Although, according to the etiology of the disease, a detailed investigation of a particular organ may be necessary, it is always advisable to perform a general necropsy protocol in an orderly, complete and systematic manner.
In the case of fish, necropsies will be treated as collective, since they are especially important in the identification of the causal agents of a disease in aquaculture production systems. In this case, in addition to collecting a sufficient number of live individuals from the same community that may present remarkable clinical signs, a sample of healthy animals should also be examined. After euthanasia by humane methods (Directive 86/609/EEC), necropsy is subsequently carried out.
Of particular importance in this case is the speed with which the necropsy is carried out, which should be done as soon as possible, since the processes of autolysis and putrefaction occur much faster in fish compared to mammals and birds.
First of all, a careful inspection of the entire body surface of the fish should be performed, in order to assess the presence of any grossly visible abnormalities (alterations of the spine like kyphosis, lordosis, etc.).
In this sense, the importance of skin coloration, the possible presence of ocular alterations (exophthalmia, hemorrhages, etc.), dermal alterations (hemorrhages, loss of scales, dermal erosions and ulcers) and in the fins (frayed, absence, hemorrhages, etc.), and the presence of external parasites adhered to the skin or gills are emphasized.
Once the possible external lesions have been located and samples have been taken, the fish is internally examined.
The animal is placed in the right lateral recumbence position, with the head facing to the left of the pathologist who will perform the necropsy, with the exception of flatfish.
A transverse incision is made on the ventral edge, immediately anterior to the anus, between the anus and the pelvic fins, avoiding any damage to the intestine. The tip of a pair of scissors is introduced into this incision and a cut is made in the abdominal wall in a cranial direction following the ventral edge of the animal, until reaching the level of the gills (isthmus).
Subsequently, a second cut is made from the initial incision in a cranial-dorsal direction describing a semicircular parabola until reaching the operculum to cut the dorsolateral wall of the musculature covering the abdominal cavity.
With these two cuts the costal wall is lifted in a cranial direction, taking special care to carefully detach any connective tissue union between the costal wall and the organs.
After this, a last cut is made from the dorsal region of the operculum to the isthmus, joining cranially both previous cuts in order to remove the entire left costal wall, leaving the organs visible in situ.
Examination of the coelomic organs
Once opened, abnormal liquid contents, fibrin, etc., are identified. in the coelomic cavity. Then a gross examination of the internal organs is performed to detect any abnormality in them. If a microbiological culture is desired, it should be performed at this point, as soon as possible to avoid possible later contamination.
The liver (or hepatopancreas), spleen and complete gastrointestinal tract are removed, making two cuts: at the level of the esophagus and anus. In this way the kidney, gonads and swim bladder are exposed for examination. The kidney is carefully removed by lifting it from the dorsal part of the cavity, where it is intimately connected to the spinal column.
Next, the heart is visualized in the dorsal-ventral portion of the cavity, located immediately posterior to the gills and separated from the rest of the organs by a fibrous septum (primitive embryonic transverse septum).
Finally, a cut is made with removal of the operculum, allowing a comfortable and detailed inspection of the gills.
Opening of the cranial cavity:
The final necropsy operation consists of exposing the telencephalon, with removal of the organ and eyeballs. For this purpose, a series of incisions has to be made in the epiaxial muscles, as well as several cuts at the level of the animal's neurocranium, removing the upper bony part of the skull.
Rapid diagnostic tests
It is mainly used for the detection of ectoparasites.
A scraping of the surface of the body and fins in the cranial-caudal direction is performed, either with a slide or scalpel. The extension is performed with a drop of the same tank or in its absence with saline, covering the extracted tissue and visualized under the microscope.
Staining can also be performed on this tissue (Diff-Quick, Gram, Giemsa...) and/or mounting of the preparation for later observations.
Fresh preparation of gills
Similar to skin scrapings, ectoparasite detection is sought.
The first gill arch is discarded and one or more of the other 3 gill arches are excised and the gill lamellae are removed. These lamellae are placed in a slide, separating them and adding saline solution or physiological serum, and then covered and observed under the microscope.
Staining can also be performed on this tissue (Diff-Quick, Gram, May Grünwald-Giemsa, etc.) and/or mounting of the preparation for later observations.
Its objective is the detection of parasites and/or bacterial forms in the sampled tissue. A small portion of the tissue is squashed by placing it between two slides and exerting pressure. Subsequently, saline is added or the tissue is stained for microscopic observation.