HYGIENE AND DISEASE IN SALMONID CULTURE

by

Tibor Takács

The anatomy and physiology of salmon and trout from a
pathological point of view

The fish's body is made up of cells. Even in multicellular organisms, cells are quite independent, small self-contained factories, in which the metabolic processes take place. The protoplasm containing the organelles of the cell (nucleus, Golgi-apparatus, ribosomes, etc.) is encapsulated by the cell membrane. Through the membrane a continuous exchange of materials takes place, supplying the cell with raw materials on the one hand and excreting metabolic wastes on the other. The cell membrane is a lipoprotein bilayer of porous structure, and is very vulnerable to abnormal changes in the environment (pH, ion concentration, ion distribution, etc.).

The pores of the cell membrane are electrically charged, and therefore the transport of materials into and out of the cell normally needs energy. Transport of these materials also depends on their solubility.

Pathogens (living agents or other factors) cause disease by disturbing the cell function (or the function of certain groups of cells) and the transport processes. They can facilitate or inhibit cell activity, or cause it to function in an abnormal way.

Since the cells in a multicellular organism are integrated into the various tissues, injury to the cells manifests itself in the malfunction of the affected tissue.

The vulnerability of the tissues depends on many factors (internal: age, body condition, protective capacity, immunological responsiveness, etc.; and external: hygienic conditions, nourishment, water quality and farming environment, etc.).

The disturbances of tissue function and injury to its cells can cause clinical symptoms of a certain disease, which can be more or less characteristic of the agent, and can even lead to pathomorphological changes in the affected organs.

Organs are built up from various tissues integrated to perform a certain function. The organs together make up the organ systems, which carry out a special series of functions, e.g., the digestive system, which comprises the mouth, gullet, stomach, pyloric caeca, intestine and accessory glands.

The organ systems and their functions in salmonid fish are as follows:

Integumentary system

This means the skin and its appendages, the fins and scales. The skin is covered by the epidermis, which has the function of keeping water out and tissue fluid in. This function is essential for permitting homeostasis of the body fluids to be maintained, i.e., a constant state or at least relative stability of the internal environment (pH, ion concentration, electric potential, etc.).

The epidermis possesses the so-called goblet cells which secrete the protective mucous coating of the skin. The mucus serves as an infection-resistant slippery fluid. Any injury to the epidermis (mechanical, parasitic, or in some cases chemical) or a lack of mucus for any other reason or deterioration in the mucus quality (e.g., at the time of sexual maturation), reduces the resistance of the skin and can open up a pathway for bacterial and fungal infection of the organism. Therefore it is very important to keep the skin surface intact. In fish farms this means careful handling of stock, avoiding sharp surfaces in construction, correct vitamin and trace element supplementation in the diet, etc.

The skin also plays a role in oxygen uptake (in salmonids 3–9% of the oxygen requirement enters through the skin), as well as in the excretion of inorganic ions.

A careful appraisal of the condition of the body surface, especially the intactness of the skin, is very important from a clinical point of view and the following points must be checked: Are there any lesions, ulcers, furuncles, mechanical injuries, parasites, haemorrhages, rashes, or spots on the surface? Are the amount and the colour of the mucus normal, or is there a discoloration of the epidermis? Are the fins and the tail intact? A clinical examination of the body surface also covers the condition of the scales (their position, are there any missing? etc.).

It is important to understand that the skin, like a mirror, can reflect the general clinical state of the organism. Discoloration is often an indication of disease.

Circulatory system

Central to the circulatory system of salmonid is the two-chambered heart. The heart pumps the blood through the gill lamellae, from where it passes to the rest of the body via the blood vessels.

Fig. 1 Schematic illustration of the circulatory system
1. ventricle, 2. atrium, 3. bulbus arteriosus, 4. gill, 5. head, 6. body, arteries and veins

One of the functions of the blood circulatory system is the transportation of materials. Dissolved materials leave the blood flow (or intravasal space) in the blood capillaries and enter the intercellular space through pores.

They are then taken up by the cells according to their requirements into the intracellular space. The balance between the three fluid spaces (intravasal, intercellular and intracellular) is of great importance to the transport processes. Agents which increase the permeability of the capillaries and the osmotic pressure of the tissues (e.g., viruses, bacterial toxins) can cause an inbalance in the water distribution. This is called oedematization or dropsy, and can make the affected organs swollen.

Fig. 2 The water spaces of the organism and the transport through the capillary wall

The injury to the capillary wall can be so severe that even the red blood cells pass through it, causing haemorrhages or bleeding within the tissues. This occurs with diseases such as IHN, VHS, IPN, aeromonas and pseudomonas infection, and from environmental causes like low pH, leaf toxin poisoning, etc.

