AQUACULTURE DISEASE AND HEALTH MANAGEMENT’ Fred P. Meyer2
U.S. Fish and Wildlife Service, La Crescent, MN 55947 ABSTRACT
Disease problems constitute the largest single cause of economic losses in aquaculture. In 1988, channel catfish producers lost over 100 million fish worth nearly $11 million. Estimates for 1989 predict even higher losses. The trout industry reported 1988 losses of over 20 million fish worth over $2.5 million. No data are available on losses sustained by producers of shellfish. Bacterial infections constitute the most important source of disease problems in all the various types of production. Gram-negative bacteria cause epizootics in nearly all cultured species. Fungal diseases constitute the second most important source of losses, especially in the culture of crustaceans and salmon. External protozoan parasites are responsible for the loss of large numbers of fry and fingerling fin fishes and are a cause of epizootics among young shellfish. The number of therapeutants approved by the Food and Drug Administration is limited. Research to support the registration of promising therapeutic agents is urgently needed. Key Words: Aquaculture, Fish Farming, Fish Diseases, Zcfulurus puncfufus, Trout, Shellfish J. Auim. Sci. 1991. 69:42014208 Introduction
Allied Aquacultures, Auburn University, AuAL). burn, Aquaculture, the water farming of fish and Harvests from natural waters have declined shellfish, represents the fastest growing animal or, at best, remained static (U.S. Office of husbandry industry in the United States and in Aquaculture, 1986). As natural stocks have many other parts of the world. Over 30 aquatic declined, aquaculture has become increasingly species are currently being cultured to produce important as a source of fishery products. The protein as a human food source. U.S. aquaculture industry has five major The demand for fish and shellfish continues facets: trout and salmon, channel catfish, bait to rise. While the per capita consumption of minnows, pet and ornamental fishes, and red meat is declining, that of fish and shellfish shellfish (crustaceans and mollusks). reached a record high in 1986 (the last year for Although the commercial production of which data are availabIe). Consumption has trout and baitfish has existed for many increased annually since 1980 and represents decades, aquaculture as a significant food growth of about 20% over the past 5 yr (E-€. K. industry has developed within the last 25 yr. Dupree, personal communication, U.S. Fish The channel catfish (Zcralurus punctarus) is the and Wildlife Service, Stuttgart, AR). In 1987, major species under culture and the annual the per capita consumption was projected at production exceeds 158 x 106 kg (O’Bannon, 7.2 kg (J. Jensen, School of Fisheries and 1987; B. Drucker, personal communication, National Marine Fisheries Service, Silver Spring,MD). The production of rainbow trout lResented at a symposium titled “Aquaculture in (Oncorhynchus mykiss) and salmon (OnAnimal Science” at the M A S 82nd Annu. Mtg., Ames, chorhynchus and Sdmo spp.) (30 x 106 kg) is IA. followed by other freshwater fin fishes, 2Retired. shrimp, oysters, lobsters, and marine fin fishes. Received August 27, 1990. Accepted April 5, 1991. About one-half of the trout and salmon are 4201
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Many of the pathogens that cause disease in fish and shellfish are facultative forms that are ubiquitous in aquatic systems. In nature, a high percentage of apparently normal and healthy animals harbor potential pathogens without evidence of clinical signs or overt disease (Wedemeyer, 1970). The development of disease in aquaculture systems usually occurs as the end result of a disruption of the normal environment in which the animals are being reared. Unfavorable conditions, such as crowding, temperature fluctuations, inadequate dissolved oxygen, excessive handling, physical abuse, inadequate diets, or toxic substances Losses Caused by Dlseases may stress the animals (Wedemeyer et al., Losses incurred by fish farmers are related 1976). If the level of the stress exceeds the to disease, floods, oxygen depletions, preda- ability to adjust, the effect can be lethal. Less tion, chemical poisoning, theft, and miscellane- severe stresses may affect the rate at which ous causes. Disease is by far the most body defenses act and antibody formation significant factor. Estimates of dollar losses takes place (Roberts, 1975). Thus, “stress” is due to disease are conservative figures because considered to be an important predisposing fingerling fish have a significantly higher factor in most bacterial diseases of fish and value per unit of weight than food-sized fish. shellfish and any situations that result in For example, fingerling trout are priced at “stress” will often be followed by clinical $10.20/kg, compared to $2.40/kg for food- disease. sized fish (USDA, 1989). Also, most economic The culture of crustaceans frequently inloss estimates fail to include the cost of labor, volves short-term holding of dense numbers of interest, lost production time, expenses of animals in pens or tanks before marketing or to treating and disinfecting, and restocking. induce molting. Such concentrations are highly The Ca@sh Journal (Anonymous, 19%) reported that 115 million catfish were lost to stressful and provide ideal conditions for the disease during the first half of 1989. About transmission of pathogens. The most severe 90% of the losses (104 million) were among economic losses in shrimp, lobster, and crab fry and fingerlings. Nearly 4.54 x lo6 kg of cultures often occur during these brief but catfish fry and fingerlings died. They had a critical periods (Sindermann, 1974). Effective programs of fish health manageminimum value of $8 million. Food fish losses ment focus on 1) keeping stressful conditions totaled 2.04 x 106 kg during this time and had to a minimum, 2) prevention of the introduca market value of $3.6 million. If the same rates of loss continued in the second half of the tion of pathogens, 3) prompt use of effective year, the total cost of losses to disease in 1989 drugs, and 4) use of vaccines when available. It is axiomatic that well-nourished fish reared would reach $23 million. The catfish industry reported that 39% of in highly favorable environmental conditions the 100 million fish lost in the 1988 produc- will be resistant to most pathogens tion season were killed by disease (Anony- (Wedemeyer et al., 1976). In many cases, mous, 1989). In economic terms, at $1.75/kg, prompt reduction of the stressful conditions the 6.2 x 106 kg of catfish killed by disease in may lead to selfcures without the need to 1988 represented a minimum loss of nearly resort to chemotherapy. Obligate fish pathogens can cause disease $11 million. The trout industry reported losses during even among healthy fish reared in good 1988 of 20.7 million fish, 50% of which were environmental conditions. Strict measures are lost to disease (USDA, 1989). The 1.04 x 106 needed to prevent the introduction of such kg of trout lost to disease cost producers a organisms via infected fish or contaminated water. In fish hatcheries, the best action for minimum of $2.5 million (at $2.40/kg). No economic data are available concerning dealing with diseases of this type may be to the extent of losses incurred in crustacean and remove all live fish, to thoroughly disinfect the molluskan culture systems. facility, equipment, and water supply, and to slaughtered and processed directly for the table. The remainder are used to stock fishing waters, for stock to be grown to food sizes, or to stock ponds where anglers pay for recreational fishing (USDA, 1989). It is estimated that 30% of all salmon now consumed are produced on fish farms and that by the year 2000 farm production may exceed the wild harvest (Van Dyk, 1990). Presently, 60% of fish and shellfish products consumed in the US. are imported (J. Jensen, personal communication).
