Return to the Table
of Contents
Next Section of
the Tumor Manual
Previous Section
of the Tumor Manual
1.1. Current questions regarding tumors in Great Lakes fish
There are almost no historic data describing the occurrence of tumors in Great Lakes fish. Scientific and public interest in this issue began slowly in the 1960s, and by 1986 there were 6 environmental agencies and three universities conducting systematic tumor surveys in the Great Lakes basin.
The urgency of this research was driven by significant advances in analytical chemistry and aquatic toxicology. Contaminant surveillance programs reported that the Great Lakes environment was receiving and accumulating an array of organic and inorganic contaminants (NRC & Royal Soc. of Canada in Black & Baumann 1991). Some of these chemicals, acting alone or synergistically with other chemicals, are recognized mammalian carcinogens. The presence of carcinogens, combined with fish tumor induction data (Metcalfe et al. 1988; Black 1983) and reports of tumors in wild fish populations (Baumann et al. 1991; Smith et al. 1989), provided strong circumstantial evidence that chemical carcinogenesis was occurring at some sites in the Great Lakes.
The occurrence of elevated frequencies of neoplasms in Great Lakes fish populations may have implications for the entire Great Lakes ecosystem. Not surprisingly, the health, diversity, and abundance of fish are used as indicators of ecosystem health. The presence of dead and diseased animals is quickly interpreted as a symptom of an unhealthy ecosystem. Concern focuses on public safety. Recreational fishers, consumers, and residents are uncertain of the risks associated with handling or eating tumored fish, or even non-tumored fish taken from the same population. In a sense, this concern is misdirected (Black and Baumann 1991). After all, it is possible to induce a liver tumor in a rainbow trout through embryonic exposure to nanogram quantities of aflatoxin B1, yet in the adult tumor bearing trout, the presence of the chemical that provoked the tumor development is no longer detectable (Black and Baumann 1991). In this situation even though a large liver tumor might be evident in the adult fish, we would expect that consumption of this fish would pose no significant risk, though not a very attractive idea to most of us. Furthermore, since most freshwater fish is cooked before consumption, it is expected that the risks of direct transmission of an oncogenic virus would be unlikely (even assuming cross infectivity between fish and man). The concern should really be focused on the chemical contaminants rather than the presence of the tumors per se, particularly in those cases where cancerous fish are taken from heavily polluted waters, where the fish are chronically exposed to a wide range of chemical contaminants (Black and Baumann 1991).
In addition to human health issues, fisheries agencies are concerned with the health of the fish and the fishery. Few monitoring programs have described the relationship between fish tumors and fish mortality, physiology, and behaviour. We know for example, that lymphosarcoma in muskellunge kills almost 100 percent of the affected fish (Sonstegard 1976a). However, there are no data describing the adverse effects of epidermal papillomas, squamous cell carcinomas, dermal fibrosarcomas, or liver neoplasia on longevity or reproductive success. There are some data suggesting that tumored fish often die and that survivors frequently show symptoms of other diseases (Sonstegard 1976).
1.2 Purpose of this manual
The purpose of this manual is to help fisheries agencies develop a tumor database that will enable fisheries managers to address relevant issues and focus resources into areas of concern. To develop such a database, field biologists need to know how to examine fish for tumors, be able to recognize gross signs of tumors, and know how to prepare samples for histopathology. Thus, this manual explains necropsy and tissue preparation procedures and describes and illustrates a variety of fish tumors. With this manual in hand, the field biologist should develop skill in recognizing the presence of commonly occurring tumors in fish and in preparing samples for further diagnosis. Because tumor diagnosis is a specialized science requiring pathological training, this manual will not enable positive identification of tumors. However, through the use of selected photographs and text, this manual should enable the prepared biologist to recognize the gross signs of neoplasia.
