Stress is debilitating to fishes and greatly increases their susceptibility to various diseases, as shown below according to Wedemeyer and McLeay (1981):
| Disease | Environmental stress factors predisposing to disease |
| Furunculosis (Aeromonas salmonicida) | Low oxygen (4 mg 1-1); crowding; handling in the presence of A. salmonicida; handling for up to a month prior to an expected epizootic |
| Bacterial gill disease (Myxobacteria spp.) | Crowding; unfavourable environmental conditions such as chronic low oxygen (4 mg 1-1); elevated ammonia (0.02 mg 1-1 unionized); particulate matter in water |
| Columnaris (Flexibacter columnaris | Crowding or handling during warm (15°C) water periods if carrier fish are present in the water supply; temperature increase to about 30°C, if the pathogen is present, even if not crowded or handled |
| Kidney disease (Renibacterium salmoninarum) | Water hardness less than about 100 mg 1-1 (as CaCO3); diets containing corn gluten or of less than about 30% moisture |
| Haemorrhagic septicaemia (Aeromonas and Pseudomonas spp.) | Pre-existing protozoan infestations such as Costia, Trichodina; inadequate cleaning leading to increased bacterial load in water; particulate matter in water; handling; crowding; low oxygen; chronic sublethal exposure to heavy metals, pesticides or poly-chlorinated biphenyls (PCBs); for carp, handling after overwintering at low temperatures |
| Vibriosis (Vibrio anguillarum) | Handling; dissolved oxygen lower than about 6 mg 1-1, especially at water temperatures of 10–15°C; brackishwater, of 10–15‰ salinity |
| Parasite infestations (Costia, Trichodina, Hexamita) | Overcrowding of fry and fingerlings; low oxygen; excessive size variation among fish in ponds |
| Spring viremia of carp | Handling after overwintering at low temperatures |
| Fin and tail rot | Crowding; improper temperatures; nutritional imbalances; chronic sublethal exposure to PCBs; or to suspended solids at 200–300 mg 1-1 |
| Coagulated yolk of eggs and fry | Rough handling; malachite green containing more than 0.08% zinc, gas supersaturation of 103% or more; mineral deficiency in incubation water |
| “Hauling loss” (delayed mortality) | Hauling, stocking, handling in soft water (less than 100 mg 1-1 total hardness); mineral additions not used; CO2 above 20 mg 1-1 |
| Blue sac disease of eggs | Crowding; accumulation of nitrogenous metabolic wastes due to inadequate flow patterns |
Prevention, rather than treatment, should be the aim of the fish culturist. Good management of fish farms is of primary importance in avoiding disease and parasite problems. The following general principles should be followed.
Provision of pathogen-free water
When pathogen and germ-free water from springs or wells is used to supply fish farms, fish can be reared under parasite-free conditions for a relatively long time. Such water is essential for hatcheries, and it is also necessary that fry rearing ponds obtain water of the best possible quality. Ideally, water should not flow from one pond to another, though due to water shortage this rule cannot often be fully observed. As second-best, water flow should be from ponds containing young, less-infected fry towards those with older, potentially more infected fish.
Control of wild fish
Wild fish living in the canals and ponds of fish farms are hosts and vectors for pathogens. Their access can be hindered by placing wire-mesh screens over the water inflows to ponds. If necessary, chemicals can be applied to eradicate them.
Deliberate introduction of fish from other hatcheries should preferably be avoided. When this is not possible, the fish should be obtained from a hatchery which has no history of serious disease problems; and also, be treated for ectoparasites prior to stocking.
Stocking density
Avoid overcrowding fish at any time, and particularly during hot weather.
Water quality
Fish pond and hatchery water must be provided in sufficient quantity, at a suitable temperature, free of pollutants, and rich in oxygen.
Age segregation
Fish of different age groups must never be stocked together in the same pond. Older fish are generally infested by parasites, which they can easily transmit to young individuals. Similarly, so-called “after stocking” is bad practice, since fingerlings stocked 2–3 weeks after the initial stocking of the pond can become heavily infested by the older fish, sometimes resulting in massive losses.
Where artificial hatching is practised, fry can develop under parasite-free conditions for longer. If natural spawning and hatching is inevitable, preventive treatment of breeders against parasites is of paramount importance, as is their removal from the pond as soon as possible after spawning.