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begin anew with clean fish or eggs from a necrosis, and viral hemorrhagic septicemia are disease-free source. the major viral diseases of trout and salmon Unfortunately, few production facilities are (Post, 1983; Wolf, 1988). able to maintain ideal rearing conditions and Infectious hematopoietic necrosis causes adverse circumstances may arise in spite of the serious problems in salmonid culture and it is best efforts of fish culturists. An epizootic estimated that this disease costs trout among the cultured stock, if left untreated, producers over $5 million dollars annually (R. may cause serious economic losses. Prompt Busch, personal communication, Biomed, Inc., medication with an effective drug is required Bellevue, WA). in conjunction with corrective action to reduce Viral hemorrhagic septicemia affects fish in the predisposing, stressful conditions. their second year of life, as well as fry and Overviews of culture systems and disease fingerlings, so it can cause serious economic problems associated with the various types of losses, as well as the death of many fish. Viral animals produced in aquaculture may be found hemorrhagic septicemia is endemic to Europe. in the following references: trout, Busch Despite rigorous attempts to prevent its intro(1987); salmon, Harrell (1987); catfish, Beleau duction into North American waters, the and Plumb (1987); shrimp, Bell and Lightner disease was discovered in 1988 and 1989 (1987); and mollusks, Brown (1987). among Pacific salmon in Washington (Eaton The scope of this workshop is directed and Hulett, 1990; Stewart et al., 1990). The toward food animals. Discussions of diseases virus has been detected among hatchery-reared among fish and shellfish will thus be limited to “salmonids” and retuming adult coho salmon the problems of species produced for human (Oncorhynchus kisutch) from three hatcheries consumption. Persons interested in baitfishes, and two rivers (Eaton and Hulett, 1990). pet fishes, or ornamental fishes wiII find that Current efforts to control the disease involve many of the diseases of catfish are also quarantine and destruction of all eggs and fry common to these species. known to have been exposed to the virus or to have originated from infected adult fish. To Vlral Diseases date, no epizootics have been reported, but the Viral diseases cause serious problems in occurrence of viral hemorrhagic septicemia every aspect of aquaculture. If precautions are disease has serious adverse implications for not taken to prevent the introduction of viral aquaculture production of salmonids in the agents, severe economic losses can occur. In Pacific Northwest. ChanneI catfish virus seems to be endemic viral diseases, regardless of the species being cultured, the only recourse is to quarantine and throughout the South Central and Southeastern destroy the infected stock. Rigorous cleaning m s of the United States. Although the virus followed by disinfection of all facilities, seems to be present in most cultured stocks of equipment, and water supplies must precede catfish, expression of the disease seems related further attempts to produce animals. When to stressful environmental conditions, the presstarting anew, the culturist must have access to ence of infected adult fish in the water distribution system, and the presence of a vhs-free stocks (Wolf, 1988). In aquaculture, the presence of a viral bacterial copathogen. When epizootics occur, disease in endemic wild stocks can pose a losses among fry and fingerlings can be very virtually insurmountable obstacle to net-pen high, but outbreaks of the disease have been farming of animals in open waters. Likewise, only sporadic and have not limited the the entry of wild “carrier” animals into the commercial production of catfish (Plumb and water supply of artificial culture systems is Gaines, 1975; Post, 1983; Wolf, 1988; Anonylikely to introduce viral agents that can cause mous, 199Ob). In shellfish culture, viral diseases have not mortalities as high as 90% or more in been a major cause of losses, but a number of susceptible stocks. Among trout and salmon, viral diseases viruses have been associated with epizootics. have received extensive study. Most are The diseases include herpes-like viral disease serious threats to the survival of fry and and reo-like viral disease of shrimp and crabs; fingerlings and have caused major losses baculovirus disease of shrimp; and herpes-type among young-of-the-year fishes. Infectious virus disease of oysters (Sindermann, 1977; hematopoietic necrosis, infectious pancreatic Sindermann and Lightner, 1988).
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Bacterial pathogens probably cause more disease problems overall than all other causes combined. In virtually every type of aquaculture, bacterial diseases rank number one among etiological agents. Septicemias, cutane ous lesions, and destruction of the shell are among the manifestations of bacterial infections. In each type of culture and for virtually every species, specific bacterial pathogens are responsible for serious disease problems. Gram-negative bacilli are the most frequent cause of bacterial diseases in finfish. Although only a few Gram-positive forms affect finfish, such bacteria cause serious diseases among crustaceans. The major bacterial diseases of trout and salmon include furunculosis (Aeromnas salmonicida), bacterial hemorrhagic septicemia (A. hydrophila), vibriosis (Vibrio spp.), enteric redmouth disease (Yersinia ruckerii, columnaris disease (Cytophaga columnaris), bacterial gill disease syndrome, and bacterial kidney disease (Renibacterium salmoninarum). Other serious, but less common, diseases are caused by Cytophaga spp., Nocardia spp., Mycobacterium sp., Streptococcus sp., Pseudomonas spp., Flavobacterium sp. (Roberts, 1982; Post, 1983). and Pasteurella sp. (Sindermm, 1977; Sindermann and Lightner, 1988). A new bacterial disease, salmonid rickettsial septicemia, has appeared among coho salmon (Oncorhynchus Kisutch) on commercial fish farms in Chile. Losses in 1989 were estimated at 1.5 million, 5- to 10-kg fish being reared to market size. Mortalities at rearing facilities reached 70%. The cause of the disease is an unidentified Gram-negative, pleomorphic coccus considered to be a rickettsia (Cvitanich et al., 1990). In catfish culture, enteric septicemia disease of cattlsh (caused by Edwardsiella ictaluro has emerged as the principle disease problem. Some workers estimate that this disease alone costs fish farmers over $10 million annually (J. Jensen, personal communication; Anonymous 19!30a,b). If the estimate is accurate, Edwardsiella may cause nearly half of all economic losses due to disease incurred by catfish farmers. Other major bacterial diseases of catfish include bacterial hemorrhagic septice mia (A. hydrophila), columnaris disease (C. columnaris), and Edwardsiellosis (E. tar&).
