by
Prof. O.N. Bauer
Dr. of Biol.Sci., GosNIORKH
Parasitic and infectious diseases of fish cause serious damage to fisheries, as follows:
from heavy mortality in fish population, to complete extinction;
decrease in the growth rate, fattiness, conditions and other bad qualities which sometimes lead to rejection of the product on the market;
disturbances in the reproduction system resulting in reduced fecundity of the fish, or lack of spawning at all.
The forms of damage can be illustrated by two examples.
Cyprinidae in the artificial reservoirs are subject to intensive infestation with the larvae of Cestodes belonging to the family of Ligulidae.
The bream (Abramis brama) in the Tsimlansky Reservoir are infested with Digramma interrupta. The infestation results in a much lower growth rate of fish, the difference between the mean lengths of infested and non-infested fish being 2 to 6 cm. It may also cause serious losses in weight. If the infestation occurs during the first summer of fish life, the reduction in weight may amount to 25 to 30 percent. The total fat content in the body of the fish is reduced, on the average, by 38 percent. In addition, heavy mortality in bream due to infestation and predation by piscivorous birds is observed within the first two years of life. In the long run, the fish infested with D. interrupta do not spawn, since the development of their gonads ceases when they are at the second stage. The annual losses in the bream fishery in the Tsimlansky Reservoir are an estimated 1,200 t, the annual catch averaging 4,000 to 4,500 t (Reshetnikova, 1959, 1965; Kosareva, 1960; Kosareva and Reshetnikova, 1960).
The white fish (Coregonus) from the lakes of north Europe, Asia and America are intensively infested with plerocercoids of Triaenophorus crassus, which attain sexual maturity in the intestine of pike (Esox lucius). The plerocercoids are localized in muscles and lessen the commercial importance of the valuable fish, though they are harmless to human beings. In some lakes of Canada the fishery even ceased altogether in the forties, since the infested fish were rejected by inspectors. The Canadian fishery suffered heavy losses (Miller, 1952).
To introduce measures aimed at controlling diseases on a scientific basis, it is necessary to make a precise diagnosis of a case and ascertain its cause. If the disease is caused by an agent it is necessary to identify its genus and species. Sometimes it is not so easy to do, because of the great variety of the parasitic fauna in fishes. To obtain a comprehensive knowledge of parasite varieties, a large-scale research program should be undertaken. This is likely to extend for decades and involve the collective efforts of many scientific specialists such as parasitologists, zoologists, microbiologists, botanists and many others.
A regular study of fish parasites was started in the thirties in the U.S.S.R. It was sponsored by the late Professor V.A. Dogiel, an outstanding zoologist. After his death in 1955, the research continued. Actually, the investigations on parasites of freshwater fish have been mainly completed. Now more than 1,300 species of parasites related to Protozoa, Monogenea, Cestoda, Trematoda, Nematoda, Acanthocephala, Copepoda and some other groups are registered in fish inhabiting U.S.S.R. fresh waters and a key with detailed identification has been compiled (Bykhovsky, 1962).
It is worth mentioning that the U.S.S.R. ranks first in the world in investigations on fish parasites. The achievements are due to the systematic character of the research and to the widely adopted Dogiel' method of “complete parasitologic obduction”, suggesting the examination of all organs of fish and registration of representatives of all groups of parasites.
Extensive research on parasite fauna of fresh-water fish is being carried out in North America but some regions and groups of parasites have not yet been studied adequately. As a result, no theoretical generalizations or comprehensive conclusions can be made. Thanks to kind information received from Dr. Glenn Hoffman, it is known that he has completed his work on compiling the “Key of fish parasites of North America”. The tables, unfortunately, include identifications only up to genera, which, of course, is insufficient to make a precise diagnosis.
The study of fresh-water fish parasites of South America is at the initial stage of collecting material and it is premature to speak of any general, though preliminary, conclusion. Nevertheless, a substantial amount of data on certain groups of parasites are available. For example, thanks to the research of Woodland, Lynsdale, Vigueras, Szidat and Nani, cestodes of suborder Proteocephalata have been studied in detail. According to the latest report of Frese (1965), a total of 18 genera and 46 species of this suborder of fishes from the Amazon and some other rivers of Latin America are described, whereas only four genera and 18 species are known for the fishes of the U.S.S.R. As to Monogenea which constitute at least 20 percent of the total number of species in the ichthyo-parasite fauna of the U.S.S.R. only two species are known for South America and described by Kohn and Paperna (1964). The species are found on aquarium fish of South American origin. Actually it is difficult to study fish parasites from this continent, since the ichthyofauna of its fresh-water bodies, the Amazon River, Parana, etc. is extremely diverse.
