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Management of Fish


When working with fish, disease prevention is always more rewarding than treatment. Once fish are sick, accurately identifying all problems present can be difficult, and treatment must be administered early in the course of an epizootic to be effective. In most cases, a comprehensive program of fish health management should be based on the principles of water quality, nutrition, sanitation, and quarantine. Efforts to maintain as clean an environment as possible including minimal accumulation of organic debris, proper disinfection of nets and equipment, and thorough disinfection of fish-holding units between groups of fish will help minimize disease outbreaks.

Sanitation of tanks and equipment starts with general cleaning and removal of organic debris. Household bleach (3–6% NaHClO) delivered at 35 mL/gal. of water creates a concentration of 200 mg/L. A 1-hr contact time at this concentration will destroy most organisms of concern, including most viruses. Bleach should not be used in closed areas containing live fish as the volatile compound may get into solution and kill fish in nearby tanks. Mycobacteria are refractory to bleach disinfection because of their waxy cell wall. Spraying equipment and contact surfaces with alcohol following treatment with bleach should be effective in eliminating mycobacteria. Quaternary ammonium compounds are also excellent disinfectants and can be used at concentrations of 500 mg/L for 1 hr.


Fish are poikilothermic, and all physiologic processes are greatly influenced by water temperature. In freshwater, the internal tissues of fish are hyperosmotic, whereas in saltwater they are hypoosmotic. Surface injuries to the skin make osmoregulation more difficult and may be of serious consequence due to loss of fluid balance and circulatory collapse.

Fish lack organized lymph nodes and Kuppfer's cells. Phagocytic tissue is located in the hematopoietic tissue of the spleen and kidney and often in the atrium of the heart. The structure of the fish kidney varies with the species; generally it is divided into an anterior “head” kidney and a posterior “caudal” kidney, located retroperitoneally, ventral to the vertebral column. Hematopoietic, renal, and endocrine tissues are found in the kidney, with hematopoietic tissue located cranially and excretory tissue located caudally. Divalent ions are excreted principally via the kidney, and monovalent ions and nitrogenous excretions via the gills. Accordingly, lesions of the kidney and gills may seriously interfere with respiration, excretion, and fluid balance.

The swim bladder in bony fish, which originates as an appendage of the foregut, regulates body buoyancy and may also be used for sound production. Physostomas fish have an open connection between the swim bladder and the GI tract, whereas physoclistis fish do not. Gas is either secreted by or absorbed into the swim bladder to maintain buoyancy or specific gravity and balance. The swim bladder is 2-chambered in koi and goldfish and 3-chambered in cod. A sensory lateral line system along the sides of the body and head receives stimuli from the aquatic environment and mediates adaptive responses through the CNS.

Fig. 1

A humoral antibody system occurs in all fish but varies considerably between classes. Although antibody production often is temperature dependent, specific serum antibodies can be demonstrated. B lymphocytes, found in the spleen and liver, are responsible for production of immunoglobulins found in the serum and tissue fluids of fish. However, fish lack the potent immunoglobulins similar to IgG of other animals. Fish do increase production of IgM, similar to other vertebrates, when responding immunologically to many infectious agents. Fish depend on increases in environmental temperature for efficient antibody production during infections (or after vaccinations), when most pathogens are replicating at a more rapid rate. The optimal temperature for antibody production varies with the species of fish (warmwater or coldwater). Extremes in environmental temperature (above or below that of the natural habitat) inhibit antibody production. T lymphocytes of fish, like those of other vertebrates, are responsible for cell-mediated immunity. Immunity is not as age dependent in fish as it is in other animals; young fish are usually immunocompetent and can be vaccinated successfully. Antibodies are found in the mucus of the fish skin and GI tract.

While anamnestic immunologic responses have been documented in fish, the duration of acquired immunity appears limited. Immunity lasts longer with individual parenteral administration of antigens than with mass bath methods. Although vaccination of fish against specific diseases has been economically important in preventing losses, there is a need for improved methodology. Advances include increased use of autogenous vaccines; several companies will work with veterinarians and their clients to develop custom vaccines for specific situations. A few vaccines (eg, Aeromonas salmonicida) are available or in development for pet fish, particularly koi.