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In crustacean culture (shrimp and lobsters), shelldestroying bacteria and septicemic infections cause the most serious problems. Shell diseases are caused by kucothrix, Benecka, Vibrio, and Pseudomonas spp. Septicemic diseases are caused by Vibrio, Aerococcus, and Pseudomonas spp. (Sindermann, 1977; Sindermann and Lightner, 1988). Filamentous bacterial disease (kucothrix spp.) is a common but less serious disease of shrimp. Bacterial diseases among oysters and clams occur, but they are not major causes of economic losses in this type of aquaculture. Bacillary necrosis caused by Vibrio spp. affects both clams and oysters (Sindermann, 1977). Fungal Diseases
Fungal growths on the surface of eggs and larvae of fish and shellfish can cause extensive direct mortalities. They also occur as common secondary invaders in wounds, lesions, or abrasions caused by bacterial pathogens, parasitic organisms, abusive handling, or unfavorable environmental conditions. Some fungal agents are primary pathogens, especially of crustaceans, and they are the cause of mortalities in rearing units. In incubating fish eggs, dead eggs provide a fertile substrate for fungal growths. If dead eggs are not removed, resultant fungal growths may cover adjacent healthy eggs and suffocate them. If the mycotic growth is not removed physically or by chemical treatments, entire lots of eggs may be lost. Adult Pacific salmon enter freshwater as early as 6 mo before their spawning date. Broodfish are usually captured and held in large ponds or raceways until they become sexually matme-sometimes for 3 to 5 mo. Secondary fungal infections on adult salmon frequently become a cause of mortality if therapeutic measures are not taken. Fungal infections among channel catfish are usually associated with bacterial lesions, parasitic infestations, or handling. They are regarded as secondary invaders, rarely as true pathogens of catfish and incubating eggs. The fungus Lagenidium infects larval crustaceans and can cause 100% mortalities among shrimp, crabs, and lobsters. Some infections may become systemic and invade all body organs, others localize in gill tissues. Fusarium spp. have been reported as gill
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infections of shrimp and lobsters (Sindermann, 1977; Sindermann and Lightner, 1988). In oysters and clams, Sirolpidium zoothorum causes systemic infections that rapidly cause mortality among larval populations in artificial rearing systems (Sindermann, 1977; Sindermann and Lightner, 1988). The fungal agents involved in fish culture are described by Post (1983) and Neish and Hughes (1980). Those affecting shellfish are detailed by Sindermann (1977) and Sindermann and Lightner (1988). Trout and salmon are attacked by Saprolegnia and Achlya spp.. catfish by Saprolegnia spp. Shrimp are infected by Lagenidium and Fusarium spp., lobsters by Lagenidium, Fusarium and Haliphthoros spp. In shellfish culture, Sirolpidium and Lubyrinthomyxa spp. cause disease in oysters and clams. Parasltlc Dlseases
Wild fish and shellfish are normal hosts to a wide array of parasitic forms. In nature, hundreds of species have been reported from fish and shellfish, but they seldom affect the survival of populations. Only in occasional cases of hyperparasitism do parasites cause epizootics in nature. In culture situations, conditions are favorable only to relatively few parasite species, but their impact is far greater than it would be in natural waters. The high density of available hosts facilitates easy transmission and enhances the. likelihood of epizootics. Fry, fingerling, or larval stages are exceedingly vulnerable to adverse effects of parasitism. In addition to mortality, parasites may cause cessation of feeding, r e d u d growth, susceptibility to bacterial or fungal pathogens, and physical deformities. It is not the purpose of this discourse to discuss all the parasitic forms that have been observed in aquaculture. Important forms associated with the various types of culture will be listed with general comments concerning their importance. A variety of protozoans are external parasites of finfishes. Most are cosmopolitan in distribution and attack a wide variety of fishes. Each genus is composed of a number of species adapted to particular fish hosts or temperature ranges. Several, such as Ichthyophthirius, Zchthyobodo, and Chilodonella, can cause mortaIity at all life stages, including adulthood (Post, 1983). Ichthyophthiriasis probably causes greater eco-
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nomic losses among catfish than any other parasite in warm-water fish farming (Dupree and Huner, 1984). It is also a serious problem in salmonid culture. Chilodonella also has the potential to cause mortality at all life stages, but this parasite has more narrow temperature ranges in which it is problematic. Cryptokryon is a significant problem in the culture of marine finfishes (Sindermann, 1977). Except for the species listed a b v e , the impact of external protozoans on finfish is primarily on young-of-the-year. Large numbers of fry and fingerlings may die of protozoan parasitism, but the economic loss is not as great as the numbers lost might suggest (Meyer, 1970). Sporozoan parasites are well known among fish and shellfish, Several may cause epizootics with severe numerical and financial losses. However, annual losses to parasites of this type are, at present, not great. Because there is no known treatment, most aquaculture operations where outbreaks occur quarantine and destroy the infected stock and then sterilize the entire culture facility to begin anew with clean eggs or stock Helminths are problematic only in the production of fry, fingerlings, and larvae. Monogenetic trematodes are the type of worm parasites most frequently associated with culture problems. They seldom are a direct cause of mortality but frequently contribute to the death of their hosts due to other infectious diseases. The primary impact of monogeneans is reduced growth, stress, and increased susceptibility to bacterial and fungal pathogens. Although other types of helminths (Digeneans, Cestodes, Nematodes, and Acanthocephalans) commonly occur in fishes and shellfish, they currently do not cause major losses in aquaculture. Copepod parasites cause serious problems in both freshwater and marine aquaculture of finfishes. When they are abundant, they are debilitating and may cause serious emaciation. Wounds caused by feeding or attachment of these parasites are often the loci of secondary bacterial and fungal infections. Copepod parasites are generally not host-specific, so the presence of wild fish in the water supply system or their access to rearing units can introduce these pests. Net pen culture systems incur significant probiems of this nature because of the constant reintroduction of the parasites immediately after treatment.
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In the culture of trout and salmon, the external protozoans Ichthyophthirius, Ichthyobodo, Chilodonellu, and Trichodina cause significant health problems at all life stages (Roberts and Shephard, 1974; Post, 1983; Meyer et al., 1983). Sporozoans also represent serious health concerns among salmonid fishes. Myxosoma cerebralis, the cause of whirling disease, affects young-of-the-year fish, but it may also cause severe economic losses among larger fish because of deformities it may induce in survivors of epizootics. Cerutomyxa shusta infects young salmonids, but its current distribution is limited to the Pacific Northwest. Proliferative kidney disease is a disease of fingerling salmonids that is becoming increasingly important. The etiological agent is believed to be a haplosporidian. Monogenetic trematodes (Gyraiactylus spp. and Ductylogyrus spp.) attack fry and fmgerling trout and salmon and can cause considerable losses if left untreated. Parasitic copepods are serious problems in grow-out ponds, raceways, and net pens. Lernuea is a serious problelil in freshwater cultures; Lepeophiherius attacks fish in marine culture systems. Both will attack fish at the large fingerling stage or older. Producers of channel catfish encounter a variety of parasitic diseases (Meyer, 1970; Dupree and Huner, 1984). The external p r e tozoans Ichthyophrhirius, Ichthyobodo, and Chilodonella attack all life stages. Trichodina is a problem among fry and fingerlings. Sporozoans of the genus Henneguya cause serious disease in fry, fingerlings, and yearling fish. The monogeneans Cleidodiscus and Gyrodactylus pose health threats to fingerling catfish. Copepods are generally not a problem in catfish culture. In crustacean culture, the impacts of parasitic organisms center on early life history stages and on food-sized animals confined in holding pens (Sindermann, 1977). Two external protozoans, Zoothamnium and Epistylis, cause problems in shrimp. Lagenophrys is pathogenic to crabs, and Anophrys attacks Nosema, lobsters. The sporozoans Pleistophoru, and Thelohaniu are capable of causing epizootics and serious economic losses. Control Measures and Therapeutants