As more data regarding fish tumors becomes available, regulatory agencies will then be able to determine the magnitude of a particular epizootic. To deal with these issues it is important to know not only which species exhibit neoplasms but whether the prevalence is increasing or decreasing. The most obvious question to answer is whether there is a relationship between tumor formation and chemical exposure. Some field studies have already demonstrated a relationship between certain types of pollutants and the presence of certain kinds of neoplasia in fish, especially liver neoplasia, but in other types of neoplasia, very little information is currently available. Other questions, not without significance, deal with the effects of the tumor on the health of the fish, whether seasonal patterns of tumor occurrence exist, and the relationship between tumor frequency and age of the fish.
1.3 Fish as sentinel animals
Chemical induction of cancer was first suspected in 1775 by Dr. Percival Pott, an English physician, who documented the occurrence of scrotal cancers in chimney sweeps exposed to soot (Pott 1963). One-hundred and forty years later, Yamagiwa and Ichikawa (Black 1984) demonstrated the first chemically induced tumors in rodents. Although neoplasms in fish were reported in the scientific literature before 1915, the concept of chemical induction of cancer in wild fish populations was slow to develop. It is interesting that the high prevalence of liver neoplasia recently found in brown bullheads and white suckers from polluted sites was not reported in the literature before 1960.
In 1962, Drs. Clyde Dawe and Merle Stanton of the National Cancer Institute, together with Frank Schwartz of the Chesapeake Biological Laboratory, examined fish from Deep Creek Lake, Maryland, when this lake was treated with a fish toxicant (rotenone) by the Maryland Fisheries Department (Dawe et al. 1964). Liver neoplasia was discovered in three white suckers (Catostomus commersoni) and in one brown bullhead (Ictalurus nebulosus) collected from this lake. This study by Dawe and his associates is, to the best of our knowledge, the first report in which chemicals were suggested as the cause of cancer in a wild fish population. Dawe further recommended the use of such bottom-feeding species as "indicators of environmental carcinogens;" i.e., sentinel animals.
Fish have already served mankind as indicators of environmental carcinogens. The discovery of the extremely potent carcinogenic action of aflatoxin B1, a common mould metabolite, resulted when a nearly world-wide epizootic (epidemic) of liver cancer developed in hatchery-raised rainbow trout (Oncorhynchus mykiss) (Sinnhuber 1967). The liver cancers were ultimately traced to trout food pellets containing mould-contaminated cottonseed meal. Due to this largely accidental discovery, peanuts and other foods which the mould can easily infect are now stored under conditions which inhibit mould growth. In addition, the U.S. Food and Drug Administration routinely monitors some foods such as peanut butter for the presence of the mould metabolites.
1.4 Cancer in fish
Fish share considerable evolutionary commonality with mammals, including man. Most of the basic pathological conditions that are recognized in mammals, e.g. inflammation, wound repair, hemorrhage, necrosis, septicemia, atrophy, metaplasia, hyperplasia, and neoplasia, are also observed in fish.
Specific DNA sequences (oncogenes) that have been conserved in evolution, can lead to formation of cancer (Mix 1986). These genes, which are similar to cancer causing genes in mammals, have been identified in some fish tumors (Mix 1986). While the precise details of the process by which normal cells become cancerous are not yet fully resolved, the broad outlines are fairly well understood. The mechanisms whereby some chemicals interact with the cell to cause cancer is a high research priority for the medical community.
Many chemicals require metabolic activation before they can induce neoplasia. As part of the detoxification and excretion process, some chemicals, such as benzo[a]pyrene, are converted to polar metabolites by liver enzymes (Fabacher and Baumann 1985). These metabolites are highly reactive and interact with critical components of the cell to initiate the cancer process. The liver is the primary source of the detoxification enzymes, referred to collectively as the Mixed Function Oxidase system (Fabacher and Baumann 1985). Thus it is not surprising that the liver is a frequent site of cancer in fish that have been exposed to chemical carcinogens in the laboratory. Again, it appears that fish metabolism of carcinogens is strikingly similar to that of mammals, including man.