As with all species, a good history is critical in establishing a diagnosis. Questions of particular interest for fish cases include the number of animals affected, whether one species or multiple, the chronicity of the problem, and a thorough description of animal housing and care including the volume and design of the system, number and size of animals stocked, species, new additions, use of quarantine, and previous medications. Clients can be asked to bring fish and water samples into the clinic, or the practitioner may wish to visit the site. Site visits allow the system to be more accurately evaluated, and the behavior of fish is readily observed. If fish are to be brought into the clinic the client should provide an animal showing the signs of concern. A live animal can be transported in a cooler with a battery powered aerator. A separate water sample should be provided in a plastic bag and transported on ice. A minimum of 1 quart of tank water should be requested for analysis. If desired for recovery after anesthetizing the animal, a larger volume of tank water may be requested. Recently deceased specimens have diagnostic value and may be submitted to the veterinary clinic or the client may be directed to submit these to a lab experienced in fish necropsy and diagnostic testing (see Necropsy and Diagnostic Techniques). Water samples should also be submitted with necropsy specimens.

Necropsy and Diagnostic Techniques

While the same principles are used in necropsy of fish as in other animals, great emphasis is placed on an accurate and thorough history, premortem signs, fresh necropsy material, and direct microscopic examination of fresh tissue smears and squash preparations. Fish decompose quickly, and many saprophytic microorganisms reproduce rapidly in the decaying tissues, which complicates isolation of pathogens unless samples are collected immediately after death. A general fish necropsy may include blood collection (premortem); biopsy of gill, skin, and fin tissues; bacterial or viral culture of internal organs; and histology. A veterinary clinic or diagnostic facility that is familiar with fish necropsy protocols and aquatic microbiology should be used. Whenever possible, fish should be submitted alive. If the fish has just died, the eyes should be clear and the gills normal in coloration and texture. There should be no “dead fish” odor. Freshly dead fish can be wrapped in a moist paper towel, placed in a plastic bag, and submitted on ice. A water sample should always be submitted with the fish. An animal that has died and been placed in a freezer has limited diagnostic value, but freshly dead frozen fish may be useful for bacteriology, virology, or toxicology testing.

Fresh tissue samples of gill filaments, skin mucus, and fins should be collected, prepared as a wet mount, and examined under a light microscope at 40×, 100×, and 400×. Fresh water should be used to prepare wet mounts of external tissues from freshwater fish, and saltwater should be used to prepare wet mounts from marine fish. If uncertain, use water from the tank or from the submitted water sample. Ensuring that the salinity used to prepare mounts is similar to the salinity present in the environment should allow organisms to remain viable long enough for identification. Tissue should be examined for morphology and for the presence of parasites, fungi, or bacteria. Microscopic examination of internal organs is also recommended if the fish has been euthanized. Unstained sections of stomach and intestine should be examined for the presence of parasites. Unstained sections of spleen, kidney, and liver should be examined for the presence of parasites, granulomas, or other anomalies.

Blood can be collected from the caudal vein of fish larger than 25–100 g, depending on species, and hematologic parameters measured. For smaller specimens that are to be euthanized, blood can be collected in a hematocrit tube immediately following euthanasia by severing the caudal peduncle and exposing the caudal vein. Use of hematology and serum chemistry is limited because normal values are not readily available; however, the information may still be clinically useful. Lithium heparin is the anticoagulant of choice for fish blood, and plasma may be used for biochemistry tests. Serology may be diagnostic in certain cases (eg, heavy metal toxicity). Whole blood (1–2 drops) can be incubated in brain heart infusion broth at room temperature on an electric rotator. If cloudiness indicative of bacterial growth develops, a loop of the blood-broth mix can be used to attempt primary isolation of a systemic bacterial pathogen.