No successful measures can be taken to control parasites without a full understanding of their biology, development cycles, life cycles, ecology of various stages of development and, primarily, their initial stages of development. The problems of the association of parasites with a host species, final or intermediate (or a group of species), are also of interest.
These are labor-consuming investigations and they require a wide application of experimental methods. That is why they should refer, first of all, to agents of mass diseases. Extensive research along this line is conducted in the U.S.S.R., and the biology of agents of mass diseases is well covered (Dogiel, Petrushevsky, Polyanski, 1958 /English edition in 1961/Bauer, 1959, English edition in 1962). Soviet scientists widely use international experience in the field of ichthyopathology and collect most interesting data from the scientific literature of the world (Schäperclaus, 1954; Davis, 1956; Reichenbach-Klinka, 1957, et. al.).
Making no reference to the details of the biology of some individual agents, it is recommended to study the biology of a certain species. As is ascertained, even in closely related species belonging to one genus and inhabiting the same host, the biologies are sometimes different. For example, monogenean Dactylogyrus vastator infecting the gills of carp and wild carp (Cyprinus carpio), represents a thermophilic form which is resistant to oxygen deficiency, whereas D. extensus of the same host is relatively thermophilic and oxyphilic (Bauer, 1959; Isumova, 1956; Prost, 1963; Paperna, 1964).
Methods of disease control can be successfully applied on a scientific basis if the biology of agents and epizootological data are well studied. The method should include prophylactic measures for preventing a disease and adequate treatment aimed at reducing the abundance of the parasite or its complete destruction. In contrast to medical, and partly to veterinary practice, ichthyopathology can apply no individual treatment. As a rule, the whole population of fish in a water body is treated. Furthermore, ichthyopathologists cannot observe infested stock inhabiting the midwater layers and can only judge its epizootic condition by examining samples of fish.
All preventive measures can be roughly divided into two groups. The first group will include measures for killing parasites by biological, physical and chemical methods. The second group includes measures aimed at strengthening resistance in fish and improving their protective features. It was Dogiel who stressed these peculiarities (Dogiel and Bauer, 1955). He supported the evidence that well nourished fish, characterized by a fast rate of growth are more resistant to diseases caused by ectoparasites, e.g., Chilodonella cyprini, Costia necatrix, Gyrodactylus, than malnourished specimens with a low growth rate, though they belong to the same stock.
It is found that immune-biological reactions are exhibited more distinctly in nourished specimens than in lean fish.
The efficiency of fish disease control is determined not only by the measures used but also by the characteristics of the body of water.
Fish diseases cannot be controlled in seas and oceans but rivers, lakes and artificial reservoirs are sometimes subject to efficient treatment.
As mentioned above, the infestation of bream in the Tsimlansky Reservoir has been reduced by 3 to 4 times due to special catches of young fish infested with Digramma interrupta.
Conditions in pond fisheries and aquaria allow us to intervene actively in the course of the parasitic disease or even to anticipate it, as fish are available there all year. The fish can either be removed from the pond and treated or the pond can be drained, dried and disinfected with chemicals. Carrier hosts and intermediate hosts should be killed. Considering the comparatively small scale of facilities in which the fish are held, chemicals may be introduced with food or dissolved in the water.
It is important to note that fish culture involves the possible development of epidemic diseases in fish, the agents of which occur in natural waters but are not so dangerous as a rule. It is the crowded conditions in a rather restricted space that facilitate outbreaks of diseases in ponds inhabited by one, or less often, two or three species from the same age group. As a consequence, when environmental conditions (water temperature, worsening of physical condition of fish due to unsuitable diet, etc.) become favorable for mass production of parasites, the disease may spread very quickly. In the space of 3 to 4 days, the entire or almost entire population of the pond, or not infrequently of the whole fish farm, can be wiped out. Diseases caused by Ichthyophthirius multifiliis, Costia necatrix, Myxosoma cerebralis, and some representatives of genus Dactylogyrus etc., spread particularly easily. Besides, hydropsy (rubella) of carp, salmon furunculosis and trout virus kidney disease are among the most infectious deseases.