The other function of the blood is the defence against invading living agents. This protection is performed by one of the two types of blood cells. The erythrocytes or red cells transport the oxygen by means of their haemoglobin content, whilst the white cells serve the protective function. The so-called monocytes and granulocytes are capable of ingesting bacteria and other particles. This process is called phagocytosis. In certain cases these cells can migrate through the capillary wall to reach the site of infection. The lymphocytes are responsible for the immune response of the organism. They produce gamma-globulin, which is a protective protein. The gamma-globulin reacts with the antigens of the infective agents and the reaction leads to the lysis, precipitation or agglutination of the agents. The concentration of the erythrocytes is almost constant (about 1.5 mill/mm³), whereas the number of white blood cells varies greatly (normally some thousands/mm³). The blood proteins total about 4–5% of its volume. Blood cell formation, or haemopoiesis, occurs in the kidney and the spleen. In the event of damage to haemopoietic tissues, e.g., by the diseases VHS, BKD, PKD, or vibriosis, the number of cells can be dramatically decreased, resulting in anaemia. The same problem can occur when the fish loses a lot of blood due to bleeding, parasites, ulcers or injuries.

In anaemic fish the gills become pale, and the defence mechanisms of the organism are depressed.

Some poisons, e.g., excess zinc and copper, as well as insufficient supplementation with iron and various vitamins, can also lead to anaemia.

The respiratory system

The respiratory system of fish is quite different from that of mammals. Fish breathe by means of gills and take up oxygen from the water into the blood through the gill wall. The CO₂ follows the opposite pathway. The wall of the gill is a very delicate, thin and vulnerable tissue. It is covered with mucus produced by the goblet cells of the gills.

Besids the CO₂, other materials are also excreted from the blood through the gills.

In specific cases the gills can show a variety of clinical signs, e.g., injury (due to Dactylogyrus, Cyrodactylus, or Salmincola invasion); the presence of the parasites themselves; discoloration (in anaemia, or in cases of inflammation due to myxobacterial, costial or fungal infections); haemorrhages, etc.

Whatever the cause, fish with injured gills show the signs of lack of oxygen, i.e., gathering at the water inlet, swimming at the surface, nervous signs, gasping, tightly opened operculas.

Muscular and bony system

Muscles consist of blocks (myotomes) which are attached to the spine. Most of the fish's muscle is white; only the fins and some segments along the lateral line have red muscles. The skeleton is originally formed of cartilage, which becomes calcified later in life (in trout when they are 6–8 cm in body length).

In certain cases cavernous lesions may be present in the muscles, e.g., with BKD or Heneguya infections. The skeleton may show severe deformity, as in the chronic form of whirling disease.

The digestive system

The digestive system is a tube-like chain of organs. In salmonids the mouth is designed merely for the capture of prey, and the ingested food passes quickly through the gullet to the stomach.

Fig. 3 The structure of the gill

1. The location of the gill
   a. gill arch, b. operculum

2. Structure
   a. gill arch, b. operculum
   c. gillrake, d. lamellae

3. Circulatory system of the gill
   e. artery, f. vein (enriched in O₂)

4. Histological structure
   g. respiratory epithelium,
   gy. capillaries with blood cells

The low pH of the stomach plays an important role in the defence against living pathogenic agents.

The healthy gut wall is covered by a thin mucous layer. Its colour is bright pink and its lumen usually contains an amount of partially digested food.

Any changes in the appearance of the gut, e.g., presence of haemorrhages, red colour, presence of parasites, an empty gut or the presence of excess fluid of various colours, are of value in diagnosing certain diseases.

The normal liver is a brown-coloured soft organ, situated just in front of the stomach. In addition to its function in digestion, the liver plays a central role in the metabolism of the organism. It is in the liver where the majority of synthesis of body proteins, fats and carbohydrates, takes place. Discoloration of the liver (e.g., in cases of lipoid degeneration, a pale colour), swelling, or the presence of parasites can be observed in diseased fish. Granulomas, proliferation, or haemorrhages may also occur on the surface of the liver. The gall-bladder can also be invaded by parasites (e.g., Hexamita).

The pancreas is scattered as microscopic islands amongst the fat surrounding the pyloric caeca. In addition to releasing digestive enzymes, the pancreas produces hormones which facilitate carbohydrate metabolism. Certain diseases cause haemorrhages in the pancreatic area, e.g., IPN, septicaemias, etc.

The swim bladder is the hydrostatic organ of the fish. In salmonids it is connected to the back of the throat via a duct. The blockage of this duct due to the presence of excess fluid, or parasites in the bladder (e.g., cystidicola) may result in excessive accumulation of air. The bladder can them become distended like a balloon, leading to more or less severe swimming problems.

The excretory system

In salmonids the kidney can be seen as a long black organ at the top of the abdominal cavity, partly covered by the swim bladder.

The kidney works as a filter, extracting harmful waste materials from the blood. The excretory parts of the kidney are the glomerulus and the tubular system. The urine drains to the urinary bladder via the ureters.

The fish kidney also plays a part in haemopoiesis. It contains two endocrine glands: the adrenal and the corpuscles of Stannius.

Any disease of the kidney not only causes excretory problems, but also damages haemopoiesis and leads to possible disturbances of endocrine functions. Due to reduced blood cell formation, the natural defences of the fish may also be severely affected. Granulomas (BKD, tuberculosis, internal fungal infections), swelling (PKD, VHS), discoloration (visceral granulomas, VHS), liquefied structure (vibriosis, aeromonas infection), or haemorrhages (septicaemia) may occur in the kidney.

The reproductive system

Both the ovaries and the testes are located at the anterior end of the abdomen in immature fish and in the resting state of mature ones. At sexual maturity the gonads develop until they extend throughout the full length of the abdominal cavity. This process is under the control of the hormones produced by the pituitary gland. The matured ova (eggs) produced by the ovaries are of orange colour and about 5 mm in diameter. The ovarian fluid which surrounds them must be clean. Cloudiness in this mucus indicates infection, and such eggs cannot be used for hatchery purposes.

