Deficiency diseases are of two types, deficiency or imbalance of the macro- nutrients in the diet—the protein, carbohydrate, lipid, and fiber—and deficiency of the micronutrients—the vitamins and minerals. In terms of macronutrients it is usually in the lipid component of the macronutrients that the most serious problems arise, in terms of practical diets, whether in relation to deficiency, oxidation, or imbalance. Among the micronutri- ents, any of a wide range of components can exert an effect, especially in fast-growing, younger fish.
8.3.1. Protein
All fish require relatively high levels of protein as a source of amino acids for protein synthesis and, generally, for gluconeogenesis as well. Thus fish diets must contain high levels of high-quality protein. Since protein is one of the most expensive components of the diet, feed manufacturers have to optimize diet formulae to allow economies while still maintaining an ade- quate complement of protein. The principal feature determining protein quality for fish nutrition is the level and availability of the essential amino acids (EAA). Deficiency of one or more of these leads to deficiency disease.
Normally essential amino acids within a formulated diet will be conserved, but if amino acid intake is restricted, then they may themselves be metabo- lized to form nonessentials. Thus to avoid deficiency it is necessary for diets to contain a sufficiency of both essential and nonessential amino acids.
Deficiency diseases can still arise even in the presence of apparent luxus.
This is because certain amino acids may be rendered biologically unavail- able or inactive, even when still chemically measurable, during the course of processing of the diet. Lysine, for example, may form an addition com- pound with carbohydrate in the feed, rendering it unavailable to the fish (Hardy 2001).
Several specific amino acid deficiency conditions have been described for fish under experimental conditions. Lysine deficiency has been specif- ically related to dorsal fin erosion, often with secondary flavobacterial in- fection (Waltonet al.1984). Spinal deformities have been associated with a variety of amino acid deficiencies including tryptophan, leucine, lysine, arginine, and histidine (Ketola 1983; Mazidet al.1978; Waltonet al.1982;
Halver and Shanks 1980). Lenticular cataract, commonly an early indica- tor of marginal deficiencies, has been associated with both methionine and tryptophan deficiency.
Under normal farming conditions, however, it is unusual to encounter acute single-amino acid deficiencies and affected fish will usually show a range of clinical features, all associated with poor growth and darkening of the skin. Such deficiency conditions can arise as a result of improper for- mulation or from using ingredients with intrinsic specific amino acid defi- ciencies or in imbalanced proportions. They may also result from improper processing of the diet, generally as a result of excessive heat or chemical treatment during preparation.
8.3.2. Carbohydrate
Since fish have a much more limited capacity for carbohydrate metabo- lism than higher vertebrates, there is little point in increasing carbohydrate levels, and only limited information is available on the effects of high- carbohydrate levels ( Jauncey 1982). Many reports have indicated that ex- cessive carbohydrate intake will result in excessive glycogen deposits in the liver, and continued intake results in extensive lipid deposited in the viscera (Halver 1972). Hess (1935) reports that excessive dietary carbohydrate in ornamental cyprinids results in hepatic degenerative changes.
8.3.3. Fats
Dietary disease problems associated with the lipid component of the diets appear to be among the most serious and prevalent of all nutritional problems in fish. Fish tissues contain predominantly fatty acids of theω-3 series and so fish diets must provide sufficient essential fatty acids of theω-3 (linolenic) andω-6 (linolenic) series as well as a general lipid contribution to calorie requirements. Deficiency syndromes result when sufficient levels of ω-3 orω-6 or longer-chain members of the series are unavailable. Linolenic acid is particularly important for normal growth. The ability to elongate and desaturate 18 : 3ω-3 (linolenic) seems to vary between species. In freshwater species such as rainbow trout, for example, oleate or linoleate incorporated into the diet can be converted, at least to some degree, to arachidonate (Castellet al.1972). The findings of Coweyet al.(1976), however, show that, in turbot at least, little conversion of linoleate to arachidonate is possible when they are fed diets high in such lipid, e.g., corn oil. In such circum- stances, major changes also take place in the structure of the fat storage cells of the animal. The fat cells surrounding the lateral lymphatic sinus are particularly affected, with thickening and deformity of the lipid cell walls, increased vascularity, and deposition of a hyaline material between the cells (Fig. 8.3).
FIG. 8.3
(A) Normal perilymphatic fat cells from a turbot given a diet containing cod liver oil.
(B) Perilymphatic fat cells from a fish a given diet containing hydrogenated coconut oil. Extensive thickening of the cell membrane, deposition of hyaline material between cells, and increased vascularity are apparent. H&E,×500.
The deficiency signs in essential fatty acid deficiency almost always relate to a swollen, pale liver with fatty infiltration, and there is a consistent anemia, putatively associated with the lack of secretion of hemopoietin by the com- promised liver. Mortality is also high, particularly in young, fast-growing fish (Castellet al.1972; Takeuchi and Watanabe 1977, 1982; Farkaset al.1977;
Takeuchiet al.1983; Bellet al.1985).