Drying, freezing and disinfection of pond bottoms
Most parasites develop in annual cycles. Their eggs or spores survive in the soil of pond bottoms. Periodic drying of a pond kills these eggs and spores, as well as their intermediate hosts (e.g., snails), thus interrupting the parasite's life cycle. In intensively managed fish farms, occasional drying of ponds is essential. In ponds dried and subjected to freezing, even the most resistant parasites are killed, and the effect of long-lasting drying of ponds under tropical conditions is almost the same. In each case the disinfection of any remaining pools of water with lime is necessary. If there is no possibility for drying, the whole pond bottom should be disinfected with 2.5 t/ha of lime.
Preventive elimination of parasites from fishes
Transferring fish into a bigger pond itself decreases the chance for parasites to reproduce. Bath treatment applied to fish before stocking into a new pond improves the situation further.
Even on fish farms where management practices and feeding standards are good, disease and parasite problems can occur from time to time, and control measures become necessary. According to the type of pathogen, culture system used, degree of incidence and intensity of infection, and other circumstances, one or more of the following control measures may be appropriate.
Test and slaughter
Where tests indicate the presence in the fish of an incurable, virulent infectious agent, slaughter of the entire stock is sometimes the only way of eliminating the pathogen from the farm and ensure it does not spread to other units. This extreme remedy is most often necessary in cases of viral infection. Some countries have laws requiring compulsory slaughter of stock when certain diseases are diagnosed, though most of them do not provide the financial compensation which would be payable to farmers of terrestrial livestock in the same circumstances.
During slaughter, all ponds, tanks, etc., must be thoroughly sterilized before re-stocking with new fish. Often fish slaughtered because of disease problems must be disposed of in lime pits or otherwise hygienically destroyed. However, for certain diseases where spread from dead carcasses is extremely unlikely, and when no public health risks are involved, slaughtered fish can be sold on the market in the normal way.
Quarantine and restriction of movement
Quarantine is when fish which are to be moved from a suspected or infected geographical area to a non-infected geographical area must be held in detention for some time, at least as long as the incubation period of the suspected disease. They can then be moved into the new geographical area only if the suspected disease does not develop.
Restriction of movement is when all transfers of fish from infected to non-infected areas are prohibited. Frequently, sales of fish to the market are also forbidden.
Quarantine and restriction of movements can be used successfully only through a good cooperation between fish culturists, fish health specialists, state agencies and everyone interested in controlling fish diseases. The most effective use of quarantine and movement restriction for disease control has been through legislative regulation of the intercontinental, interstate or interprovincial movement of fish. Quarantine and restriction of movement have been found to be especially effective when combined with test and slaughter or sanitation and disinfection.
Immunization and disease resistance
Immunization and disease resistance have been of limited use for control of infectious diseases of fish. The reasons why fish immunization has been of limited success are:
fish are not as immunologically competent as higher animals, especially at lower temperatures; and
the technology for mass immunization of cultured fish is limited. Injection of all fish in a fish culture facility is usually impossible.
The oral method of immunizing large numbers of fish against bacterial pathogens has been of limited use. The procedure used for oral immunization has been to add the immunizing agent to fish food and to feed at prescribed time intervals. Mass immunizing techniques in which fish are first submersed in a hyperosmotic solution followed by immersion in a hypoosmotic bacterin are more adaptable to the requirements of the large numbers of fish in most culture facilities. Fish are placed in a bacterin which is contained in a pressure vessel. A partial vacuum is established inside the vessel and then released. The rapid change in pressure causes some of the bacterin to enter tissues of the fish. This method has been used experimentally but holds little promise for mass immunization of fish. Immunization of large numbers of fish has been accomplished by placing them into water suspensions of immunizing agents and allowing direct absorption.
Spray methods may also be used by spraying the immunizing agents against the fish. These two methods of mass immunizing of fish are being developed rapidly because both are less traumatic to the animals. Resistant strains of fish species have been used to control disease. Attempts have been made to develop strains of fish which are resistant or immune to certain pathogens. This method of disease control holds promise but requires much more research to be effective.
