These notes follow the tape entitled, Fish Necropsy Procedures, which was produced at Purdue University several years ago. (Note: This procedure should only be performed by fish health professionals). It is important to note that this necropsy demonstration procedure was performed using Hybrid Striped Bass, so necropsy procedures performed on other species of fish may be slightly different, due to species differences. The single most important way to perform a necropsy is based solely on the individual performing the technique. Therefore, regardless of the technique used, be certain to be consistent and be thorough.
The necropsy procedure, per se, should not be performed until all of the
other procedures such as gill and fin clippings, skin scrapings, and blood
collection have been performed. Also,
lengths and weights of the fish as well as any identification photographs or
recording of identification (wire tags) should be noted prior to the necropsy.
EXAMINATION OF THE GILLS AND HEAD
In order to expose the gills, the operculum is cut transversely and removed. Gill tissue samples may be collected for histopathology by transection of the gills at the gill arches. Normal gills are bright red and are of uniform size and shape. Abnormal gill lesions include discoloration (paleness associated with anemia, or brownish color associated with nitrite posioning). Other gill lesions include hemorrhages, white foci, excessive mucous accumulation or, blunting, fusions and erosions associated with gill parasites as well as bacterial infections. Gills are not very suitable organs for bacteriologic or virology culture, since they are commonly contaminanted with their aquatic environment. The pseudobranch is an endocrine organ which is commonly collected for histopathology, however, lesions are rarely observed in this organ. The brain is commonly collected for bacteriologic culture as well as histopathology. In order to remove the brain, bilateral cuts should be made medial and dorsal to the globes. These transections are connected cranially and caudally by another series of transections and the calvarium can be removed. Unless the brain is to be sampled for virology or bacteriology evaulation, it is usually placed in fixative whole and transected after fixation has occurred. Both globes can be removed from the fish by transecting the external globe muscles between the globe and the bony socket. The globes should also be fixed similar to the brain prior to sectioning for histopathology processing.
The heart is usually the first organ examined. It lies in the pericardial cavity, which is just anterior to the peritoneal cavity in the throat region of the fish and just posterior and ventral to the gills. The heart normally is red, highly muscular and subdivided into two major chambers the ventricle and the atrium. It empties into the aorta via the white elastic tissue, known as the bulbous arterious.
Next, the liver is evaluated grossly. In some species, the pancreatic tissue is incorporated into the liver, and for this reason, this organ is also known as the hepatopancreas. The color of the liver will vary based on the species of fish. In many species it will appear tan, similar to that observed in mammalian animals, such as dogs. However, in other species, such as the hybrid striped bass, the liver will be diffusely pale and/or mottled. This is due to the varying amounts of fat and glycogen within the liver. The clinical significance of these variations is unknown, but is thought to be minimal. The liver should be thoroughly examined by slicing through the parencyma in 0.5 cm intervals to look for the presence of abnormalities such as necrosis, abscesses, parasite migration tracts or neoplasms.
The spleen is bright red to black and located within the mesentery. This is a very important organ for immune system function and regulation in the fish. The size and shape of this organ varies, dependent upon the species, but is usually flattened similar to that observed in other species. The spleen and kidney are the best tissues to culture when a bacterial septicemia is suspected.
Next, the gastrointestinal tract is removed from the fish. Care must be taken not to contaminate the remaining internal organs with material from within the lumen of the gastrointestinal tract prior to taking samples for bacteriologic culture. The stomach of the fish is usually noted only as a simple diverticulum from esophagus, however, in some species, such as salmonids, numerous pyloric ceca extend from the stomach as additional digestion chambers. These are blind-ended pouches which may be confused as parasites to the novice fish pathologist. If the fish is healthy, the remaining intestinal tract may be obscured due to large amounts of mesenteric fat, which can be stripped away easily. The intestinal tract is not easily divided into a small and large intestine as it is in mammals. The distal portion of the intestinal tract ends at the anal opening. The intestine tract should be incised and examined for parasites attached to the mucosal surface or free within the lumen, inflammatory foci associated with bacterial or viral infections and rarely, neoplasms.
The swim bladder is a white, shiny to translucent organ which lies near the dorsum, just ventral to the black posterior kidney which lies just ventral to the vertebral bodies. The primary function of this organ is to regulate buoyancy which is acheived by the amount of gas allowed into this organ. Abnormalities of the swim bladder may include focal to diffuse thickenings which may cause the fish to swim erratically.
The gonads of a fish are paired, however, the sex of the fish may be very difficult to determine if the fish is young, i.e., not yet reached sexual maturity. In immature fish, the reproductive organs are small ribbon-like structures which lie just ventral to the swim bladder and extend thoroughout the length of the peritoneal cavity. In adult females which are close to spawning, much of the abdominal cavity may be filled with mature eggs within the ovaries.
As previously mentioned, the posterior kidney is the dark linear organ which runs the entire length of the peritoneal cavity, and is dorsal to the swim bladder and ventral to the vertebral bodies. This organ is very different from kidneys in mammals and is called the posterior kidney, since some species of fish have an anterior or “head” kidney. The kidney of the fish is responsible for the production of all of the hematopoietic tissue, analogous to the bone marrow in mammals. For this reason, it contains a large amount of melanin pigment scattered throughout the tissue, which gives it this diffuse black color. This tissue is normally much more friable than mammalian kidneys. Finally, remember that this is a target tissue for bacteriologic culture in those fish suspected of bacterial kidney disease or any bacterial septicemia.
In summary, remember, there is no one correct procedure to necropsy a fish. The most important technique is the one which is thorough and concise. It should be noted that the above necropsy procedure is only effective in fish which weigh 4-6 ounces or greater and that smaller fish (shorter than 3 inches) are usually not transected, but rather fixed whole and sectioned in toto. Bacteriologic samples can be obtained from these fish simply by transecting the distal 1/3 of the fish and taking a microswab sample from the kidney. Virology samples are best obtained from these small fish by grinding the entire fish prior to filtering the sample to remove bacterial agents.