In any animal husbandry, measures to prevent the introduction or onset of disease are always the most effective, cost-efficient, and
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long-lasting. Successful preventive measures in aquaculture center on 1) preventing the introduction of pathogens, 2) maintenance of good water quality, 3) avoidance or reduction of environmental stressors (low dissolved oxygen, temperature control, density control, and removal of metabolic wastes), 4) adequate nutrition, 5 ) isolation of cultured animals from feral stocks, and 6) immunization, if available. It has long been recognized that poor water quality, environmental and physiological stressors, and poor nutrition are the primary causes of disease outbreaks. Unfortunately, human errors, imperfect culture systems, and inadequate diets continue to be a part of current aquaculture operations, regardless of the species being cultured. It is inevitable, thus, that diseases will continue to be a limiting factor and that therapeutants will be needed. Chemotherapy should be considered as an emergency or last-resort measure. Although chemicals may reduce the incidence of pathogens or control the abundance of facultative organisms, they also may have negative effects on desirable pond biota and on the flora of biological filters. Some chemicals may be hazardous to the user or leave undesirable or harmful residues in the cultured animals. Proper use of chemotherapeutants begins with an accurate diagnosis of the disease and causative agent. This information must be coupled with a sound understanding of the physical and biological system in which the animals are being reared. When considering a possible drug or chemical, the following questions must be answered: 1. Is the drug registered for use in aquacul-
ture against the etiological agent? 2. What is the toxicity of the drug to the host animal? 3. Will the available methods of treatment deliver effective levels of the drug to the site of infection? 4. What hazards does the drug pose to the Usel? 5. How will the drug affect desirable biota or biological filter systems? 6. Will the drug leave harmful or undesirable residues in the flesh of treated animals? The application of chemicals to animals or to their environment is regulated by the U.S. Food and Drug Administration (FDA) or the
AQUACULTURE AND DISEASE
U.S. Environmental Protection Agency (EPA). All internal and external uses of drugs and anesthetics are controlled by FDA and applications of chemicals and pesticides to the environment are regulated by EPA. The number of drugs and chemicals a p proved for treating diseases of fish and shellfish is limited. Currently, 39 chemicals are approved for use in aquaculture: 9 therapeutants, 4 disinfecting agents, 6 water treatment compounds and tracer dyes, 3 anesthetics, 15 herbicides and algicides, and 2 piscicides (Schnick et al., 1986). Five other compounds (povidone iodine, quatemaq ammonium compounds, potassium permanganate, copper sulfate, and diquat dibromide) are approved for use in treating cutaneous bacterial infections or external parasites (Meyer and Schnick, 1989). Many of the therapeutants used to control diseases in other food animals have shown efficacy against pathogens encountered in fish and shellfish production. Although the research to extend their registration for uses in aquaculture has not been completed, a number of promising drugs is known. Two review papers (Alderman, 1988; Meyer and Schnick, 1989) discuss the efficacy of various compounds and their potential to be registered for aquaculture uses. Before any therapeutant is used for purposes not approved by FDA or EPA, veterinarians or fish health specialists should consult FDA’s Center for Veterinary Medicine, RockviUe, MD for guidance. Implications
An urgent need exists for regulatory approval of therapeutic drugs for use in combatting diseases in aquaculture. Even though aquaculture is a rapidly growing industry, the fact remains that the individual types of culture are still too small to be able to afford the cost of developing their own needed therapeutants. Increased federal support and greater caoperation and involvement by regulatory agencies are vital if producers are to be able to control the economic losses presently caused by diseases. Without such support, the aquaculture industry in the United States will fail to achieve its full potential and be unable to compete in the world market.
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Anonymous. 1989. U.S. catfiihlosses 1988.Water Farming J. 4(3):16. Agonymous. 199Oa. ESC cure may be in disease. Catfish J. 4(6):16. Anonymous. 1990b. S M y shows ESC and Winter kill are top problems. Cagkh News. June, 1990. 4(11):1. Anonymous. 19%. Disease top cause of losses in 1989. Catfrsh J. 4(7):8. Beleau, M.H.and J. A. Plumb. 1987. Channel catfish culture methods used in the United States.Vet. Hum. Toxicol. 29(Suppl. 1):52. Bell, T. A. and D. Lighmer. 1987. An outline of penaeid shrimp culture methods including infectious disease problems and priority drug treatments. Vet. Hum. Toximl. 29(Suppl. 1):37. Brown, C. 1987. An outline of bivalve mollusks culture methods including disease problems and treatments. Vet. Hum. Toxicol. 29(Suppl. 1):35. Busch, R A. 1987. Trout culture methods in the United States. Vet. Hum. Toxicol. 29(Suppl. 1):45. Cvitanich, J., 0. Garate and C. E. Smith. 1990. Etiological agent in a Chilean coho disease isolated and confirmed by Koch’s postulates. Fish Health Section Newsletter, Am. Fish. Soc. 18(1):1. Dupree, H. K. and J. V. Huner. 1984. Third Report to the Fish Farmers. U.S. Fish and Wildlife Service, Washington, Dc. Eaton, W. D. and J. Hulett. 1990. The fourth (and fifth?) isolation of viral hemorrhagic septicemia virus in Washington state. Fish Health Section Newsletter, Am. Fish. SOC. 18(1):3. Harrell, L. W. 1987. Salmonproduction in the United States. Vet. Hum. Toxicol. 29(Suppl. 1):49. Meyer, F. P. 1970. Seasonal fluctuations in the incidence of disease on fish farms. In: S. F. Snieszko (Ed.) A Symposium on Diseases of Fishes and Shellfishes. pp 21-29. Am. Fiih. Soc., Spec. Publ. No. 5., Bethesda, MD. Meyer, F. P. andR. A. Schnick. 1989. A review of chemicals used for the control of fish diseases. Crit. Rev. in Aq. Sci. 1:693. Meyer, F. P.,J. W. Warren and T. G. Carey. 1983. A guide to integrated fsh health management in the Great Lakes basii. Great Lakes Fishery Commission, Spec.Publ. 83-2. Neish, G. A. and G. C. Hnghes. 1980. Diseases of Fishes: Pungal Diseases of Fishes. TF.H. Publications, Neptune, NJ. O’Bamon, B. K. 1987. Fisheries of the United States. U.S. Dept. of Commerce, Current Fishery Statistics No. 8700, 1988. Plumb, J. A. and J. L. Gaines. 1975. Channel catfish virus disease. In: W. E. Ribelin and G. Mgaki (Ed.) The Pathology of Fishes. pp 287-302. Univ. of Wisconsin Press, Madison. Post, G. 1983. Textbookof Fish Health. T.F.H. Publications, Neptune, NJ. Roberts,R. J. 1975. The effects of temperature on diseases and their histopathological manifestations in fish. In: W. E.Ribelin and G.Migaki (Ed.) The Pathology of Fishes. pp 477494. Univ. of Wisconsin Press,
Madison. Literature Cited
Alderman, D. J. 1988. Fisheries Chemotherapy: A review. Ln: J. F.and R. I. Roberts (Ed.) Recent Advances in Aquaculture. pp 1-60. Timber Press, Portland, OR.
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Roberts, R J. 1982. Microbial Diseases of Fish.Academic
Press, New York. Robats. R. J. and C. J. Shepherd. 1974. Handkook of trout and salmon diseases. Fsbing News (Books) Ltd., surrey, UK.
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Schnick,R.A., F. P. Meyer and D. L. Gray. 1986.A guide to approved chemicals in f d production and fshery resource management. US. Fish and Wildlife Service and Univ. of Arkansas CooperativeExtension Service. Little Rock, AR. M P 241-l1M-1-86. Sindermaun, C. J. 1974.Diagnosis and control of mariculture diseases in the United States. Technical Series, No. 2., National Marine Fisheries SnVice, U.S.Dept. of Commerce, Washington, DC. Sindermann, C. J. and D. V. Lightner. 1988. Disease Diagnosis and Control in North American Marine Aquaculture. Elsevier, New York Sindennann, C.J. 1977. Disease Diagnosis and Conlrol in North American Aquaculture. Elsevier, New York. Stewart, B. C., C. Olson, and S. Lutz. 1990. VHS virus detected at Lummi Bay sea ponds, Belhgbatn, Washington. Fish Health Section Newsletter. Fish Heallh Section, Am. Fish. SOC. 18(1):2.