The abdominal cavity which contains all the organs mentioned above must be free of excess fluid, parasites and bleeding on its walls. If any of these is present, the belly often become swollen. The pathological changes in the abdomen can readily be seen in dissection (e.g., Diphyllobothrium, filariid worms, excess fluid in septicaemias).

The nervous system and the sensory organs

The brain is responsible for the behaviour of the fish. Pathogens, toxins and poisons affecting the nervous system can cause severe behavioural disorders (e.g., whirling disease, botulism, VHS, etc.).

The olfactory and the optical area of the brain are the most developed units of the cerebrum.

The olfactory area connects directly to the nostrils.

The eyes have no eyelids but are otherwise of similar structure to those of mammals. In case of some diseases they may show exophthalmy (pop-eye) as if they were swollen (VHS, TB, etc.). Opacity of the eyes may be seen due to cataract of the lens in case of Diplostomum infection (eye fluke).

The ear has no outlet and mainly serves as an organ of balance in fish. The cerebellum coordinates the motion of the fish.

The brain extends backwards as the spinal cord, from which arise the nerves serving the various organs.

A sensory organ characteristic of fish is the lateral line. It detects pressure waves in the water.

Endocrine system

The endocrine system is coordinated by the pituitary gland, which is located on the base of the brain and produces the so-called tropic hormones. The tropic hormones act on the activity of the other endocrine glands (thyroid, adrenal and the gonads). The pancreas also has an endocrine function in regulation of sugar and protein metabolism.

The functions of some endocrine glands, e.g., corpuscles of Stannius pseudobranch, ultimobronchial gland, urophysis and the pineal, are still not fully understood.

The body's defence mechanism against attack by pathogens

The first line of defence is the body surface. If the surface defences (mucus, epidermis, stomach pH) cannot resist a bacterial, viral, fungal, or parasitic invasion, then the fixed and mobile phagocytes try to ingest the invading agents with the help of protective proteins which are present in the blood (non-specific proteins). Later the organism tries to isolate the site of infection by producing a demarcation line (inflammation, or a cyst) around the invading agents to separate them and damaged cells from the healthy tissues. These processes can be considered as the second line of defence of the organism.

In the course of time an immune response develops specifically against the antigens of the challenging agents: this is the third step in the defence. Where the defence is successful the organism defeats the challenge and a full reparation of the damaged tissues will ensue.

However, if the above mechanisms are not completely successful the invading agents can cause severe diseases which may even kill the host. The complicated mechanisms of defence require considerable energy, and are also influenced by environmental factors. Therefore the resistance of the animals and their ability to fight against pathogens is both energy- and environment-dependent.

Fish in good condition, kept in favourable circumstances can resist disease much better than those on a poor feeding regime and living under stressful conditions.

Stressful conditions block the defence mechanisms and may cause a sudden outbreak of disease. Sudden stresses affecting the adrenal gland, which produces adrenalin and noradrenalin, can cause nervousness, hypermotility, hyperventilation and increase the frequency of the heart contractions. In extreme cases these minute changes can lead to the rapid death of the fish, due to shock and rapid energy mobilization as a consequence of which the fish loses its energy balance.

On the other hand, even small stresses which permanently affect the organism increase the corticoid hormone production of the adrenal gland. The corticoids block the protective mechanisms of the fish (phagocytosis and immune responsiveness), allowing any living agent to more easily penetrate the body and cause damage.

Correct feeding, appropriate stocking density, and good hygienic and technological conditions all help to avoid these problems in fish farms.

An example of a stress condition is the so-called “sea transfer problem”. The osmotic stress can result in very high losses during the first few days after transferring smolts from freshwater to the sea.

Sick fish are pale in colour, move listlessly, and show tetanic contractions. The cornea often becomes opaque. Frequently only a small proportion of fish can recover unless prompt measures are taken. Fish which have lost scales during transfer die more easily due to the lack of surface defences.

To avoid this problem, a preliminary small-scale transfer of smolts is recommended as a trial of the ability of the stock to survive the trauma. The stress of transfer can also make fish vulnerable to certain diseases, e.g., furunculosis, IPN, vibriosis.

Knowledge of immunity helps with prevention of some diseases. By injecting antigens of a particular agent into the fish, the immune response can be artificially provoked. Thus treated fish can resist natural infection due to acquired immunity (e.g., against furunculosis, vibriosis, Ich, IHN, IPN, etc.).

Hygiene in the husbandry of salmonid fish

Animal hygiene is the science which deals with the relationship between environmental factors and the organism. For successful farming of trout and salmon, a good knowledge of biological and environmental requirements is essential.

Temperature of the water and the oxygen content

The temperature of the water determines the speed of the metabolic processes in the fish, and thus affects the oxygen requirement of the organism. Since the dissolved oxygen concentration of the water falls with rising temperature, in extreme cases it may be necessary to starve fish in high temperatures in order to minimize their oxygen demand. Above about 20°C the oxygen level of the water becomes limiting for salmonids.