The most important problem with the lipid component of fish diets is the propensity which the high concentration of polyunsaturated fatty acids (PUFA), which includes the essential ω-3 andω-6 fatty acids, has for be- coming autoxidized by atmospheric oxygen unless antioxidant protection
is incorporated in the diet. Autoxidation not only reduces the availability of fatty acids to the host, but also is deleterious to the fish in its own right, because rancidin, a product of the oxidation process, induces high levels of free radicals, peroxides, aldehydes, and ketones which are not only toxic to the fish but also react with other dietary components. Control of oxidative rancidity is one of the most serious of all of the problems facing the feed compounder and the pathological effects which develop in fish fed ran- cid diets can be extremely serious. For this reason, farmed fish diets often include a significant luxus ofα-tocopherol as well as other antioxidants.
The most serious and frequently reported clinical sign associated with rancid fat within feeds is the syndrome in farmed rainbow trout known as fatty liver disease, or lipoid liver degeneration. Many species of fish use the liver as a major lipid-storage organ during the periods of heavy feeding, and at such times, extensive lipid infiltration of the hepatic parenchyma is seen. However, in species such as salmonids and mullets, the liver does not play this role to any significant degree, and when excessive unsuitable lipid is incorporated in the diet, the pathological syndrome of lipoid liver degeneration occurs.
Lipoid liver degeneration often occurs in fish fed trash fish or pelleted diets, in which the lipid component is excessive or has been partially oxi- dized. The rancid lipids exert their effect by their inherent toxicity and also by reacting with the dietary proteins to lower their biological value. They also reduce the activity of vitamins which are not themselves antioxidants.
The particular features of each outbreak of lipoid liver disease will vary de- pending on the contribution of the various products of the oxidation to the pathological effects.
The clinical features of lipoid liver disease include extreme anemia, with a concomitant pallor of the gills of affected fish. The liver appears swollen and bronzed and has rounded edges (Fig. 8.4). Histologically there is extreme lipid infiltration of the hepatocytes. This may, in the later stages, appear as ceroid or lipofuscin depositions, resulting in loss of cytoplasmic staining and distortion of hepatic muralia. In addition, there is inactivity of renal and splenic hemopoietic tissue, and the melanomacrophages are replete with high levels of pale-staining pigment (Fig. 8.5). Depending on the extent to and length of time for which the condition has been extant, the degree of oxidation, and the type of fat in the diet, there may also be varying degrees of infiltration of the liver by macrophages, which also contain ceroid. All salmonids, and possibly other top predators, are susceptible to the condi- tion, although it has been described principally in rainbow trout fed on trash fish. When fish are lightly affected, starvation can lead to complete re- covery, but when severe liver or hemopoietic damage is present, fish rarely recover to the level of satisfactory feed conversion. Jacksonet al.(1984) have
FIG. 8.4
Rainbow trout affected by lipoid liver disease. The liver is swollen, with rounded edges, and there is considerable deposition of abdominal fat.
FIG. 8.5
Ceroid infiltration of hepatic cells in lipoid liver degeneration. The hepatic structure is distorted and hepatocytes are loaded with yellow pigment. H&E,×250.
reported the effect of a range of diets prepared from clupeid silage with and without the antioxidant ethoxyquin. Their findings showed that, at a sub- optimal level, those diets with high levels of hydroperoxides and secondary breakdown products (namely, unprotected) consistently induced patholog- ical changes, although these were limited to the morphological appearance and distribution of the eosinophilic granule cells of the intestine.
Other signs associated with the feeding of oxidized lipids include con- gestion, hemorrhage, splenic hemosiderosis, exophthalmia, steatitis, dark- ening of the skin, focal hepatic necrosis, and skeletal myopathy (Soliman et al.1983; Murai and Andrews 1974; Park 1978; Mocciaet al.1984).
The condition of Atlantic salmon in Norway, known as Hitra disease, is often considered to be associated with oxidized lipid or vitamin E deficiency (Fjolstad and Heyeraas 1985; Poppeet al.1985), although there is now strong evidence that it is associated primarily with pathogenic vibrios (Egidiuset al.
1981; Hjeltnes and Roberts 1993).
8.3.4. Fiber
The role of dietary fiber in animal nutrition has become a major area of investigation, and Davies (1985) has reviewed the evidence for its sig- nificance in fish nutrition. There does not appear to be any pathological effect associated with excessive or low levels of fiber, although it may af- fect the growth rate. There is, however, concern that the removal of fiber from higher-protein diets, or the semipurified diets used in research, may affect the value of such diets. Some species, namely, catfish, need substantial amounts of fiber in the diet to move nutrients along the absorptive pathways slowly.
8.4