It is necessary to add that most of the fish diseases at hatcheries are brought in with water inhabited by wild fish, which play the role of carrierhosts. This refers primarily to parasites characterized by a wide specificity. For example, Ichthyophthirius, Costia and Argulus may infest almost all fresh-water fish. In view of this fact, preventive measures should be applied at all stages of rearing fish. The type of a fish farm, water supply pond location and biotechnique used should all include provisions for prophylactic measures.
When designing new ponds or fish farms, all rules important from the point of view of epizootology should be observed.
The choice of a suitable source of water is of great importance. The best source for a water supply should not be inhabited by fish, so agents will not be brought into the pond with the water. For example, a trout farm or salmon hatchery should be placed near spring water to avoid any kind of disease, primarily parasite diseases.
But, unfortunately, springs can supply only comparatively small ponds with water. Besides, the water is usually too cold and can only be used for cold-water fish such as salmon. Normally, when constructing ponds for thermophilic fish, such as carp, Far East hebivorous fish etc., large water bodies inhabited by wild fish are used as water supply sources.
Since large water reservoirs, lakes and ponds are more heavily infested with parasites, it is better to choose small rivers with fast running water, inhabited by a limited number of fish.
To protect ponds from wild fish and their parasites water supply sources should be provided with special facilities. As to fish, this problem has been more or less solved by installation of special screens and filters preventing wild fish from coming into ponds. The screens may be driven electrically, mechanically or hydraulically. It is more difficult to combat minute organisms, e.g. Cyliospores, Ichthyophthirius (40 microns long and 20 microns wide) spores of Microsporidia or active stages of Costia which are even smaller - 3 or 5 microns. Laboratory experiments show that such parasites can be completely detained with a sand filter, the sand layer being 40–50 cm deep. Unfortunately, such filters have not been designed yet. Some preliminary experiments show that ultrasound, infrared and ultraviolet rays can kill the majority of parasites in the water (Goncharov, Nechaeva, Elpiner, 1956). But these methods have never been practised.
When designing new ponds, provisions should be made for each pond to be independently supplied with water and drainage. Underestimation of some factors in designing and construction of ponds will lead to severe losses. It is also important that a proper number of special ponds should be provided. If the right number of types of ponds is not sufficient, the fish culturist has to use other ponds in an improper way, e.g., wintering ponds used for spawning, nursery ponds for stocking, etc. This results in accumulation of agents and worsening of the epizootic conditions in the ponds. A certain number of quarantine and isolation ponds should be provided. The former are used for holding fish for a long time, while the latter are designed for infested fish or those suspected of having some epidemic disease.
When designing ponds, one should not reduce the cost of construction for reasons of simplification of the layout, unreasonable extension of each pond or partial or complete elimination of special ponds from the project. This would surely cause difficulties in the operation of the fish farm and so increase the cost of production.
The choice of biotechnical methods is very important for disease prevention. No contacts are permitted among fish of different ages, since experiments show adult fish are more intensively infested with parasites than the young (Dogiel, Petrushevski, Polyanski, 1958). Larvae should be separated from spawners and held in another pond. This has been practised when rearing salmon. The eggs are taken from spawners, fertilized, incubated in some apparatus and the larvae hatched are held in special basins, where no older fish are reared.
The procedure is different for warm-water fish like carp. The mature fish are placed into the spawning ponds, where spawning, incubation of eggs and hatching of larvae take place. As a rule, the spawners are removed from the spawning ponds only when the ponds are drained and the larvae caught to be held in nursery ponds. Though larvae and adult fish are held together only for 4 to 10 days the former can have already become infested with a great number of parasites and caught some infectious diseases. The larvae of carp can be infested with Ichthyophthirius on the very first day of their life and with parasites such as Trichodina, Gyrodactylus, Dactylogyrus, Eimeria etc., on the fourth or sixth day after hatching. To avoid infestation it is recommended that spawners be removed from the pond immediately after spawning, but it is not always done. That is why the so-called industrial method of obtaining eggs and larvae of carp, herbivorous and some other fish, which spawn in spring and have sticky or pelagic eggs, is highly appropriate (Woynarovich, 1962; Tez, 1963; Konradt and Sakharov, 1963 et al.).