The optimal temperature range for growth of trout and salmon is about 14°–17°C, and for incubation of eggs 6°–9°C.

The minimum level of dissolved oxygen necessary in the water for growing trout and salmon is about 5.5 ppm (mg/l) and for eggs about 7 ppm (mg/l). Therefore, where possible the incoming water should be fully saturated with oxygen, but without other gases (e.g., N₂ supersaturation causes a danger of gas bubble disease; H₂S can produce toxicosis).

It is advisable to install splash boards at the inlets to ponds or raceways, and to have aerators at hand for dangerous periods. It is also very important to regularly measure the amount of oxygen present in pond water. The conditions are favourable if the oxygen content of the outflow is higher than the minimum requirement. Measurements should be done daily at least in those tanks which are the most crowded. This is particularly important when temperatures are at their highest. Table 1 illustrates the approximate oxygen and water requirement of portion-sized rainbow trout.

At high altitudes (due to the reduced atmospheric pressure) as well as in salt water (depending on the salinity) the solubility of oxygen is reduced.

Table 1

WATER REQUIREMENT FOR 1 t OF 200 g RAINBOW TROUT,
ASSUMING 100% SATURATION OF FRESHWATER WITH OXYGEN

During darkness any aquatic plants (algae) present use up oxygen, and consequently the amount of dissolved oxygen may suddenly drop to a dangerously low level. Faeces, unused food, and other organic matter entering the water decompose and take up oxygen. This can also result in oxygen deficiency, causing stress and even mortalities especially in heavily-stocked ponds.

Small fish require more oxygen per unit of body weight than larger ones.

All the above factors must be taken into consideration when calculating the optimum flow rate, stocking density, feeding rate, and chemical treatments. In connection with the last of these factors, it is important to know that formalin removes oxygen from the water. Therefore during treatment with this chemical one must ensure that the oxygen level does not drop dangerously, and aerators must be kept on hand.

pH

The pH value of the water shows its alkalinity or acidity. The range is from 0 to 14, with a neutral point at 7. Because pH is a logarithmic scale, a slight change of the value means a marked change in the state of the water.

The buffer systems of the organism strictly regulate the internal pH, resulting in a relatively stable internal milieu. To provide this constant pH, it is essential that the metabolic processes of the fish should continue normally. It is also very important to maintain a relatively constant pH close to the neutral point (6.4–8.4) in the fish's environment.

Variations in the pH, particularly sudden ones, cause stress, and changes to levels outside the optimal range can seriously harm the fish.

Low pH causes haemorrhages in the gills, decreasing the respiratory surface and the gill function, and may lead to severe losses.

In the case of high pH, the concentration of free ammonia (NH₃) in the water increases. This is harmful to the gills and also the nervous system.

Buffered waters (with high concentration of calcium salts) have a stable, but usually high pH. Falling pH can be buffered by adding lime (CaCO₃) to the water. The pH is easily measured by portable pHmeters.

Ammonia

Ammonia is produced during the protein metabolism of fish, and is excreted via the kidney and the gills.

In addition, a certain amount of ammonia may be produced in the course of the decomposition of organic matter in the water. The speed of the process depends on temperature and pH.

If the pH is high (>7) a bigger proportion of ammonia appears as free ammonia (NH₃) in the water. This is very aggressive to live tissues and causes severe gill damage. Following gill damage caused by the NH₃, various secondary infections may develop. The prevention of this demands thorough regular cleaning of the tanks, and avoidance of over stocking and overfeeding.

Regular monitoring of the ammonia level is of great value, particularly under conditions of alkaline water, high temperature and low oxygen levels.

Pollution of water with organic and inorganic materials

In addition to supplying oxygen, an adequate flow rate is needed to carry away the waste materials excreted by the fish. The excreted organic materials may be of toxic character in themselves (e.g., NH₃, CO₂) when they reach a certain level. Suspended solids (faeces, waste food particles) harm the fish by mechanical irritation of the skin and by clogging the gill surfaces.

The accumulation of these wastes also provides a culture medium for bacterial growth, thus enriching the bacterial content of the water. Decomposition of organic matter increases the NH₃ concentration on one hand and removes oxygen on the other. The biochemical oxygen demand (BOD) is the oxygen consumed (in mg/l) by the wastes over a specified time. Its value can increase greatly when organic pollution enters the water via the inflow or the air (e.g., after a storm or spate, as sewage or silage liquor). An extreme increase in BOD may asphyxiate the fish, whereas smaller increases cause stress due to O₂ deficiency.

Some waters, drawn from a spring or well, may contain insufficient oxygen due to the lack of surface oxygenation. However, these waters may be supersaturated with other gases like N₂ or H₂S, which are toxic to fish. All the above-mentioned problems can be adequately controlled by regular removal of wastes (cleaning if needed, increasing the flow rate to the appropriate level), by correct stocking density, and by the reliable aeration of the water.

It is absolutely essential to check the quality and the quantity of prospective water sources before establishing a farm.

Inorganic materials enter the water mainly from industrial pollution (acid-rains and effluents), though heavy metal ions may occur in the water after being dissolved from ores or pipes. Pipelines in a fish farm must never be constructed from zinc or copper. Metal ions are extremely toxic to fish (especially copper, lead and iron) and even to the eggs (especially iron).