In current medical terminology, the terms tumor, neoplasm and cancer are often used interchangeably (Black 1984). Generally, the term cancer denotes a malignant disease in which the cancer, unless cured by medical treatment, has the capacity to invade and disseminate throughout the body (metastasize), ultimately killing the host (Black 1984). The terms neoplasia (the disease process) and neoplasm (a tumor), include both the malignant and non-malignant (benign) forms of the disease (Black 1984). Although many definitions of neoplasia exist, probably none are completely satisfactory. The complexes recognized as neoplasia are characterized by their "relatively autonomous growth of abnormal cells, which by proliferation, press upon, infiltrate, or invade healthy tissues, thereby causing destruction of cells and organs, interference with physiological function, or eventual death of the animal" (Black 1984). In fish, as in mammals, neoplasms display a range of growth potentials from those that appear to be very-slow growing and localized, to those that are obviously invading the host's tissues and are therefore considered cancerous (Black 1984).
There is almost overwhelming epidemiological evidence, supported by laboratory induction studies, that some of the neoplasms reported in Great Lakes fishes are caused by carcinogens in the environment acting alone or in the presence of tumor promoters (Hayes et al. 1990; Metcalfe et al. 1988; Baumann et al. 1990). However, not all fish cancers are caused by chemicals. Viruses are known to cause a number of benign and malignant neoplasms found on Great Lakes fish (Smith et al. 1989). Transmission studies in laboratory experiments on lymphosarcoma in northern pike and muskellunge, combined with evidence of the seasonal nature of the disease (suggestive of a temperature response) (Sonstegard 1976), the detection of a viral enzyme (Papas et al. 1976), the occurrence of the disease in apparently uncontaminated environments (Papas et al. 1976; Sonstegard 1976), and the disease patterns in the environment suggestive of an infectious process (Sonstegard 1976), indicate that the lymphosarcoma is a virus-induced cancer. Other neoplasms believed to be associated with viruses include lymphocystis and dermal sarcomas in walleye, papillomas in carp and possibly lip and body papillomas in white suckers, although a virus etiology for white sucker papillomas has still to be confirmed (Smith et al. 1989).
2. PROCEDURES FOR NECROPSY AND TISSUE PREPARATION
2.1 Sampling
2.1.1 Live and dead fish
Fish tissues deteriorate rapidly and every effort should be made to examine fresh specimens. This requirement will have implications on the number of fish collected at each site and the methods of collection and transportation. Examination of freshly killed fish is preferred. If possible, fish should be captured alive and kept alive until the time of sampling. Tissue deterioration is substantially reduced at low temperatures and dead fish may still be useful if they are immediately chilled by packing them in ice. Placing live fish in ice just prior to sacrificing them lowers their body temperature to near freezing. This has an anaesthetic effect and helps retard tissue changes that occur after death. Only humane methods of sacrificing fish are acceptable. The preferred method is an overdose of anaesthetic (MS 222).
2.1.2 Numbers of fish and tissues
The prevalence of several reported types of neoplasms in Great Lakes fish increases with age. For example, skin and lip papillomas on brown bullheads and white suckers are rarely found on immature fish but the frequency of occurrence increases rapidly with age in mature fish (Smith et al. 1989). As a result, comparisons of tumor frequency for spatial and temporal trends must be standardized by age, and sex if necessary. Selection of a sampling strategy should be based on known prevalence data and should target older fish to optimize the possibility of finding neoplasms.
All grossly visible tumors should be sampled. However, small lesions and discolorations may escape observation and it will be necessary to sample the appropriate tissues from all of the normal and abnormal fish in the sample. For example, in a survey to detect liver tumors in white suckers, all obvious liver lesions and suspected anomalies will be sampled. In addition, several slices of liver tissue will be collected from all of the apparently normal fish. The same number of tissue slices from the same locations on the liver will also be collected from all of the abnormal fish from the same location.
2.2 Necropsy equipment
A small tool box or fishing tackle box is handy for keeping dissecting equipment. The following items are recommended for conducting fish necropsies.