The use of this method, in which the eggs are stripped from the fish and incubated in apparatus, eliminates any contact with adult fish and the consequent infestation and catching of infectious diseases. If the young fish are then reared in well disinfected ponds and water is supplied from a proper source containing no parasite agents, a healthy stock, free from parasites, can be obtained.
One should also not put the young fish together with adult fish, because the latter will again be a source of infestation.
For example, an attempt to rear yearling carp together with two-year-old carp in feeding ponds, or two-year-old carp with three-year-old in the farms with the three-summer cycle common in some countries of west Europe, usually leads to worsening epizootic conditions in the ponds.
At the same time mixed rearing of some different species is very appropriate for example, carp, trout and whitefish in the north zone of carp culture; yearling carp with fry of pike, pike perch (Stizostedion lucioperca), and catfish (Silurus); carp with herbivorous or plankton-eating fish from the Far East, or carp with Tilapia, etc.
Fishes from distantly related systematic groups, e.g., from different genera, families and even orders, have a comparatively small number of common parasites and infectious diseases.
This is why the absolute increase in the density of stocking, when different fishes are reared together results in reducing the density of population of some individual species. In addition, some fish feed on other fish, carrier hosts or intermediate hosts of parasites and sometimes even on parasites. It is known, for example, that two-year-old carp eat molluscs from the family of Limnaeidae, intermediate hosts of Diplostomum spathaceum and closely related species of trematode, and agents of parasitic cataracts of the eyes of young pike perch, trout and whitefish. It is recommended that carp ponds where many leeches Piscicola occur, be stocked with perch (Perca fluviatilis) which include them in their diet.
In America, small fish of the genus Notropis, which eat Cyclopoida, copepodite stages of Salmincola included, are recommended to be placed into ponds infested with Salmincola edwardsii, which is a dangerous parasite of Salvelinus fontinalis. Thus, mixed rearing of different species contributes to the fish productivity of ponds and sometimes even improves their epizootic conditions.
The biotechnique used is aimed at restricting the strength of intermediate and final parasite hosts. In combating the former it is particularly necessary to dry ponds and to leave them without water for one vegetative season at least. Due to this, total mortality occurs in both intermediate hosts of parasites, Mollusca, larvae of Insecta, Oligochaeta, Hirudinea, etc., and cysts and spores of the agents. Various disinfectants, such as quicklime, lime chloride, calcium cyanamide etc. are used in the treatment of ponds.
The evidence on the efficiency of this method in combating fish diseases has been supported by many workers in Europe, America and Southeast Asia (Furtado, 1961). Also, to combat some parasites developing without intermediate hosts and forming no cysts, spores or eggs, it is recommended that the pond be filled with water after the fish are removed and left for an adequate period.
For example, it is found that Ichthyophthirius may survive without a host for not more than four days at the water temperature of +18° to +20°C. Then all cylliospores which got out of the reproduction cycle perish. This so-called method of “treatment” should be applied only in case the biology and life cycle of the parasite are well known.
The biotechnical method should prevent ponds from being overgrown with plants, which not only reduce the useful area of the pond but also provide protection for piscivorous birds, e.g., seagulls, herons, cormorants, etc. The birds affect the size of the fish population by eating young fish, sometimes even big fish, and they carry fish helminths. They are final hosts of helminths, which, being at the larval stage, infest fish and cause various serious diseases, especially to young fish.
To understand the damage caused to reared fish by piscivorous birds it is worth mentioning such trematodes as Postodiplostomum cuticola, Hysteromorpha triloba, Clinostomum marginatum, various species of genera Diplostomum, Cotylurus, and Cestoda of the family Ligulidae, common in different countries of the world, including Latin America and Southeast Asia. That is why such birds are quite intolerable at pond fisheries, especially at the ponds where the young are reared.