The wide use of chemicals (pesticides, herbicides, minerals, etc.) can pose a serious danger to the natural environment and thus for fish. Acute losses may occur if these chemicals are not used carefully enough.

It must not be forgotten that the chemicals used for treating fish diseases are also toxic except in very low concentrations. Therefore the special precautions recommended by manufacturers must be strictly adhered to.

Miscellaneous factors

Ultraviolet radiation can cause severe sunburn on the skin of salmonids, particularly at high altitudes and when the water is free of suspended materials (borehole or well water). The eggs are also sensitive to UV radiation. To avoid the risk of damage by exposure to UV, it is advisable to provide shadow for salmonid fish and to cover incubator trays.

The food must be as free as possible from contamination with toxins, poisons, fungal spores, bacteria, photosensitizing materials, etc. Pellets must be stored in dry conditions and moist food must always be fed fresh.

Any rotting of the raw materials leads to bacterial contamination. It can also cause production of toxins, and spoils the dietetic value of the food as well as its palatability.

To estimate the health state of a fish stock, regular checking of the food conversion rate (FCR) is necessary. The FCR also reflects on the food quality.

Technological requirements and precautions

Daily removal of dead eggs or the use of fungicides is necessary to prevent the spread of fungi to healthy eggs. Brought-in eggs must be treated in a suitable disinfectant solution before introduction to the farm. Before transport it is necessary to wait until the eggs become eyed and are more resistant to the trauma of handling. After hatching, the egg shells should be removed and it must be ensured that water flows are adequate to supply oxygen and remove metabolic wastes. This becomes even more critical once the fry have commenced feeding.

In order to avoid whirling disease, young rainbow trout cannot be reared in earth ponds until they reach about 7 cm in length.

Regular grading is very important during the rearing period to prevent an excessive spread in unit weight amongst a particular stock. Grading must be done by careful handling in order to minimize the stress.

Important daily routines on fish farms are: cleaning inlet screens, filters, and tanks; repairing channels, taps and banks. After emptying, the tank must be scrubbed and cleaned thoroughly before disinfection. After disinfection, tanks should be left fallow prior to the next stocking. It is also important to wash out any possible remnants of disinfectants from the tanks before restocking. Where numerous predators are present it is very important to protect the stocks against them with screens, nets or other methods.

Infectious diseases

According to their aetiology, infectious diseases (i.e., diseases caused by living agents) of salmonids can be divided into four groups:

1. viral diseases

2. bacterial diseases

3. fungal and algal diseases

4. parasitic diseases

Though heavy losses in fish culture are caused directly by living agents, in many cases inadequate environmental conditions or poor husbandry practices can be found in the background of the disease outbreak.

It is important to understand that, besides obligate pathogenic agents, the majority of pathogens can cause disease and losses if the susceptibility of the fish is enhanced by unfavourable farming conditions. This latter group of agents are called facultatively pathogenic.

Apart from the susceptibility of the fish, the probability of a disease developing also depends on the number of the pathogenic agents present and their virulence. Knowledge about pathogenic agents helps with protection and therapy against specific diseases.

Viral diseases

Viruses are organisms on the border between the living and nonliving world. They are incapable of independent multiplication, and can only multiply within a host, i.e., within a more developed organism. All animals and plants from bacteria to man can act as hosts for viruses. Thus the viruses are all parasitic, but some of them are of no clinical significance (the so-called orphan viruses). The origin of the viruses is still in doubt.

At its centre, the virus has a piece of nucleic acid, the so-called core. All other living organisms contain both DNA and RNA, but not viruses. Their nucleic acid (NA) can be either DNA (Desoxy-ribaNA) or RNA (Ribo-NA). The core is encapsulated by a protein structure which is characteristic of the strain of virus. This protein capsule is called an envelope. One function of the envelope is the protection of the NA core and the facilitation of the penetration of the virus into the host cell. The envelope is also responsible for the antigenic effect of the virus.

After entering the host cell, the NA of the virus reaches the host cell's genetic material, giving information for reproducing the virus itself and its envelope. This way the host cells are forced by the virus NA information to change their original activity to virus production. This process leads to the disturbance of cell function and the destruction of the cell with all its consequences.

In general, viral diseases of salmonids are of acute character, with sudden heavy mortalities in most cases.

The majority can spread horizontally from the infected animals to others within a short time, and there is also the possibility of vertical infection via the sexual products.

The viral diseases cannot be successfully treated. Therefore the only way to combat them is by prevention using all the available techniques for protection against epidemics.

In some cases active immunization is available for prophylactic purposes. The main features of the most important viral diseases are summarized in Table 2.

There are also three less important viral diseases affecting salmonids. Viral Erythrocytic Necrosis (VEN) is not yet considered as being economically significant. However, in some cases the presence of the virus can cause difficultues for export of fish to certain countries. The Leukaemia and Fibrosarcoma viruses are responsible for development of various tumours in the fish. Their occurrence is sporadic.

Bacterial diseases

Bacteria are very common in the aquatic environment, and they can multiply very rapidly. Some are capable of growing in the host, sometimes leading to clinical diseases. These are the parasitic bacteria; the others are of saprophytic habit.

Table 2

DISEASES CAUSED BY VIRUSES

The so-called obligate pathogens can only live inside the host. Facultative pathogens on the other hand are also capable of independent life. They cause diseases only when circumstances render the host susceptible.