1. Knife, a thin blade 9-inch fillet knife.
2. Dissecting scissors, 2-inch and 4-inch blades with sharp and blunt points (several spare pairs).
3. Bone shears (or side cutters).
4. Forceps, several pairs of different sizes.
5. Scalpels, medium and large disposable blades.
6. Sharpening stone or steel.
7. Dissecting platform and measuring board.
8. Small ruler and dividers.
9. Scales for weighing fish (to 0.1 grams for organ weights and 1 gram for fish weights).
10. Buffered formalin (10 percent), chilled.
11. Specimen bags and tissue cassettes.
12. Soft pencils for labeling the tissue cassettes.
13. Adjustable fluorescent desk lights for indoor work.
14. Magnifying glass for close inspection of small lesions.
15. Ice.
16. Absorbent tissues or cloths.
17. Scale envelopes.
18. Anesthetic (MS 222) and exposure tank.
19. Data sheets.
20. Tape recorder (foot controlled). This is optional, but verbal data are more detailed and much easier to record during the autopsy.
21. Information describing the type of tumors associated with different species and factors such as age or sex that are known to affect tumor prevalence.
22. Camera with macro lens and/or a video camera.
2.3 Necropsy procedures
2.3.1 External examination
After sacrificing the fish, make a careful examination of the external surface including the oral and gill cavities. If necessary, remove the operculum. All abnormalities such as growths, ulcers, parasites, fin erosion, color changes, and malformations including abnormal scale patterns, spinal curvature, and missing or damaged fins, barbels, or eyes, should be recorded and described. Visual inspection may not be sufficient to detect small skin lesions but these can frequently be found by touch. In general, external neoplasms are usually detectable by gross examination, providing the observer has knowledge of the normal anatomy and morphology of the fish. It should be born in mind, that most neoplasms appear grossly as nodules or masses of tissue that have no counterpart in healthy animals. However, many of the lesions, particularly in younger animals, are small and may appear insignificant. There is a tendency for manuals and publications to select large, easily observable neoplasms for their illustrations. As a result, biologists frequently overlook the small (1-3 mm) flat lesions that are noticeable only as a focal color change. We have had good success recognizing tumors from gross observation once the biologists were aware of subtle changes in the shape and color of affected tissues.
2.3.2 Internal examination
To conduct the internal examination, it is important to open the body cavity in such a way as to expose all the internal organs (Figure 1) and yet not damage any of them (by cutting them with the knife or tearing them by excessive torsion, pressure, etc.). Although different fish species exhibit variation in their anatomy and morphology, most fish can be dissected using a ventral incision which runs from just anterior to the anus, forward to the area between the pelvic fins. A V-shaped dissecting board, equipped with several barbless hooks on elastic bands is a useful tool for holding open the body cavity allowing examination of the organs as they appear intact. An alternative is to carefully remove the entire left flank of the fish by cutting from the anus, dorsally and anteriorly slightly below the lateral line to the posterior margin of the operculum and through the bony arch of the pectoral girdle. This incision permits the entire flank to be folded back, exposing the viscera. Following careful examination of the organs in situ, the entire viscera can be removed from the body cavity by cutting the esophagus at a point just anterior to the liver followed by severing the intestine just anterior to the anus. In those fish which have a discrete liver such as bullhead, perch, trout, etc., the liver can now be removed and examined carefully on all surfaces for abnormalities. The gall bladder, containing the bile (normally a dark green liquid), should be removed by careful dissection. Slicing the liver at 3-5 mm intervals and examining the cut surfaces is the only method for detecting neoplasms which may not be grossly visible on the surface of the liver.
In some fishes such as members of the Catostomidae (sucker family) and the Cyprinidae (minnow family), the liver is not a discrete organ and is present as hepatopancreatic tissue enveloping the digestive tract, spleen, and gall bladder. In these types of fishes it is helpful to remove and examine the entire viscera as one unit. Without prior fixation, it is impractical to examine the liver by the cut surface technique, because this would also sever the intestine, releasing feces and digestive juices, and in the process, making a thorough examination of the digestive tract more difficult. If suspected liver cancers are present, it is fairly easy to excise samples of the lesion(s) using a scalpel or scissors to remove plaque-like 10 mm diameter by 3 mm thick pieces for histology. Take care to not cut into the digestive tract. In some cases, because of the size or nature of the lesion (eg. a suspected tumor of the digestive tract), it is advantageous to fix the entire viscera, remembering to cut into the visceral tissues at 3 mm intervals to accomplish good fixation.