Drug therapy and disinfection
Drug therapy and sanitation is the method of disease control which normally comes to mind whenever treatment of a disease is mentioned. Therapeutic drugs have been found to be effective in the control of certain diseases of humans and domestic animals. However, drugs used for systematic therapy, disease prophylaxis and disinfection procedures have been of limited use for diseases of fish. There are an extremely large number of known therapeutic compounds available from pharmaceutical manufacturers which have not been applied to the control of fish diseases. There are a great number of synthetic compounds prepared by chemists and pharmacologists each year which may be potentially effective in the control of pathogenic organisms. Only a very limited number of all of these compounds have been examined for possible usefulness as therapeutic compounds for fish disease control.
Those compounds which have been tested and licensed for use with fish can conveniently be considered according to their method of application, as follows:
Individual treatments (e.g., injections). Except for very valuable breeders, treatment of individual fish is not normally feasible or economic. Normally mass, or stock, treatment is applied in fish farms both for preventive and therapeutic purposes. Medicaments against parasites can be applied in the form of baths or in medicated feeds. However, intraperitoneal injection can be very effective against threadworms.
Bath treatments. External parasites can best be eliminated by treatment with antiparasitic medicines dissolved in water. Three types of bath treatments can be given.
Short bath: Infested fish are placed into the bathing solution for between one minute and one hour. For this purpose, tanks made of wood or synthetic material are suitable. Metal tanks are less satisfactory, since poisonous components can dissolve from their surfaces during the treatment. For treatment in ponds, different dilutions of common salt and formalin are used.
A 15-minute bath in a 2.5% salt solution can be used against various unicellular parasites, fish lice and leeches. This cheap procedure, based on the effects of osmotic differences, is easy to apply and, though it does not give perfect results, it meets the requirements of practical purposes. Bathing in 1:5 000 formalin for 45 minutes is effective against unicellular parasites, e.g., Chilodonella and Costia.
Long bath: Very dilute solutions of malachite green or organic phosphate esters are used for treatment in ponds. Since water exchange is slow, contact time will be long.
Malachite green (zinc-free), which was originally used as a staining material, is now widely employed against unicellular parasites. It is the only efficient chemical against “ich” disease. Malachite green can be used at a concentration of 0.1–0.2 mg/l for 24–48 hours in small ponds. The required amount of chemical can be dissolved in lukewarm water and poured in at the pond inflow. Inflow water subsequently carries the chemicals to every part of the pond. The outlet sluice must be closed when the green colour can be seen there. For common carp, concentrations as high as 0.4–0.8 mg/l can be used. However, herbivores, catfish and eels, are more sensitive to this chemical, and concentrations of only 0.1–0.2 mg/l are recommended for them. Since the toxicity of malachite green varies between batches, it should be tested on a few fish before widespread use.
A 24-hour bath will free fish of Chilodonella and Trichodina, while a 48-hour treatment also kills all the developmental stages of Ichthyophthirius multifiliis. Malachite green is of no use against metazoans and spore-bearing protozoans.
Various copper compounds are used in cases of fungal infestation. For the disinfection of water, copper sulphate is applied at the rate of 1–2 kg/ha (where the water column is 1–2 m); malachite green can be used to eliminate fungi from eggs and fish. Because the zinc content of malachite green can be toxic, test bathing should be done before the full treatment.
During decomposition, dilute malachite green solution becomes toxic. Bathing solutions should therefore always be freshly prepared. This also holds true for most of the other compounds used for bath treatments. Due to its intensive and lasting staining effect, malachite green must be applied with much care. Fish bathed in this chemical will retain the colour for 6–8 days, and should not be consumed until after that period.
Organic phosphate esters: Low concentrations of various derivatives of these chemicals (Ditrifon, Flibol, Neguvon, Dipterex, Masoten) are extremely effective against Dactylogyrus, leeches, fish lice and Lernaea when applied as a bath for 24–48 hours. Most of the parasites die within 24 hours. One advantage of these chemicals is that, at the low concentrations used, they decompose very quickly in the pond. It is therefore not necessary to change the water after treatment. Their disadvantage, however, is that they also damage food organisms. Consequently, increased attention must subsequently be paid to proper feeding.
In-transit bath: This is a very practical way to eliminate parasites while fish are being transported from one place to another. Organic phosphate esters are especially suitable for this; during a 0.5–2 hour-long transport, a 0.1 g/1 Ditrifon solution will free fish of monogeneans and parasitic copepods. Protozoans can also be eliminated in a similar way with appropriate concentrations of formalin or malachite green.