Fish should be euthanized and opened under sterile conditions. A sterile swab of posterior kidney, or other organ of interest, may be shipped to a lab in transport media, but primary isolation directly onto an enriched media (ie, tryptic soy agar enriched with 5% sheep's blood) is preferable. Although blood agar supplemented with salt is helpful for marine fish, it is not necessary if an enriched blood agar is used. Ordal's or similar cytophage media should be available for isolation of myxobacteria (slime bacteria, including Flavobacterium columnarae). Sabouraud's is an excellent all-purpose media for the isolation of fungal agents. Lowenstein's or Middlebrook media are recommended for isolation of Mycobacteria spp. Mueller-Hinton is the media of choice for sensitivity testing of most common bacteria isolated from fish. If abscesses or other obvious anomalies are visible, those sites should also be cultured. As a general rule, bacterial or fungal cultures taken from fish tissue should be incubated at room temperature (25°C). Some agents of concern will not grow at all at 37°C, the standard temperature for incubation of cultures taken from mammals. Agents of zoonotic concern, such as Mycobacteria, can be dual incubated at both 25 and 37°C. An acid-fast stain should be available for bench-top staining of granulomatous material which, when positive, is strongly suggestive of Mycobacteria. If fish are seen spinning before death, or showing other behavioral indications of neurologic disease, brain cultures are indicated.

If viral disease is suspected, appropriate tissues may be collected and frozen for laboratory submission. Several viral diseases are of regulatory concern to veterinarians practicing fish medicine in the USA (See Table: Fish Diseases of Regulatory Concern in the USATables).

Table 1

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Therapeutic Considerations

Therapy for pet and ornamental fish is often based on environmental management followed by the use of targeted therapy to control specific pathogens. Use of prophylactic medication in the absence of diagnostic testing is strongly discouraged and may contribute to resistant bacterial infection and other complications.

Drug therapy can be provided via several routes of administration, including exposure by bath (adding medication to water), medicated feed, injection, or topical administration. Generally speaking, bath and topical treatments are most useful for external infections, whereas medicated food and injection are most appropriate for internal infections.

Using a bath treatment requires accurate measurement of the volume of water to be treated. To calculate volume in a rectangular tank, measure length, width, and depth, and multiply the 3 measurements. To calculate volume of a circular tank, multiply 3.14 by the radius squared by the depth. To calculate directly into liters, measure in cm and multiply by 0.001. If the measurements are made in feet, the result will be in cubic feet; to convert cubic feet to gallons multiply by 7.481. If treating an odd-shaped container, volume may be calculated mathematically, but it may be easier to purchase a flow meter to measure the volume required to fill the tank. Alternatively, the volume of in-flowing water per minute can be measured by determining how long it takes to fill a 1-L graduated cylinder (or 5-gal. bucket). Using that information, determining how long it takes to fill a tank or ornamental pond can provide a fairly accurate assessment of volume.

Some medicated feeds can be purchased commercially for aquaculture or pet fish. Custom-made medicated feeds can be made for use in ornamental fish. Flake, pellet, or gel diets can be used as a base for medicated food for pet fish. Cooking oil spray is an effective binder for use with pelleted or flake foods for ornamental fish. The addition of medication to commercially available gel diets is easily done as the gel cools. In general, medication should not be added when the gelatin is hot, as some medications, particularly oxytetracycline, are heat labile.

Injections can be given either IM or IP. IM injections can be given in the epaxial muscles, lateral to the dorsal fin. IP injections can be given anterior to the pelvic fins, just off the ventral midline. Topical treatments, usually in ointment form, should be applied directly to the lesion of concern. The fish can be manually restrained, usually without removing the entire animal from the water. The area to which the treatment is applied should be held out of the water for several seconds (<1 min) to allow some absorption before returning the fish to the water. Repeated applications may be necessary.

Sunburn can occur in surface-swimming fish or can be induced (even in bottom-dwelling species) by feeding photodynamic drugs such as phenothiazine, although ultraviolet light penetrates water poorly. Affected fish will have visible lesions along the dorsal surface. Providing shade solves the problem.

FDA-approved therapeutic options in fish are limited; however, there has been significant progress in new approvals in the past few years and this trend is expected to continue. The FDA web site (FDA web site) is the best resource for current information on the status of drugs and chemicals, particularly those intended for aquaculture use. FDA-approved drugs commercially available for use in food fish are listed in see FDA-Approved Drugs for Aquaculture Use in the USA (2007)Tables. In addition, the FDA has listed several compounds as being of “low regulatory concern.” These compounds, though not fully approved, are considered innocuous enough for use in food fish. Of these, salt is the most important. A few compounds, including copper sulfate and potassium permanganate, are not FDA-approved, but are used in aqua-culture under the provision of “moderate regulatory concern.” Finally, there are several non-FDA approved compounds that are used in pet fish practice under controlled conditions. These have no legal status at present and have no place in food animal practice. In addition to being aware of FDA concerns, fish practitioners should be familiar with environmental regulations. Federal and state environmental regulations are of greatest concern when treating outdoor ponds.