Systematically examine all other major organs including the gas bladder (including the inside surfaces), kidney, spleen, gonads, and digestive tract. Whenever possible, the entire digestive tract should be opened and the lining mucosal surface examined for elevated, round, or plaque-like growths. The presence of an enlarged, sometimes fluid-filled, distended intestine may be indicative of intestinal lesions. If suspected tumors are present in any of the fish, corresponding tissues must also be sampled from apparently normal fish from the same site. The occurrence of gross lesions in some fish may indicate a high prevalence of pre-neoplastic lesions in the population. The absence of "normal" tissue severely limits data interpretation.
2.4 Tissue diagnosis
Presently, the only technique for positively identifying cancer in fish is through the use of histopathology. In this technique, samples of the suspect tissue are first preserved or "fixed" by placing them in a fixative solution such as 10% neutral buffered formalin. After fixation has been completed, the tissue is dehydrated through a graded series of alcohol solutions. Following dehydration, the alcohol is replaced with an organic solvent and the tissue is infiltrated with melted paraffin. After the paraffin has solidified, thin sections of the paraffin-tissue block (5-7 microns) are cut using a microtome.
These tissue sections are further processed and stained prior to microscopic examination. In making a diagnosis, the pathologist looks for a variety of subtle alterations in the cells comprising the lesion to decide whether or not a lesion is neoplastic.
Because all diagnoses must be made by a qualified pathologist, it should be emphasized that this manual will not enable positive identification of a neoplasm. Based solely upon their gross appearance, there are a number of non-neoplastic conditions which can easily be mistaken for tumors. However, occasionally a particular type of neoplasm, such as the lip papilloma on white suckers, may have a characteristic gross morphology and occur with sufficient frequency in a given species, such that only a representative sample of the diseased animals need be submitted for diagnosis. The gross signs of a neoplasm are often readily detectable, providing the observer has a thorough knowledge of the normal fish anatomy and morphology. However, some neoplasms show no gross abnormality, and it is usually necessary to sample all tissues for histological diagnosis.
2.5 Tissue sampling
Ideally, fish tissue samples should be not more than 3 mm thick (to permit rapid penetration of fixative and prevent tissue compression in the cassette) and should contain the suspicious lesion or growth, plus the surrounding normal tissue. Some tumors are more than several centimetres in diameter and it is not practical or necessary to save the entire lesion. Histological diagnosis is determined by the type and shape of cells and the cell patterns, staining properties, mitotic figures, and invasion or expansion into surrounding tissue. In almost all cases, pathologists prefer normal and abnormal tissue on the same slide to facilitate comparisons. It is particularly important to include the dermal layers in epidermal lesions. Inclusion of adjoining normal tissue beside and below the lesion will aid the pathologist in determining the degree of local invasiveness and the cell of origin. Tissue sections should be cut with sharp instruments to avoid pulling, tearing, and distorting the tissue.
2.6 Fixation
Phosphate buffered formalin is an excellent general fixative for fish tissue. Buffered formalin is prepared as a 10 percent solution and buffered with phosphate salts (Table 1). Formaldehyde is acidic (pH 3-5) and the pH of the final solution should be between 6.9 and 7.4. Other fixatives are available and effective. The histopathologist who will be examining the preserved tissues may have a preferred fixative and should be consulted before fish collections are made.
Two rather handy techniques can be employed to fix tissues collected in the field. One simple method is to use commercial tissue cassettes which are made with snap closures and ventilated sides to allow fixative penetration. They are available from most scientific supply companies and have a label area that can be easily marked with a soft pencil. After closure, the cassette containing the tissue sample is placed in a large non-breakable container of fixative. Another procedure that works well, and has the added advantage of being compatible with fixation of large tissue specimens, is the use of small cloth bags. These can be pre-numbered with a waterproof marker, and closed with a single overhand knot prior to being placed in a large container of fixative. Cloth bags used for tissue fixation and storage should be prewashed to remove water repellents and ironed to facilitate labeling.