Medicated feeds. Parasitic diseases can be cured with medicines mixed into feed or with premixes. This type of therapy is especially effective against tapeworms, but is also suitable for the elimination of nematodes and coccidia. For the treatment of Bothriocephalosis in carp farms, feed containing 0.1–0.2% Devermin can be fed for 1–3 days.
In bacterial diseases, Furane derivatives (furazolidone, nitrofurazone, sulphonamides), sometimes in combination with trimethoprin, or antibiotics (oxytetracycline, chloramphenicol, neomycin), are applied with feed. Before treatment begins, tests for drug resistance should be done. In applying these drugs, the recommended dosage and period of treatment should be respected, so as to prevent the development of bacterial resistance. For myxobacterial infections, the drugs are routinely administered mixed with feed, only rarely dissolved in water.
Disruption of parasite life cycles
Destruction or reduction of a link in the transmission cycle has been used to control infectious diseases of fish involving animal parasites. Metazoan parasites of fish have a definite transmission cycle involving the fish at some stage of development. Many of these parasites require one or more other animal host species to complete the life cycle. Each stage of development in each host offers a possible means of disrupting the transmission of the parasite. However, in practice eliminating a link in the transmission cycle may not be feasible, because it may mean the elimination of a protected mammal or bird, or of a crustacean or mollusc which can be difficult to eliminate.
Food has a decisive role in fish farming. On surveying the different fish farming operations, from the extensive ways up to the intensive systems, feed will show an increasing importance. Consequently the risk of diseases caused by food quality problems or by pollution of water also increases.
Impact of food quality on the aquatic environment
This problem is of most significant importance in intensive fish rearing systems where biologically complete feeds are used. Certain compounds - e.g., nitrogen, phosphorus, protein - in these feeds will dissolve in water if not properly stabilized, thereby increasing its oxygen uptake and ammonia level. This also favours the propagation of several harmful bacterial and algal species in fish rearing systems.
It is thus important to reduce the disintegration of these mixed feeds to a minimum. Fats and carbohydrates in the diet not only provide concentrated metabolizable energy, but stop the compounds dissolving and reduce phosphorus content to less than 7 g/kg.
Relationship between the quality of food and the general resistance of fish to diseases
Compounds of the food absorbed from the intestine pass through the liver. There they become utilizable for the organism, are detoxified, and sometimes stored. During this process serum proteins important for the organism, and other biologically active compounds ensuring a general resistance to diseases, will also develop here. Therefore any damage to the liver, including those caused by the food, lead to a decrease in quantity of these compounds and to impairment of the general resistance of the organism. This favours the rapid growth of saprophytes and facultative pathogens, and thus to development of disease.
Deficiency diseases
Deficiency diseases are encountered predominantly in intensive rearing systems where natural food is not available and the fish are reared entirely on pelleted feed.
Deficiency diseases caused by lack of inorganic micro- and macro-elements are rarely found even under intensive conditions, because fish can take them from the water or the feed. However, iodine deficiency has occasionally been observed in trout-fry rearing. Genuine deficiency diseases are most often caused by lack of vitamins. Most experience has been gained in trout farming.
Diseases caused by toxic compounds in the feed
Unfortunately it is not uncommon in many parts of the world to find feeds which have been spoiled by microbial action or polluted with chemicals or pesticides being fed to fish, in the mistaken belief that they can be fed without any risk under semi-intensive conditions. On the contrary, however, these polluted feeds cause serious inflammation of intestines, liver damage and sometimes fish kills. In addition, they cause poor appetite, impaired growth, and retard weight gain of fish for 2–3 weeks. Thus a few such episodes during a growing season can cause irreparable damage to productivity.
Fats become rancid through the oxidation of unsaturated fatty acids. In the process Vitamins A and E are decomposed and damage to the liver and genital tissues occurs.
Cereal grains treated with caustics (e.g., mercury TMTD) are dangerous since the toxic agents accumulate in the fish and might be harmful to humans. Fungi such as Aspergilus and Fusarium produce toxins such as aflatoxin, T2 toxins and ochratoxin. These toxins attack various organs of the fish when they enter it with polluted feed. Aflatoxin causes liver cancer in trout, F2 toxin impairs the reproductive capacity of carp, inhibiting the genital production. T2 toxin causes poor appetite, impaired feed conversion ratio, and prolonged exposure injures the immunity system. The above problems can be avoided by using only feeds of the best possible quality.