The bacteria are of cellular character, surrounded by the cell wall. In unfavourable conditions the cell wall may become thicker, producing a spore which helps the bacteria to survive. Spore-forming bacteria are very resistant, whilst species which are not able to develop spores are much more sensitive to environmental factors and are therefore more easily killed.

Bacteria damage the host mainly by producing toxins. The toxins may be produced by the living bacteria and excreted into the environment. These are the exotoxins. The other type of toxins are released from the bacteria only after they are damaged or destroyed. These are called endotoxins. Both kinds of toxin are of antigenic nature, being composed of protein or mucopolysaccharides.

Since the harm caused by the bacteria usually depends on the susceptibility of the host fish, maintenance of an adequate environment in the fish farm is of great importance in avoiding the onset of bacterial diseases or in decreasing the losses due to these diseases.

Though bacteria, like viruses, may precipitate severe losses, the onset of disease is not quite so sudden. If they enter the blood stream, bacteria cause septicaemia. In this case they can be isolated from the various tissues, cultured and identified in a laboratory.

At present, vaccines are available against some bacterial pathogens, i.e., Aeromonas hydrophyla, Ae. punctata, Vibirio anguillarum, and Yersinia ruckeri. All the bacterial diseases can be treated with chemicals (antibiotics or sulphonamides). The bacterial diseases are summarized in Tables 3 and 4.

In addition there are some diseases for which the aetiology is not fully known, but which commonly have bacterial infection in the background. These diseases are presented in Table 5.

Table 3

BACTERIAL DISEASES I

x = staining properties

Table 4

BACTERIAL DISEASES II

x = staining properties

Table 5

DISEASES WITH POSSIBLE BACTERIAL BACKGROUND
(or unknown aetiology)

Parasitic diseases

Parasites are primitive animals which spend part or all of their lives at the expense of a host organism. Parasites are of tremendous diversity and they may be found in almost every tissue of the host.

They are particularly common on the external surfaces (i.e., the gills and skin) of fish, but they can also attack the internal organs.

Some of them have a direct life-cycle, whilst others can reproduce only via one or more intermediate hosts. The intermediate host is defined as an animal which serves as a home for the early phases of parasitic development. After invading the final host the parasites achieve their sexual maturity and produce sexual products (eggs or larvae).

Salmonids can function as either the final or the intermediate hosts of various parasites.

The damage caused by parasites can be of several types. In most cases, the hosts suffer from various and sometimes very severe disturbances when they are invaded. The harm done depends on the number of parasites present.

Parasites often irritate the host mechanically, causing a continuous problem. They damage the tissues they invade either mechanically or by digesting them. In this way they can cause inflammation at the affected site.

Further, by damaging the body surface they open up a pathway for secondary infections by bacteria or fungi. As they develop in the tissues, the parasites compress the surrounding host tissues, causing atrophy which can lead to very serious impairment of organic function. Finally, many parasites produce materials of toxic character, provoking more or less serious toxicosis in the host.

Therefore, all the parasitic diseases are of great economic and clinical importance. Though almost all of them can be successfully treated with various chemicals, the most important thing is to prevent the diseases arising in fish farms by appropriate, adequate feeding and environmental conditions, and by strict epidemiological control to avoid parasitic infections entering the stock.

Some parasites, the protozoa, can only be studied microscopically (or in some cases with a hand lens). They are single-cell parasites, which usually move by means of flagellae or cilia. The pattern of the movement often helps with species identification when they are examined under the microscope.

All other parasites are multicellular organisms, sometimes of considerable body size. In most cases these can readily be seen with the naked eye, or by the routine use of a hand lens.

Like viruses and bacteria, parasites can also provoke an immune response by the invaded organism, but prophylactic active immunization is available only against Ichthyophthirius multifiliis (Ich.)

The most important parasites affecting salmonid fishes are described in Tables 6 and 7.

Table 6

DISEASES CAUSED BY PARASITES I: PROTOZOA

Table 7

DISEASES CAUSED BY PARASITE II: METAZOA

Fungal and algal diseases

Many fungi and algae are common inhabitants of the water, but their accumulation may precipitate diseases in fish.

Fungi are abundant in the environment. They can be found in most freshwaters and also in the mud. When conditions are unfavourable they can harm fish.

Fungi multiply by producing spores. Infection of fish mostly occurs by picking up spores either on the body surface (in the case of external fungal diseases) or via the mouth into the alimentary canal (in the case of internal, or systemic mycoses).

External mycoses are usually associated with previous damage to the body surface, whereas internal ones result from injesting contaminated food.

In certain circumstances some fungi produce toxins, causing severe toxicosis in the affected fish. This is called mycotoxicosis. Algae rarely produce diseases in salmonids (with the exception of a few toxin-producing species), but they can be a potent cause of sudden large-scale losses, known as fish kills. These occur following a massive algal population explosion, or algal bloom. Fish mortality can be the result of oxygen depletion caused by algal respiration, or less commonly due to the neurotoxic effect of the algae. The diagnosis of fungal diseases can be confirmed by microscopic examination of the affected organs and identification of the fungal spores and hyphae.

The most important fungal diseases are summarized in Table 8.