Table 2

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In many cases, therapeutic management of fish other than catfish or salmonids requires extra-label use of drugs. A new mechanism called indexing is being developed by the FDA to allow for legal use of nonapproved drugs in ornamental fish. The FDA web site ( provides the most current information. Information provided here is intended for use in pet fish medicine.

There are 3 FDA-approved antibiotics currently available for use in aquacultured food fish in the USA. Some of these products can be useful in ornamental fish, especially koi, however extra-label use of FDA-approved medicated feeds is not permitted under current law. Recognizing the need to deliver medications in a medicated food for fish, the FDA has indicated that it would not normally consider regulatory action against a veterinarian using medicated fish food in an off-label manner if the following criteria are met: 1) the extra-label use is for treatment of a minor species as defined by federal law; 2) in an aquatic species, the use of medicated feed in an off-label manner is limited to products approved for use in other aquatic species; and 3) a valid veterinarian-client-patient relationship is clearly established.

Veterinarians should also be familiar with the Veterinary Feed Directive (VFD). This is of particular interest to practitioners in rural areas, who may be asked to write prescriptions for aquacultured fish. In order for a veterinarian to issue a VFD order, there must be a valid veterinarian-client-patient relationship, and the veterinarian must have examined the fish and identified a bacterial disease that would be treatable with the VFD drug (eg, florfenicol). Extra-label use of the VFD is prohibited by law.

A small group of compounds have been designated by FDA as “high regulatory priority,” ie, their use is likely to result in enforcement action by the FDA. The most important of these compounds are chloramphenicol, the nitrofurans, and malachite green. These compounds should never be used in food animals for any reason, and their use in nonfood species is discouraged.

Antibiotics can be delivered to pet fish through any of the treatment routes listed; however, medicated food is most common and usually most effective. Common antibiotics used in pet or ornamental fish include oxytetracyline, potentiated sulfa drugs, and enrofloxacin (in koi and exhibit fish). See Table: FDA-Approved Drugs for Aquaculture Use in the USA (2007)Tables for approved dosages and withdrawal times. Oxytetracycline (Terramycin® 200) can be fed at a dose of 55–83 mg/kg, sid for 10 days to control many gram-negative bacterial infections, including columnaris disease. Because of the longevity of oxytetracycline on the market, there may be significant resistance to it in some bacterial isolates. Romet®-30 (sulfa-dimethoxine and ormetoprim, see table 3) also is effective against many gram-negative organisms, with less resistance. Palatability can be a problem when feeding medicated food to sick fish, which may have a poor appetite. Aquaflor® (see table 3) is a recently approved medicated feed containing florfenicol for use in channel catfish and salmonids. It is marketed as a VFD product. This broad-spectrum antibiotic should have excellent efficacy against many gram-negative bacteria, although it was specifically targeted to treat Edwardsiella ictaluri infection of channel catfish. Erythromycin is not FDA-approved for use in fish, but is an excellent antibiotic for fish infected with gram-positive bacteria, particularly Streptococcus. Erythromycin can be fed at a dose of 100 mg/kg, sid for 14 days. Palatability may be a concern with erythromycin-medicated feed. Erythromycin has been used in management of bacterial kidney disease of salmonids and streptococcal infections in food and nonfood species. FDA permission is required for use in food animals.

Delivery of antibiotics in a bath treatment is not generally recommended due to unknown or limited efficacy and damaging environmental effects (ie, antibiotics tend to kill the biofilter). Oxytetracycline (100–400 mg/L for 1 hr, sid for 10 days) has some efficacy when delivered in a bath. Oxytetracycline in a bath treatment is chelated by hard water and is therefore ineffective in marine systems. Enrofloxacin has been used as a bath at 2.5–5.0 mg/L for 5 hr, sid for 7 days. Water changes are recommended after the 5 hr contact time, and the drug may be chelated by hard water. Kanamycin has also been used as a bath treatment at dosages of 50–100 mg/L for 5 hr, repeated every 3 days for 3 treatments, with water changes recommended after the 5-hr contact time. Nephrotoxicity is a concern in fish treated with aminoglycosides. Many other options are discussed in the literature.