Fatty liver in trout can be the result of feeding diets of high carbohydrate content. If protein content of feed is lower than 25%, the development of the disease is very likely, but is also made worse by Vitamin E deficiency. The disease manifests itself in the accumulation of fat in the liver and at the same time in a dramatic decrease of glycogen. Accumulation of fat is followed by lesions in liver cells. Lesions become infiltrated by lymphocytes. The colour of the liver becomes light, resembling that of a fat goose. Owing to the damage to the liver, the general resistance of fish decreases and they can sometimes die of quite simple causes. The disease cannot be cured, but by feeding diets of 40–50% protein content it can be prevented.
Diagnosis of diseases caused by feed
Diagnosis of these fish diseases is a very difficult task since such damage very rarely manifests itself in a clear form. Since the development of the disease is accompanied by a decrease in general resistance, common/routine diseases can appear (external parasites, facultative pathogenic bacteria, etc.). Diagnosis of these diseases needs wide-ranging testing.
First, environmental effects, primary infections and parasitic infestations, have to be ruled out. If this has been reliably done, feed-caused disease can be suspected. The presumptions concerning the cause of the disease can be confirmed by changing feed (after feeding with good quality and biologically complete feed, the fish should recover). Generally in such cases the disease can also be experimentally induced.
Fish are the best indicators of environmental pollution. All substances which the fish encounters during its lifetime can be detected in its tissues. Consequently, the examination of fish body composition will give a reliable picture of the level of pollution in the fish pond.
Pollution in fish ponds is a result of agricultural and industrial activities. In agriculture, inorganic fertilizers (those with high ammonia content are the most dangerous) and pesticides (chlorinated hydrocarbons, organic phosphate esters, carbonates) are responsible for pollution. The most dangerous pollutants of industrial origin are heavy metals, raw oil derivatives, and chlorine.
Heavy pollution of the environment can cause toxicosis and fish kill. If a high number of fish of mixed ages and species die simultaneously, toxicosis can be suspected as the probable cause. However, before starting any investigation, the possibility of oxygen deficiency should first be ruled out. This occurs either when fish cannot take up sufficient oxygen due to a gill problem, or because the oxygen level in the water is very low. Oxygen deficiency may occur if a high amount of organic material is dumped into the water (generally from pig farms, slaughterhouses, sugar processing or distillery plants). The decomposition process of this organic material uses up oxygen from the water.
Some of the above-mentioned toxic agents (ammonia, hydrogen sulphide) may be released during the decomposition of organic material, while others are carried in with the inflow water.
Lime and inorganic fertilizers are washed into the water by rain and snow. Active substances in insecticides (organic phosphate esters, chlorinated hydrocarbons) enter the water either from the fields or as a result of negligence during spraying. These substances are neurotoxins, and the symptoms of this type of toxicosis in fish are writhing and convulsive swimming. Generally, the cause of death is respiratory paralysis. No characteristic symptoms are produced by industrial pollutants (detergents, heavy metals, etc.).
No firm diagnosis can be made from the symptoms alone, and even histological examination cannot give conclusive information. Extensive investigations covering every possible parameter are necessary to establish the cause of toxicosis. During a mass fish kill bacterial diseases, parasitic infestations and oxygen deficiency can be reliably ruled out. However, toxicosis can be the cause even in cases where negative toxicological results are obtained. Examination of toxicosis needs great care and circumspection, since heavy fish losses are often followed by legal action to establish liability.
Further Reading
Amlacher, E. 1970 Textbook of fish diseases. Jersey City, New Jersey, TFH Publications Inc., 302 p.
Herwig, N. 1979 Handbook of drugs and chemicals used in the treatment of fish diseases. Charles C. Thomas Publisher, 272 p.
Pickering, A.D. (ed.). 1981 Stress and fish. London, Academic Press, 368 p.
Plum, J.A. (ed.). 1979 Principal diseases of farm-raised catfish. South.Coop.Ser. Ala.Agric.Exp.Stn., Auburn. (225);92 p.
Post, G.W. 1983 Textbook of fish health. Neptune, New Jersey, TFH Publications Inc., 256 p.