Table 8

FUNGAL DISEASES AFFECTING SALMONIDS

Fish kills

A sudden mass mortality, occurring over a very short period among previously healthy fish, is called a “fish kill”. When this happens, rapid action is necessary to save as many fish as possible and to ensure that all available information is collected for future evaluation and formulation of measures to prevent a recurrence.

The commonest cause of fish kills is the lack of oxygen. This can frequently arise as a result of rapid growth of water plants (particularly algae) or as a consequence of severe water pollution with materials which take up oxygen (e.g., sewage, silage liquor). Fish kills can also be precipitated by poisoning, e.g., with cyanide, which renders the fish incapable of oxygen consumption. Other factors such as technological or mechanical accidents and blockage of the water flow may also cause the same problem.

One of the most serious threats to salmonids in marine conditions is the algal bloom. Algal blooms can sometimes turn whole seas red, orange, or green. They take up oxygen and in some cases produce toxins which are nerve and gill poisons.

The only feasible counter-measures against fish kills are to provide maximum aeration and immediately begin harvesting the fish.

Most of the chemicals used for therapeutic purposes in fish culture are themselves toxic except in very low concentration, as are disinfectants. Therefore these chemicals must always be used carefully and under continuous supervision to avoid accidental poisoning of the fish.

Clinical and behavioural indications of disease

For diagnostic purposes, some of the diseases detailed above are considered again below, grouped according to the behavioural and clinical symptoms they produce in sick fish.

Gill problems

Fish affected by gill problems always show the symptoms of oxygen deficiency, i.e., gasping, gathering at the water inlet, riding on the surface, nervousness.

The possible causes of gill disease are as follows:

In the case of early feeders: bacterial and parasitic gill disease, fungal gill damage. Decreased pH, high ammonia concentration or high levels of solids in the water may help disease agents to gain entrance.

In growers: parasites, especially Costia, gill flukes, or glochidia, are the commonest causes, but other factors such as acid rains (pH decrease), snow melt (with chemical contamination of the water), or high levels of dissolved heavy metals (Cu, Zn, Fe), can also provoke serious gill problems.

Sudden losses can occur particularly at high temperature or after a chemical treatment (or other stressful factors) as a result of relative shortage of oxygen.

Diseases connected with abnormal swimming behaviour and buoyancy

Gas bubble disease is due to the supersaturation of the water supply with gases (usually N₂), and may lead to severe losses among early fry or even growers.

Fry swim in a disturbed fashion, frequently upside down or hanging in the water. Bubbles of gas may be observed in the skin, in the mouth and within the capillaries of the gills. Prevention is possible by appropriate aeration of the water.

“Flashing” is an abnormal swimming motion in which fish suddenly and briefly turn onto their sides, momentarily showing a silver appearance. When this occurs repeatedly, external parasitic infections with Costia, Trichodina and Scyphidia complex, Ich, Gyrodactylus, etc., can be suspected.

Scraping: With certain types of severe irritation, fish may scrape themselves along the bottom or bank of the pond. Simultaneously they frequently show symptoms of flashing and jumping.

These symptoms can be due to Ich, Gyrodactylus, Argulus or Lernaea invasion.

Disorientated swimming

Whirling - whereby the fish swim round and round in tight-circles, is characteristic of Myxosoma infection in trout growers. It is the consequence of the skull cartilage deformation caused by the parasite.

Due to infection with Diplostomum, fish show opacity in one or both eyes, making them more or less blind. They often swim at the sides of the pond and are easy prey for predators.

Botulism, due to the nerve toxin produced by Clostridium botulinum, affects the swimming behaviour of the fish. Affected trout develop nervous signs, sinking to the bottom almost lifeless and then twitching back to the surface, until they die. The toxin is produced in rotten food or organic material on the bottom of the pond.

Bloat - The obstruction of the swim bladder duct by parasites or dusty food particles can lead to the accumulation of excess air within the swim bladder. This increases the buoyancy of the body, causing the fish to float on the surface in a belly, head, or tail-up position. Inserting a needle into the bladder can solve the problem, allowing a percentage of the fish to recover. A particular cause of the disease may be overfeeding in winter. Therefore it is advisable to prevent such overfeeding or, once the disease has been seen, to starve the fish.

Other surface problems

Sunburn - Because they do not have any protective pigment in their epidermis, salmonids are particularly vulnerable to ultraviolet (UV) exposure. This is especially important at high altitudes and where ponds are supplied with borehole or well water, because clean air and water do not filter out the UV radiation. UV irradiation causes lesions on the body surface, which can become secondarily infected. The only prevention is to provide shading for the fish where necessary.

An even more dangerous form of the sunburn occurs when the diet contains photosensitizing materials, which enhance the effect of UV energy. In this case fish become sensitive to even relatively low levels of UV exposure.

The eggs are also vulnerable, and therefore the hatchery trays must be covered to exclude light.

Diseases associated with dietary anomalies

Starvation - There are two basic causes which may lead to emaciation of fish. The first is inadequate feeding, which is particularly dangerous to early feeders. The second may arise when fish are suffering from one or more chronic infections. Many acute diseases may turn into a chronic form if they are not treated properly. This results in loss of appetite, leading to deterioration of body condition. There is the additional danger that the disease agents infecting emaciated fish may spread to attack healthy members of the stock. To prevent this, it is best to catch these so-called “bad-doers” and kill them.