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United States Department of Agriculture. 1989 Trout production. National Agriculhval Statistics Service. SpCr (9-89). Washington, DC. United States Office of Aquaculture. 1986. Compelling masons for the United States to look seriously at aquaculture development. USDA, Washington, DC. Van Dyk, J. 1990. Long journey of the Pacifc salmon. National Geographic 178(1):>37. Wedemeyer, G. 1970. The role of s-s in the disease resistance of fish. In: S. F. Snieszko (Ed.) A Symposium on Diseases of Fishes and Shellfishes.pp 30-35.Am.Fish.Soc., Washington, DC.,Spec.Publ. No. 5, Bethesda, MD. Wedemeyer, G. A., F. P. Meyer and L. Smith. 1976. Diseases of Fishes: Environmental Srress and Fish Diseases. T F H . Publications, Neptune, NJ. Wolf, K.1988.Fish Viruses and Fisb Viral Diseases. Cornell Univ. Press, Ithaca, NY.
U.S. Fish and Wildlife Service, La Crescent, MN 55947 ABSTRACT
Disease problems constitute the largest single cause of economic losses in aquaculture. In 1988, channel catfish producers lost over 100 million fish worth nearly $11 million. Estimates for 1989 predict even higher losses. The trout industry reported 1988 losses of over 20 million fish worth over $2.5 million. No data are available on losses sustained by producers of shellfish. Bacterial infections constitute the most important source of disease problems in all the various types of production. Gram-negative bacteria cause epizootics in nearly all cultured species. Fungal diseases constitute the second most important source of losses, especially in the culture of crustaceans and salmon. External protozoan parasites are responsible for the loss of large numbers of fry and fingerling fin fishes and are a cause of epizootics among young shellfish. The number of therapeutants approved by the Food and Drug Administration is limited. Research to support the registration of promising therapeutic agents is urgently needed. Key Words: Aquaculture, Fish Farming, Fish Diseases, Zcfulurus puncfufus, Trout, Shellfish J. Auim. Sci. 1991. 69:42014208 Introduction
Allied Aquacultures, Auburn University, AuAL). burn, Aquaculture, the water farming of fish and Harvests from natural waters have declined shellfish, represents the fastest growing animal or, at best, remained static (U.S. Office of husbandry industry in the United States and in Aquaculture, 1986). As natural stocks have many other parts of the world. Over 30 aquatic declined, aquaculture has become increasingly species are currently being cultured to produce important as a source of fishery products. The protein as a human food source. U.S. aquaculture industry has five major The demand for fish and shellfish continues facets: trout and salmon, channel catfish, bait to rise. While the per capita consumption of minnows, pet and ornamental fishes, and red meat is declining, that of fish and shellfish shellfish (crustaceans and mollusks). reached a record high in 1986 (the last year for Although the commercial production of which data are availabIe). Consumption has trout and baitfish has existed for many increased annually since 1980 and represents decades, aquaculture as a significant food growth of about 20% over the past 5 yr (E-€. K. industry has developed within the last 25 yr. Dupree, personal communication, U.S. Fish The channel catfish (Zcralurus punctarus) is the and Wildlife Service, Stuttgart, AR). In 1987, major species under culture and the annual the per capita consumption was projected at production exceeds 158 x 106 kg (O’Bannon, 7.2 kg (J. Jensen, School of Fisheries and 1987; B. Drucker, personal communication, National Marine Fisheries Service, Silver Spring,MD). The production of rainbow trout lResented at a symposium titled “Aquaculture in (Oncorhynchus mykiss) and salmon (OnAnimal Science” at the M A S 82nd Annu. Mtg., Ames, chorhynchus and Sdmo spp.) (30 x 106 kg) is IA. followed by other freshwater fin fishes, 2Retired. shrimp, oysters, lobsters, and marine fin fishes. Received August 27, 1990. Accepted April 5, 1991. About one-half of the trout and salmon are 4201
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Many of the pathogens that cause disease in fish and shellfish are facultative forms that are ubiquitous in aquatic systems. In nature, a high percentage of apparently normal and healthy animals harbor potential pathogens without evidence of clinical signs or overt disease (Wedemeyer, 1970). The development of disease in aquaculture systems usually occurs as the end result of a disruption of the normal environment in which the animals are being reared. Unfavorable conditions, such as crowding, temperature fluctuations, inadequate dissolved oxygen, excessive handling, physical abuse, inadequate diets, or toxic substances Losses Caused by Dlseases may stress the animals (Wedemeyer et al., Losses incurred by fish farmers are related 1976). If the level of the stress exceeds the to disease, floods, oxygen depletions, preda- ability to adjust, the effect can be lethal. Less tion, chemical poisoning, theft, and miscellane- severe stresses may affect the rate at which ous causes. Disease is by far the most body defenses act and antibody formation significant factor. Estimates of dollar losses takes place (Roberts, 1975). Thus, “stress” is due to disease are conservative figures because considered to be an important predisposing fingerling fish have a significantly higher factor in most bacterial diseases of fish and value per unit of weight than food-sized fish. shellfish and any situations that result in For example, fingerling trout are priced at “stress” will often be followed by clinical $10.20/kg, compared to $2.40/kg for food- disease. sized fish (USDA, 1989). Also, most economic The culture of crustaceans frequently inloss estimates fail to include the cost of labor, volves short-term holding of dense numbers of interest, lost production time, expenses of animals in pens or tanks before marketing or to treating and disinfecting, and restocking. induce molting. Such concentrations are highly The Ca@sh Journal (Anonymous, 19%) reported that 115 million catfish were lost to stressful and provide ideal conditions for the disease during the first half of 1989. About transmission of pathogens. The most severe 90% of the losses (104 million) were among economic losses in shrimp, lobster, and crab fry and fingerlings. Nearly 4.54 x lo6 kg of cultures often occur during these brief but catfish fry and fingerlings died. They had a critical periods (Sindermann, 1974). Effective programs of fish health manageminimum value of $8 million. Food fish losses ment focus on 1) keeping stressful conditions totaled 2.04 x 106 kg during this time and had to a minimum, 2) prevention of the introduca market value of $3.6 million. If the same rates of loss continued in the second half of the tion of pathogens, 3) prompt use of effective year, the total cost of losses to disease in 1989 drugs, and 4) use of vaccines when available. It is axiomatic that well-nourished fish reared would reach $23 million. The catfish industry reported that 39% of in highly favorable environmental conditions the 100 million fish lost in the 1988 produc- will be resistant to most pathogens tion season were killed by disease (Anony- (Wedemeyer et al., 1976). In many cases, mous, 1989). In economic terms, at $1.75/kg, prompt reduction of the stressful conditions the 6.2 x 106 kg of catfish killed by disease in may lead to selfcures without the need to 1988 represented a minimum loss of nearly resort to chemotherapy. Obligate fish pathogens can cause disease $11 million. The trout industry reported losses during even among healthy fish reared in good 1988 of 20.7 million fish, 50% of which were environmental conditions. Strict measures are lost to disease (USDA, 1989). The 1.04 x 106 needed to prevent the introduction of such kg of trout lost to disease cost producers a organisms via infected fish or contaminated water. In fish hatcheries, the best action for minimum of $2.5 million (at $2.40/kg). No economic data are available concerning dealing with diseases of this type may be to the extent of losses incurred in crustacean and remove all live fish, to thoroughly disinfect the molluskan culture systems. facility, equipment, and water supply, and to slaughtered and processed directly for the table. The remainder are used to stock fishing waters, for stock to be grown to food sizes, or to stock ponds where anglers pay for recreational fishing (USDA, 1989). It is estimated that 30% of all salmon now consumed are produced on fish farms and that by the year 2000 farm production may exceed the wild harvest (Van Dyk, 1990). Presently, 60% of fish and shellfish products consumed in the US. are imported (J. Jensen, personal communication).