Several cautions apply to either method. If large tissues must be preserved, it is necessary to cut nearly through the specimen at approximately 3 mm intervals to ensure that the cut surfaces are fully exposed to the fixative. Ideally, the volume of fixative used should be 10 times the volume of tissue. In practice, smaller volumes can be used if the original solution is replaced with fresh fixative within 24 hours. If several pieces of tissue are sampled, care must also be taken to ensure that the specimens remain separated until surface fixation is complete, otherwise, the pieces will fuse together, preventing adequate fixation. Good fixation requires rapid tissue penetration to prevent post-mortem deterioration. Tissue autolysis occurs rapidly in fish and can be substantially slowed by chilling the fixative.
Table 1. Fixative solution - neutral buffered formalin
Formaldehyde (37 - 40%) 100 ml
Distilled water 900 ml
Sodium phosphate (Na H2PO4 H2O) 4 gm
Sodium phosphate (NA2 HPO4) 6.5 gm
Although the buffered formalin solution is the preferred fixative, the formaldehyde can be added directly to tap water and buffered with marble chips in an emergency. The pH of the 10 percent formalin solution should be between 6.9 and 7.4 and can be checked with a litmus. Tissues deteriorate rapidly in warm weather and the following steps are useful for minimizing autolysis.
2.7 Recording data
The following information is required to ensure that adequate and appropriate data are collected and to ensure that sufficient data are included on the tissue cassette.
2.7.1 Written records
2.7.2 Tissue cassettes
1. Adopt a routine for placing tissues in the cassette so that suspected lesions are always oriented in the same direction. Tissues change color after fixation and it is sometimes difficult to find smaller lesions in preserved material. Routinely placing the lesion facing the bottom of the cassette will help minimize this problem.
2. Each tissue cassette can usually hold several pieces of tissue. Try to space them so they are not touching each other.
3. Keep the sections thin (3 mm) to avoid compressing the tissue in the cassette.
4. Using a soft pencil, record the fish number, the site number, and the date on the tissue cassette. You may also wish to assign numbers for each organ (skin, heart, liver, etc.). Mark an * on cassettes containing tissues that have suspected neoplasms. This information allows the pathologist to quickly select fish and tissues for priority analysis.
2.7.3 Gross description of neoplasms
All abnormalities, especially tissue masses which appear grossly to be neoplastic, should be clearly described as follows.
3. NEOPLASMS OF BROWN BULLHEADS (Amerius nebulosus)
3.1 Neoplasms of the skin and mouth
Brown bullheads are commonly affected with epidermal neoplasms of the mouth and skin (Figure 2). The majority of neoplasms that occur on the lips are diagnosable as benign papillomas, but some invade the dermis and are diagnosed as carcinomas. They can be found in the corners of the mouth or on the upper or lower lips or combinations of these locations in the case of multiple lesions (Baumann et al. 1987). They may occur singly or in multiples and can vary in size from several millimetres to several centimetres, sometimes surrounding the entire mouth. The frequency of occurrence increases with age, and prevalence may exceed 40 percent in older fish (MacCubbin and Ersing 1991). When taking a tissue specimen of a skin tumor remove the entire lesion along with a portion of the underlying tissue. In some cases it is convenient to remove a section of the jaw which encompasses the neoplasm.