Vitamin, mineral and trace element deficiencies

Although these compounds are only required in small amounts, they are nevertheless essential in the diet because they make up active parts of various enzymes. Consequently the level of metabolism determines the actual requirement for these compounds.

Sub-lethal deficiencies can cause a decrease in the resistance of the fish, leading to secondary infections as well as to decreased growth performance of the stock.

The only satisfactory way to avoid deficiencies is the adequate supplementation of the diet. It is also very important to bear in mind that fish under unfavourable conditions (stressful environment, presence of diseases, transport, handling, etc.) need extra vitamins and therefore increased supplementation.

On the other hand, the presence of certain trace elements can precipitate toxicosis at concentrations above a certain limit. The most dangerous are the heavy metal ions. Therefore their concentration should be determined in the water source prior to establishing a farm.

Fat diseases

Fatty degeneration of the liver

The oxidation of unsaturated fats in the diet can lead to lipoid degeneration of the liver. The liver typically becomes bronze coloured and swollen, and a consequent muscular degeneration and anaemia occur. Mortality may be considerable.

The solution to this problem is the proper storage of food materials and addition of antioxidants to the food. Moist food must always be fed fresh.

Pansteatitis

Pansteatitis is a condition in which the body fat becomes inflamed. Additional changes are the inflammation of the swim bladder and myodegeneration. It is thought to be associated with the incorporation of unsuitable fish meal into the diet.

Gall stones

These are gravel-like, hard lumps of calcified material in the gall bladder. They are thought to be associated with the presence of certain types of lipid in the diet.

Principles for prevention and treatment of diseases

Prevention principles

All the measures listed here are essential to avoid the infection of the farm with agents originating in the environment, as well as to prevent the spread of a disease once it has broken out in a farm:

1. Only eggs of known origin and free of disease should be taken in, and even these should be disinfected before entering the farm.

2. Live fish should never be introduced to the farm unless absolutely necessary. Where it is unavoidable, the new fish must be kept in quarantine for at least a month.

3. Care should be taken to ensure that, should a disease appear in one pond, cross-infections to the others will not follow (i.e., the ponds should be independent from one another).

4. Only people working on the farm must be allowed in. Except where the business depends on them, the number of visitors should be reduced to a minimum. Disinfection of feet and hands is required when entering.

5. Lorries entering the farm should be disinfected and the traffic minimized.

6. Transfer of fish between ponds should be minimized.

7. Dead fish and fish in a moribund condition should be regularly removed and buried.

8. Ponds and tanks should be thoroughly disinfected before stocking, and dip nets should always be disinfected before use.

9. The water supply should be continuously monitored, and predators kept out.

10. Diligent record-keeping is necessary, based on day-to-day monitoring. It should cover the following:

    • water flow
    • fish weight
    • water temperature
    • oxygen, pH, ammonia, salinity
    • food consumption
    • treatments
    • losses
    • unusual events

Treatment principles

All the precautions listed here are absolutely necessary for the successful treatment of any disease:

1. Do not feed the fish for 24 h prior to treatment.

2. Use only plastic buckets for mixing the treatment solution.

3. Double-check your calculations (i.e., weight of fish, feeding rate, water volume, flow rate, etc.).

4. Treat in the morning when temperatures are lowest (applies to dips, flushes and baths).

5. Carry out a preliminary trial with a few fish first.

6. Ensure that the trial was successful and then treat the stock.

7. Watch the stock closely during the treatment.

8. Monitor oxygen level throughout the treatment and use aerators if necessary.

9. Repeat the treatment only in case of absolute necessity, and anyway not within 30 h.

10. Fish which have been under antibiotic treatment must not be slaughtered until 1–4 weeks after the last treatment.

Methods of treatment in fish culture

There are three ways in which fish can be treated. Chemicals can be added to the water or to the food, and in special cases it is possible to treat the fish individually by injection, etc.

1. Adding chemicals to the water

1. Dip treatment

   • The fish is dipped in an appropriate solution for a few seconds in a suitable container.

2. Flush treatment

   • Chemicals are added to the inlet of the pond so that they run through the pond as a flush. (In raceways higher concentrations are necessary.)

3. Bath treatment

   • The water flow is stopped, and after adding the chemical to the pond water fish are bathed for the required time.

4. Flowing method

   • A constant supply of chemical is injected into the incoming water by means of a delivery pump or siphon for the required time.

2. Adding chemicals to the food

1. The drug can be mixed with the food on the farm with the help of gelatine or oil.

2. Food containing the required drug is made by commercial feed manufacturers.

3. Individual treatment

1. Intramuscular injection

2. Intraperitoneal injection

3. Treatment of the body surface.

The commonly used chemicals and drugs, dosages and methods of treatment are summarized in Tables 9 and 10.

Table 9

CHEMICALS ADDED TO THE WATER I

CHEMICALS ADDED TO THE WATER II

Table 10

DRUGS ADDED TO THE FEED

Table 11 shows the most commonly used disinfectants and chemicals for special purposes (anaesthetization, decreasing the oxygen uptake of organic matter, snail and mussel destruction).

Table 11

CHEMICALS USED IN SALMONID CULTURE

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