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begin anew with clean fish or eggs from a necrosis, and viral hemorrhagic septicemia are disease-free source. the major viral diseases of trout and salmon Unfortunately, few production facilities are (Post, 1983; Wolf, 1988). able to maintain ideal rearing conditions and Infectious hematopoietic necrosis causes adverse circumstances may arise in spite of the serious problems in salmonid culture and it is best efforts of fish culturists. An epizootic estimated that this disease costs trout among the cultured stock, if left untreated, producers over $5 million dollars annually (R. may cause serious economic losses. Prompt Busch, personal communication, Biomed, Inc., medication with an effective drug is required Bellevue, WA). in conjunction with corrective action to reduce Viral hemorrhagic septicemia affects fish in the predisposing, stressful conditions. their second year of life, as well as fry and Overviews of culture systems and disease fingerlings, so it can cause serious economic problems associated with the various types of losses, as well as the death of many fish. Viral animals produced in aquaculture may be found hemorrhagic septicemia is endemic to Europe. in the following references: trout, Busch Despite rigorous attempts to prevent its intro(1987); salmon, Harrell (1987); catfish, Beleau duction into North American waters, the and Plumb (1987); shrimp, Bell and Lightner disease was discovered in 1988 and 1989 (1987); and mollusks, Brown (1987). among Pacific salmon in Washington (Eaton The scope of this workshop is directed and Hulett, 1990; Stewart et al., 1990). The toward food animals. Discussions of diseases virus has been detected among hatchery-reared among fish and shellfish will thus be limited to “salmonids” and retuming adult coho salmon the problems of species produced for human (Oncorhynchus kisutch) from three hatcheries consumption. Persons interested in baitfishes, and two rivers (Eaton and Hulett, 1990). pet fishes, or ornamental fishes wiII find that Current efforts to control the disease involve many of the diseases of catfish are also quarantine and destruction of all eggs and fry common to these species. known to have been exposed to the virus or to have originated from infected adult fish. To Vlral Diseases date, no epizootics have been reported, but the Viral diseases cause serious problems in occurrence of viral hemorrhagic septicemia every aspect of aquaculture. If precautions are disease has serious adverse implications for not taken to prevent the introduction of viral aquaculture production of salmonids in the agents, severe economic losses can occur. In Pacific Northwest. ChanneI catfish virus seems to be endemic viral diseases, regardless of the species being cultured, the only recourse is to quarantine and throughout the South Central and Southeastern destroy the infected stock. Rigorous cleaning m s of the United States. Although the virus followed by disinfection of all facilities, seems to be present in most cultured stocks of equipment, and water supplies must precede catfish, expression of the disease seems related further attempts to produce animals. When to stressful environmental conditions, the presstarting anew, the culturist must have access to ence of infected adult fish in the water distribution system, and the presence of a vhs-free stocks (Wolf, 1988). In aquaculture, the presence of a viral bacterial copathogen. When epizootics occur, disease in endemic wild stocks can pose a losses among fry and fingerlings can be very virtually insurmountable obstacle to net-pen high, but outbreaks of the disease have been farming of animals in open waters. Likewise, only sporadic and have not limited the the entry of wild “carrier” animals into the commercial production of catfish (Plumb and water supply of artificial culture systems is Gaines, 1975; Post, 1983; Wolf, 1988; Anonylikely to introduce viral agents that can cause mous, 199Ob). In shellfish culture, viral diseases have not mortalities as high as 90% or more in been a major cause of losses, but a number of susceptible stocks. Among trout and salmon, viral diseases viruses have been associated with epizootics. have received extensive study. Most are The diseases include herpes-like viral disease serious threats to the survival of fry and and reo-like viral disease of shrimp and crabs; fingerlings and have caused major losses baculovirus disease of shrimp; and herpes-type among young-of-the-year fishes. Infectious virus disease of oysters (Sindermann, 1977; hematopoietic necrosis, infectious pancreatic Sindermann and Lightner, 1988).
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Bacterial pathogens probably cause more disease problems overall than all other causes combined. In virtually every type of aquaculture, bacterial diseases rank number one among etiological agents. Septicemias, cutane ous lesions, and destruction of the shell are among the manifestations of bacterial infections. In each type of culture and for virtually every species, specific bacterial pathogens are responsible for serious disease problems. Gram-negative bacilli are the most frequent cause of bacterial diseases in finfish. Although only a few Gram-positive forms affect finfish, such bacteria cause serious diseases among crustaceans. The major bacterial diseases of trout and salmon include furunculosis (Aeromnas salmonicida), bacterial hemorrhagic septicemia (A. hydrophila), vibriosis (Vibrio spp.), enteric redmouth disease (Yersinia ruckerii, columnaris disease (Cytophaga columnaris), bacterial gill disease syndrome, and bacterial kidney disease (Renibacterium salmoninarum). Other serious, but less common, diseases are caused by Cytophaga spp., Nocardia spp., Mycobacterium sp., Streptococcus sp., Pseudomonas spp., Flavobacterium sp. (Roberts, 1982; Post, 1983). and Pasteurella sp. (Sindermm, 1977; Sindermann and Lightner, 1988). A new bacterial disease, salmonid rickettsial septicemia, has appeared among coho salmon (Oncorhynchus Kisutch) on commercial fish farms in Chile. Losses in 1989 were estimated at 1.5 million, 5- to 10-kg fish being reared to market size. Mortalities at rearing facilities reached 70%. The cause of the disease is an unidentified Gram-negative, pleomorphic coccus considered to be a rickettsia (Cvitanich et al., 1990). In catfish culture, enteric septicemia disease of cattlsh (caused by Edwardsiella ictaluro has emerged as the principle disease problem. Some workers estimate that this disease alone costs fish farmers over $10 million annually (J. Jensen, personal communication; Anonymous 19!30a,b). If the estimate is accurate, Edwardsiella may cause nearly half of all economic losses due to disease incurred by catfish farmers. Other major bacterial diseases of catfish include bacterial hemorrhagic septice mia (A. hydrophila), columnaris disease (C. columnaris), and Edwardsiellosis (E. tar&).