As previously mentioned, an adequate food supply is very important for maintaining good epizootic conditions for fish. This can be achieved by organic and mineral fertilization, which stimulate the development of food invertebrates. Various fertilizers, which improve the physical condition of fish and increase their growth rate, are suitable. But any excess content of organic chemicals in ponds leads to oxygen deficiency and to the outbreak of some infectious diseases. The disease caused by the fungus Branchyomyces, which infest the blood vessels of the gills of carp, pike perch and sometimes whitefish results in the formation of a thrombus, damage of gills and eventual death. The mortality due to this fungus is of a sudden and massive character. Experiments show this disease occurs only in case some excess in the organic content of the water is recorded.
The improvement of physical condition of fish and increase in their growth rate can be achieved through artificial diet. Trout, especially rainbow trout, willingly eat artificial food of animal origin, and carp and Chinese grass carp prefer artificial vegetable food. But in this case there is a definite danger that some alimentary diseases may occur. If the diet of trout comprises fatty food of inadequate quality, lipoid or ceroid degeneration of the liver is observed (Factorovitch, 1961; Ghittino, 1962). Recently such cases have been recorded in Chinese grass carp.
This method also enables introduction of some drugs into the diet of fish for prophylactic purposes. For example, the addition of arsenic drugs (stoversol and osarsol) to the diet of trout, contributes to combating the twist disease (Scolari, 1953; Uspenskaya, 1957). Antibiotics, mainly chloramphenicol, added to the diet with the food moistened with the solution of methylene blue, are used to combat hydropsy (Schäperclaus, 1956; Volf and Havelka, 1957). It has recently been found that the addition of 5 percent phosphatides into the diet of rainbow trout prevent the ceroid degeneration of the liver (Privolnev et al., 1965).
One should be extremely careful when some wild fish or fish from other hatcheries are restocked. Experience shows that a rapid spread of diseases among fish farms and hatcheries is often tied in uncontrolled transplantation of fish. Some cases are recorded for neighbouring countries and even for different continents.
For example, at the end of the 19th century Dactylogyrus extensus (Müller and Van Cleave, 1932) and, apparently Caryphyllaeus fimbriceps were introduced with carp from Europe to North America. In the same way, Lernaea carassii, which caused a mass mortality of carp in fish hatcheries, seemed to be transplanted with goldfish to North America from Japan (Tidd, 1934; Davis, 1956). Ichthyophthirius, which caused a heavy mortality among fish, was introduced with trout to Australia. Cestoda Khawia sinensis, nematoda Philometra lusiana and some other parasites were brought from the Far East, together with wild carp from the Amur River, into ponds of the European part of the U.S.S.R. In recent years the cestoda Bothriocephalus gowkongensis was brought in with young grass carp to different countries (U.S.S.R., Ceylon and others) from China. It now infects various species cultured in ponds and inhabiting commercial water bodies (Muselius, 1963; Babaev, 1965; Fernando and Furtado, 1962 et al.). B. gowkongensis intensively infest carp, especially the young, and cause heavy mortality.
As a rule, agents newly introduced into fish farms appear to be virulent, soon increase their numerical strength and cause mass mortality in local populations of fish. The lack of immunity towards infection and infectious diseases is responsible. The agent whose development is not limited by immunity reproduces vigorously and causes mass mortality in fish. The fish population later becomes immune to the disease and ceases suffering from it. Unfortunately, the problem of development of immunity to parasites in fish is still obscure. Experimental data support the evidence on the development of some immunity in fish infested with Ichthyophthirius (Bauer, 1959) and Dactylogyrus vastator (Paperna, 1964; Bauer, Vladimirov, Tez, 1965). As to infectious diseases, the problem of breeding new species of fish characterized by a higher resistance towards mass diseases such as hydropsy in carp, furunculosis in trout and septicemia, deserves a great deal of attention.
To prevent spreading of fish parasites it is necessary to minimize the scope of fish transplantation. The number of fish reared in the farm should meet local stocking requirements. Transplantation of fish is recommended only in case of extreme necessity to prevent interbreeding by renewing the blood and in cases when new species are intended to be acclimatized. But in both cases it is preferable to transplant fish at the eggs or larvae stage. Transplanted fish should be carefully treated with chemicals to kill parasites and held in quarantine ponds. The quarantine period should be fixed in relation to peculiarities of the species and parasites found on them. Releasing of fish into ponds should be supervised by specialists in fish diseases. This prophylactic measure should be given special attention.
