECROPSY EXAMINATION OF FISH
433
ACKNOWLEDGMENTS The authors would like to express their gratitude to Andrew Kane, Pakika de MenezesFerreira, and Ann Muhvich for their support.
REFERENCES l. Amlacher E: Textbook of Fish Diseases. Neptune, New Jersey, TFH Publications, 1970, pp 17-52 2. Groman DB: Histology of the striped bass. Bethesda, Maryland, American Fisheries Society Monograph no. 3, 1982 3. Lagler KF, Bardach JE, Miller RR, May Passino DR: Ichthyology. New York, John Wiley & Sons, 1977 4. May EB: Workshop, "The Aquatic Animal," held at the National Aquarium in Baltimore on January 16-17; 1985 5. Roberts RJ: Fish Pathology. London, Bailliere Tindall, 1978 6. Sindermann CJ, Ziskowski JJ, Anderson VT Jr: A guide for the recognition of some disease conditions and abnormalities in marine fish. National Marine Fisheries Service, U.S. Department of Commerce, Technical Series Report no. 14, 1978
Department of Pathology School of Medicine University of Maryland Baltimore, Maryland 21201