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In crustacean culture (shrimp and lobsters), shelldestroying bacteria and septicemic infections cause the most serious problems. Shell diseases are caused by kucothrix, Benecka, Vibrio, and Pseudomonas spp. Septicemic diseases are caused by Vibrio, Aerococcus, and Pseudomonas spp. (Sindermann, 1977; Sindermann and Lightner, 1988). Filamentous bacterial disease (kucothrix spp.) is a common but less serious disease of shrimp. Bacterial diseases among oysters and clams occur, but they are not major causes of economic losses in this type of aquaculture. Bacillary necrosis caused by Vibrio spp. affects both clams and oysters (Sindermann, 1977). Fungal Diseases
Fungal growths on the surface of eggs and larvae of fish and shellfish can cause extensive direct mortalities. They also occur as common secondary invaders in wounds, lesions, or abrasions caused by bacterial pathogens, parasitic organisms, abusive handling, or unfavorable environmental conditions. Some fungal agents are primary pathogens, especially of crustaceans, and they are the cause of mortalities in rearing units. In incubating fish eggs, dead eggs provide a fertile substrate for fungal growths. If dead eggs are not removed, resultant fungal growths may cover adjacent healthy eggs and suffocate them. If the mycotic growth is not removed physically or by chemical treatments, entire lots of eggs may be lost. Adult Pacific salmon enter freshwater as early as 6 mo before their spawning date. Broodfish are usually captured and held in large ponds or raceways until they become sexually matme-sometimes for 3 to 5 mo. Secondary fungal infections on adult salmon frequently become a cause of mortality if therapeutic measures are not taken. Fungal infections among channel catfish are usually associated with bacterial lesions, parasitic infestations, or handling. They are regarded as secondary invaders, rarely as true pathogens of catfish and incubating eggs. The fungus Lagenidium infects larval crustaceans and can cause 100% mortalities among shrimp, crabs, and lobsters. Some infections may become systemic and invade all body organs, others localize in gill tissues. Fusarium spp. have been reported as gill
AQUACULTURE AND DISEASE
infections of shrimp and lobsters (Sindermann, 1977; Sindermann and Lightner, 1988). In oysters and clams, Sirolpidium zoothorum causes systemic infections that rapidly cause mortality among larval populations in artificial rearing systems (Sindermann, 1977; Sindermann and Lightner, 1988). The fungal agents involved in fish culture are described by Post (1983) and Neish and Hughes (1980). Those affecting shellfish are detailed by Sindermann (1977) and Sindermann and Lightner (1988). Trout and salmon are attacked by Saprolegnia and Achlya spp.. catfish by Saprolegnia spp. Shrimp are infected by Lagenidium and Fusarium spp., lobsters by Lagenidium, Fusarium and Haliphthoros spp. In shellfish culture, Sirolpidium and Lubyrinthomyxa spp. cause disease in oysters and clams. Parasltlc Dlseases
Wild fish and shellfish are normal hosts to a wide array of parasitic forms. In nature, hundreds of species have been reported from fish and shellfish, but they seldom affect the survival of populations. Only in occasional cases of hyperparasitism do parasites cause epizootics in nature. In culture situations, conditions are favorable only to relatively few parasite species, but their impact is far greater than it would be in natural waters. The high density of available hosts facilitates easy transmission and enhances the. likelihood of epizootics. Fry, fingerling, or larval stages are exceedingly vulnerable to adverse effects of parasitism. In addition to mortality, parasites may cause cessation of feeding, r e d u d growth, susceptibility to bacterial or fungal pathogens, and physical deformities. It is not the purpose of this discourse to discuss all the parasitic forms that have been observed in aquaculture. Important forms associated with the various types of culture will be listed with general comments concerning their importance. A variety of protozoans are external parasites of finfishes. Most are cosmopolitan in distribution and attack a wide variety of fishes. Each genus is composed of a number of species adapted to particular fish hosts or temperature ranges. Several, such as Ichthyophthirius, Zchthyobodo, and Chilodonella, can cause mortaIity at all life stages, including adulthood (Post, 1983). Ichthyophthiriasis probably causes greater eco-
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nomic losses among catfish than any other parasite in warm-water fish farming (Dupree and Huner, 1984). It is also a serious problem in salmonid culture. Chilodonella also has the potential to cause mortality at all life stages, but this parasite has more narrow temperature ranges in which it is problematic. Cryptokryon is a significant problem in the culture of marine finfishes (Sindermann, 1977). Except for the species listed a b v e , the impact of external protozoans on finfish is primarily on young-of-the-year. Large numbers of fry and fingerlings may die of protozoan parasitism, but the economic loss is not as great as the numbers lost might suggest (Meyer, 1970). Sporozoan parasites are well known among fish and shellfish, Several may cause epizootics with severe numerical and financial losses. However, annual losses to parasites of this type are, at present, not great. Because there is no known treatment, most aquaculture operations where outbreaks occur quarantine and destroy the infected stock and then sterilize the entire culture facility to begin anew with clean eggs or stock Helminths are problematic only in the production of fry, fingerlings, and larvae. Monogenetic trematodes are the type of worm parasites most frequently associated with culture problems. They seldom are a direct cause of mortality but frequently contribute to the death of their hosts due to other infectious diseases. The primary impact of monogeneans is reduced growth, stress, and increased susceptibility to bacterial and fungal pathogens. Although other types of helminths (Digeneans, Cestodes, Nematodes, and Acanthocephalans) commonly occur in fishes and shellfish, they currently do not cause major losses in aquaculture. Copepod parasites cause serious problems in both freshwater and marine aquaculture of finfishes. When they are abundant, they are debilitating and may cause serious emaciation. Wounds caused by feeding or attachment of these parasites are often the loci of secondary bacterial and fungal infections. Copepod parasites are generally not host-specific, so the presence of wild fish in the water supply system or their access to rearing units can introduce these pests. Net pen culture systems incur significant probiems of this nature because of the constant reintroduction of the parasites immediately after treatment.