Various medicines are used to restrict the numerical strength of parasites in the fish reared and to conserve the population. In this respect, many more achievements have been gained during the recent 10 to 15 years than ever before. A great number of new chemicals have been applied to combating infectious diseases and ecto- and endoparasites.
Antibiotics have proved to be the most effective means to combat mass hydropsy of carp. This method is suggested and experimentally tested by Schäperclaus (1955, 1956, 1958 etc.). Of many antibiotics tested, he gives preference to those of a wide spector of action and, first of all, to chloramphenicol. It was found that it may be either injected once into the peritoneum or added to the diet of carp for prophylactic purposes or as a treatment. The danger of further infestation is lessened if carp are held in a bath of a very diluted solution (200 to 500 mg/l) of chloramphenicol for 24 hours. Some other antibiotics, e.g., syntomycin, streptomycin and terramycin, can be also used for treatment of hydropsy but with less efficacy.
Chloramphenicol is also recommended to treat trout furunculosis, pike plague and some other diseases.
The antiseptic methylene blue is suggested for hydropsy control (Volf and Havelka, 1954, 1957). The most successful results are obtained from systematic feeding of carp with food impregnated with its solution. Baths with methylene blue seem to be less effective.
Besides antibiotics for furunculosis control, various sulfa drugs, such as sulphamerosin, sulphaguanidin and furosolidon are also recommended (Denfel, 1964 et al.). The latter has been tested for hydropsy treatment, but the results obtained are still obscure.
Previously for combating ectoparasites, a method of exposing fish for a short period (5 to 15 minutes) to a chemical solution was often used for destroying parasites. It is not possible to mention all chemicals suggested. The most important are various salts (table salt included) in different concentrations, dyes, oxidants, ammonia, formalin, insecticides, dusts and phosphorus-containing compounds. It was found that a short-term bath is ineffective for some parasites but any increase in the concentration of a solution can be harmful to the fish treated.
The method of dipping fish into mild solutions prepared in special baths or tanks is laborious and involves unreasonable handling of the fish. Long-term baths were then suggested but it seemed disadvantageous because it necessitated the introduction of aeration. A trend to introduce chemicals in mild concentrations directly into the ponds is now observed. Different compounds have been tested in many countries and among them salt, malachite green perydil mercurial acetate (PMA), various insecticides such as dust lindan, neguvon, chlorofos, dipterex, etc. have proved most effective.
As a rule, these compounds are applied to small ponds or basins since it would be too expensive to distribute them over large areas.
Some other new methods have been put into practice. In feeding ponds for fish which consume food on the bottom, chemicals are introduced into feeding areas only, which are fenced off from the other area by oilskin or plastic. Slots are left near the bottom for fish to enter. Thus, the fish are subjected to the action of drug during the feeding period (Avdosoev, 1962).
For treating fish which feed from floating troughs (Chinese grass carp), specialists from China suggested use of the following method. A small bag containing a drug in the form of crystals or powder is fastened to a feeding trough. The drug is gradually dissolved and water containing the chemicals in a mild concentration surrounds the trough, so the fish are treated while feeding.
Only recently have various drugs been applied to combat endoparasites, both protozoa and helminths, inhabiting the intestines of fish. Various antihelminthes such as camalu, fenotiazin and extract of fern rhizome (din-butyl-tin-oxide etc.) have been effectively used by American and Soviet workers to remove tapeworms from carp, grass carp, bass (Micropterus) and trout (Allison, 1953, 1957; Kanaev, 1956; Mussilius, 1962; Shcherban, 1965; Hoffman, 1959). Santonin is used for nematodes (Agapova, 1957).
Kalomel and fenotiazin are widely used for controlling the diseases of young trout, caused by the flagellate Hexamita a parasite of the alimentary canal and gallbladder, adversely affecting fish culture in America (Allison, 1953; Rucker, 1957; Hoffman, 1959). The arsenic drug applied to control trout twist disease, has been discussed previously.
Thus, thanks to numerous investigations carried out mostly in recent years, various means to control mass fish diseases of different origin are at the disposal of ichthyopathology.
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