Neoplasms of the skin may occur anywhere on the body, but in some environments, they are found more frequently on or near the head (Poulet et al. 1994). Strictly speaking, papillomas of the oral epithelium can also be considered skin neoplasms, but it is convenient to separate lesions of the mouth from those which occur elsewhere on the body (Figure 3). Early stage growths may occur as solitary papules or small nodules of less than 2 mm in diameter, slightly elevated above the surrounding normal tissue (Poulet et al. 1994). More advanced growths can exceed 30 mm in diameter and can occur as single nodules or as multiple groups of coalescing nodules, raised well above the surrounding normal tissue. Growths can vary in color from more or less normal skin tones to dark brown or black. In brown bullheads, tumors of the skin usually originate from one of two cell types (Smith et al. 1989). Lightly pigmented neoplasms are usually composed of neoplastic epithelial (epidermal) cells. Black to dark brown growths, on the other hand, are usually composed of neoplastic pigment cells (melanocytes). These are diagnosed as melanomas (Figure 4). The skin neoplasms exhibit a range of growth potentials from totally non-invasive to highly invasive malignancy (Smith et al. 1989). In general, larger growths, especially those exhibiting multinodular formations tend to be more invasive. Depending upon the growth pattern revealed by microscopic examination, the growths composed of neoplastic epithelium can be diagnosed as either epidermal hyperplasia, papilloma, or carcinoma (Baumann et al. 1987). In humans, pigment cell neoplasms composed of neoplastic melanocytes are among the most malignant of all neoplasias but are diagnosed as melanomas. In fish, these pigment cell neoplasms do not appear to be as malignant, however many exhibit local invasiveness and occasional specimens have exhibited distant metastases. In addition to the pigment cell neoplasms described above, some bullheads also have exhibited variably sized irregular areas of superficial, dark brown to black, pigmentation. We have applied the term melanosis to this condition (Figure 5) since it involves an increase in superficial melanocytes that does not appear to involve direct development of melanoma. Although there is suspicion that epidermal neoplasia in bullheads may be related to pollutant exposure (Black 1983), the possible relations of the pigment cell neoplasms or melanosis to chemical exposure has not been well studied. A genetic mechanism of melanoma induction is observed in hybridization experiments using sword-tail X platyfish (small aquarium species) (Setlow et al. 1989). Since brown bullheads are known to hybridize with black bullheads, one wonders whether or not a similar mechanism might be involved in melanoma induction in the affected bullheads.
Confounding lesions
These include healing wounds and ulcers; excessive scar tissue accumulations around old injuries, particularly in the mouth; abrasion from sampling gear; bacterial infection (Aeromonas salmonicida); epidermal hyperplasia; and injuries from pectoral and dorsal spines.
Etiology
The occurrence of lip papillomas in bullheads from relatively uncontaminated sites suggests a possible viral etiology, but the presence of a virus has not been confirmed (Smith et al. 1989). Laboratory studies have successfully induced skin papillomas and carcinomas by exposing the fish to chemical carcinogens or extracts of sediments from the Great Lakes, suggesting a possible chemical etiology (Black 1983).
3.2 Neoplasms of the liver
Liver neoplasms of bile duct origin are diagnosed as either cholangioma or cholangiocarcinoma. When they originate from hepatocytes, depending upon factors such as the degree of morphological alteration, cytologic and histologic atypia, staining properties, etc. a variety of diagnoses may be appropriate.
Grossly, cholangiomas may appear as white or cream-colored foci to larger white or cream-colored nodules that may exceed several centimetres in diameter (Figure 5). They may or may not bulge slightly above the liver capsule and are difficult to detect from external examination when they are embedded within the liver. Early stage neoplasms of hepatocellular origin may be similar to the bile duct tumors in gross appearance, i.e., as white or cream-colored foci, or they may appear as pale foci just beneath the liver capsule. More advanced tumors may appear as white, gray, cream-colored, or reddish-tan colored masses bulging from, or as nodules within, the liver tissue. On occasion they may be visible only by examination of the cut surface. Any liver that is enlarged, hemorrhagic, bile-stained, or in which a smooth capsular surface is absent, is suspicious.
Confounding lesions
Encysted parasites (1-20 mm) appear as "pearly" white cysts that can be "shelled out" from the normal tissue. Often the presence of parasites can be confirmed when the mass is cut open. Granulomas may also be confused with cholangiomas during gross inspection. However, they are usually present as numerous white to creamy-white nodules that may be solid or caseous (cheesy) on the cut surface.
Etiology
Chemical carcinogens are suspected in the etiology of brown bullhead liver neoplasms (Black 1983). Tumor prevalence may exceed 25 percent in older fish living in chemically contaminated environments (Baumann et al. 1987). There is strong circumstantial evidence linking the presence of liver neoplasms with exposure to polynuclear aromatic hydrocarbons (Black 1983; Baumann et al. 1983).
Return to the Table
of Contents
Next Section of
the Tumor Manual
Previous Section
of the Tumor Manual