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In the culture of trout and salmon, the external protozoans Ichthyophthirius, Ichthyobodo, Chilodonellu, and Trichodina cause significant health problems at all life stages (Roberts and Shephard, 1974; Post, 1983; Meyer et al., 1983). Sporozoans also represent serious health concerns among salmonid fishes. Myxosoma cerebralis, the cause of whirling disease, affects young-of-the-year fish, but it may also cause severe economic losses among larger fish because of deformities it may induce in survivors of epizootics. Cerutomyxa shusta infects young salmonids, but its current distribution is limited to the Pacific Northwest. Proliferative kidney disease is a disease of fingerling salmonids that is becoming increasingly important. The etiological agent is believed to be a haplosporidian. Monogenetic trematodes (Gyraiactylus spp. and Ductylogyrus spp.) attack fry and fmgerling trout and salmon and can cause considerable losses if left untreated. Parasitic copepods are serious problems in grow-out ponds, raceways, and net pens. Lernuea is a serious problelil in freshwater cultures; Lepeophiherius attacks fish in marine culture systems. Both will attack fish at the large fingerling stage or older. Producers of channel catfish encounter a variety of parasitic diseases (Meyer, 1970; Dupree and Huner, 1984). The external p r e tozoans Ichthyophrhirius, Ichthyobodo, and Chilodonella attack all life stages. Trichodina is a problem among fry and fingerlings. Sporozoans of the genus Henneguya cause serious disease in fry, fingerlings, and yearling fish. The monogeneans Cleidodiscus and Gyrodactylus pose health threats to fingerling catfish. Copepods are generally not a problem in catfish culture. In crustacean culture, the impacts of parasitic organisms center on early life history stages and on food-sized animals confined in holding pens (Sindermann, 1977). Two external protozoans, Zoothamnium and Epistylis, cause problems in shrimp. Lagenophrys is pathogenic to crabs, and Anophrys attacks Nosema, lobsters. The sporozoans Pleistophoru, and Thelohaniu are capable of causing epizootics and serious economic losses. Control Measures and Therapeutants
In any animal husbandry, measures to prevent the introduction or onset of disease are always the most effective, cost-efficient, and
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long-lasting. Successful preventive measures in aquaculture center on 1) preventing the introduction of pathogens, 2) maintenance of good water quality, 3) avoidance or reduction of environmental stressors (low dissolved oxygen, temperature control, density control, and removal of metabolic wastes), 4) adequate nutrition, 5 ) isolation of cultured animals from feral stocks, and 6) immunization, if available. It has long been recognized that poor water quality, environmental and physiological stressors, and poor nutrition are the primary causes of disease outbreaks. Unfortunately, human errors, imperfect culture systems, and inadequate diets continue to be a part of current aquaculture operations, regardless of the species being cultured. It is inevitable, thus, that diseases will continue to be a limiting factor and that therapeutants will be needed. Chemotherapy should be considered as an emergency or last-resort measure. Although chemicals may reduce the incidence of pathogens or control the abundance of facultative organisms, they also may have negative effects on desirable pond biota and on the flora of biological filters. Some chemicals may be hazardous to the user or leave undesirable or harmful residues in the cultured animals. Proper use of chemotherapeutants begins with an accurate diagnosis of the disease and causative agent. This information must be coupled with a sound understanding of the physical and biological system in which the animals are being reared. When considering a possible drug or chemical, the following questions must be answered: 1. Is the drug registered for use in aquacul-
ture against the etiological agent? 2. What is the toxicity of the drug to the host animal? 3. Will the available methods of treatment deliver effective levels of the drug to the site of infection? 4. What hazards does the drug pose to the Usel? 5. How will the drug affect desirable biota or biological filter systems? 6. Will the drug leave harmful or undesirable residues in the flesh of treated animals? The application of chemicals to animals or to their environment is regulated by the U.S. Food and Drug Administration (FDA) or the
AQUACULTURE AND DISEASE
U.S. Environmental Protection Agency (EPA). All internal and external uses of drugs and anesthetics are controlled by FDA and applications of chemicals and pesticides to the environment are regulated by EPA. The number of drugs and chemicals a p proved for treating diseases of fish and shellfish is limited. Currently, 39 chemicals are approved for use in aquaculture: 9 therapeutants, 4 disinfecting agents, 6 water treatment compounds and tracer dyes, 3 anesthetics, 15 herbicides and algicides, and 2 piscicides (Schnick et al., 1986). Five other compounds (povidone iodine, quatemaq ammonium compounds, potassium permanganate, copper sulfate, and diquat dibromide) are approved for use in treating cutaneous bacterial infections or external parasites (Meyer and Schnick, 1989). Many of the therapeutants used to control diseases in other food animals have shown efficacy against pathogens encountered in fish and shellfish production. Although the research to extend their registration for uses in aquaculture has not been completed, a number of promising drugs is known. Two review papers (Alderman, 1988; Meyer and Schnick, 1989) discuss the efficacy of various compounds and their potential to be registered for aquaculture uses. Before any therapeutant is used for purposes not approved by FDA or EPA, veterinarians or fish health specialists should consult FDA’s Center for Veterinary Medicine, RockviUe, MD for guidance. Implications
An urgent need exists for regulatory approval of therapeutic drugs for use in combatting diseases in aquaculture. Even though aquaculture is a rapidly growing industry, the fact remains that the individual types of culture are still too small to be able to afford the cost of developing their own needed therapeutants. Increased federal support and greater caoperation and involvement by regulatory agencies are vital if producers are to be able to control the economic losses presently caused by diseases. Without such support, the aquaculture industry in the United States will fail to achieve its full potential and be unable to compete in the world market.
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Anonymous. 1989. U.S. catfiihlosses 1988.Water Farming J. 4(3):16. Agonymous. 199Oa. ESC cure may be in disease. Catfish J. 4(6):16. Anonymous. 1990b. S M y shows ESC and Winter kill are top problems. Cagkh News. June, 1990. 4(11):1. Anonymous. 19%. Disease top cause of losses in 1989. Catfrsh J. 4(7):8. Beleau, M.H.and J. A. Plumb. 1987. Channel catfish culture methods used in the United States.Vet. Hum. Toxicol. 29(Suppl. 1):52. Bell, T. A. and D. Lighmer. 1987. An outline of penaeid shrimp culture methods including infectious disease problems and priority drug treatments. Vet. Hum. Toximl. 29(Suppl. 1):37. Brown, C. 1987. An outline of bivalve mollusks culture methods including disease problems and treatments. Vet. Hum. Toxicol. 29(Suppl. 1):35. Busch, R A. 1987. Trout culture methods in the United States. Vet. Hum. Toxicol. 29(Suppl. 1):45. Cvitanich, J., 0. Garate and C. E. Smith. 1990. Etiological agent in a Chilean coho disease isolated and confirmed by Koch’s postulates. Fish Health Section Newsletter, Am. Fish. Soc. 18(1):1. Dupree, H. K. and J. V. Huner. 1984. Third Report to the Fish Farmers. U.S. Fish and Wildlife Service, Washington, Dc. Eaton, W. D. and J. Hulett. 1990. The fourth (and fifth?) isolation of viral hemorrhagic septicemia virus in Washington state. Fish Health Section Newsletter, Am. Fish. SOC. 18(1):3. Harrell, L. W. 1987. Salmonproduction in the United States. Vet. Hum. Toxicol. 29(Suppl. 1):49. Meyer, F. P. 1970. Seasonal fluctuations in the incidence of disease on fish farms. In: S. F. Snieszko (Ed.) A Symposium on Diseases of Fishes and Shellfishes. pp 21-29. Am. Fiih. Soc., Spec. Publ. No. 5., Bethesda, MD. Meyer, F. P. andR. A. Schnick. 1989. A review of chemicals used for the control of fish diseases. Crit. Rev. in Aq. Sci. 1:693. Meyer, F. P.,J. W. Warren and T. G. Carey. 1983. A guide to integrated fsh health management in the Great Lakes basii. Great Lakes Fishery Commission, Spec.Publ. 83-2. Neish, G. A. and G. C. Hnghes. 1980. Diseases of Fishes: Pungal Diseases of Fishes. TF.H. Publications, Neptune, NJ. O’Bamon, B. K. 1987. Fisheries of the United States. U.S. Dept. of Commerce, Current Fishery Statistics No. 8700, 1988. Plumb, J. A. and J. L. Gaines. 1975. Channel catfish virus disease. In: W. E. Ribelin and G. Mgaki (Ed.) The Pathology of Fishes. pp 287-302. Univ. of Wisconsin Press, Madison. Post, G. 1983. Textbookof Fish Health. T.F.H. Publications, Neptune, NJ. Roberts,R. J. 1975. The effects of temperature on diseases and their histopathological manifestations in fish. In: W. E.Ribelin and G.Migaki (Ed.) The Pathology of Fishes. pp 477494. Univ. of Wisconsin Press,
Madison. Literature Cited
Alderman, D. J. 1988. Fisheries Chemotherapy: A review. Ln: J. F.and R. I. Roberts (Ed.) Recent Advances in Aquaculture. pp 1-60. Timber Press, Portland, OR.
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Roberts, R J. 1982. Microbial Diseases of Fish.Academic
Press, New York. Robats. R. J. and C. J. Shepherd. 1974. Handkook of trout and salmon diseases. Fsbing News (Books) Ltd., surrey, UK.
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