Injection is the most effective means of controlling the amount of antibiotic delivered to a fish. Enrofloxacin can be delivered IM at a dosage of 5–10 mg/kg, and the higher dosage can be repeated every third day, minimizing handling. Three treatments are generally recommended. When using enrofloxacin, the less concentrated dose (22.7 mg/mL) is recommended, even for use in large fish, due to adverse tissue reactions with the concentrated dose. If injection volume is excessive, more than one injection site can be used. Other injectable antibiotics include amikacin (5 mg/kg, IM, every 3 days for a total of 3 treatments). As with other aminoglycosides, nephrotoxicity may be a concern. Erythromycin (10 mg/kg, IM, sid for 3 days) can be used to treat gram-positive infections in large fish.

Hydrogen peroxide (35%) has recently been FDA approved for use in finfish for the control of bacterial gill disease (caused by Flavobacterium branchiophilum) in salmonids, external columnaris (caused by F columnare) in coolwater finfish and catfish, and fungal infection (saprolegniasis) of eggs (See Table: FDA-Approved Drugs for Aquaculture Use in the USA (2007)Tables). These are common external infections in fish that may follow handling or be associated with high organic load or low water temperatures for the affected species. Hydrogen peroxide is used as a short-term, continuous-flow bath. Treatments are administered daily or on consecutive alternate days for 3 treatments. An initial bioassay is recommended prior to treating a large group of fish. Paddlefish are sensitive, and use of hydrogen peroxide is not recommended. Other sensitive species include northern pike, pallid sturgeon, and in some instances, walleye.

Use of topical ointments containing antibiotics can be practical in pet fish. Some seem to absorb fairly quickly, but if treating a substantial wound, it may be appropriate to cover it with a waterproof compound. Frequent application of antibiotic ointment (ie, twice a day) can work well to treat superficial ulcers in pet fish.

Formalin is FDA approved for use in finfish and penaeid (saltwater) shrimp (See Table: FDA-Approved Drugs for Aquaculture Use in the USA (2007)Tables). Methanol may be added to formalin products as a preservative. Formalin eliminates protozoan parasites and monogeneans from the external surface of fish. It may also have some efficacy against external bacterial and fungal infections. It can be used as a prolonged bath at concentrations of 15–25 mg/L. The lower concentration is recommended for pond use because formalin removes dissolved oxygen from the water. Vigorous aeration during formalin treatment is essential. A concentration of 25 mg/L is equal to 2 drops/gal. (useful for delivering formalin to aquarium fish). When treating at ≤ 25 mg/L, a water change is not necessary following chemical administration. At this concentration, formalin has minimal impact on biofiltration; however, if ammonia is tested using Nessler's reagent, a very high reading may be observed for several days. This is an artifact caused by the interaction of the 2 compounds. Short-term baths with formalin can be provided at concentrations up to 250 mg/L for 30 min, but close observation during treatment is essential because 250 mg/L may be lethal in some fish. At water temperatures >77°F (25°C), the concentration should not exceed ∼170 mg/L. If adverse reaction to the chemical becomes apparent, the fish should be immediately placed in clean water. If formalin is allowed to chill to <45°F, a white precipitate, paraformaldehyde, will form. Because paraformaldehyde is highly toxic to fish, formalin should never be used if a precipitate or cloudiness is observed. Formalin is carcinogenic and potentially toxic to workers; material safety data sheets should be on hand in businesses where the chemical is used, and employees should be informed of appropriate safety precautions.

Salt is categorized as of “low regulatory concern” by the FDA and has many uses in fish medicine including destruction of single-celled protozoans and management of osmoregulation. Salinity is typically measured as parts per thousand (ppt) or g/L (note that 1 ppt = 1 g/L). Seawater is typically 32–37 ppt salt. By increasing or decreasing the amount of salt to which a freshwater or marine fish is exposed, osmoregulatory stress can be minimized and many parasites eliminated. For freshwater fish, a 30 ppt dip for 0.5–10 min, depending on species, is an effective ectoparasiticide and is strongly recommended when moving fish. When fish show signs of distress, commonly manifest by rolling on their side, they should be removed from the bath. The use of salt is a quick and effective means of minimizing the introduction of protozoans into a system with new fish. A solution of 5–10 ppt salt is recommended for transportation of freshwater fish; most species will tolerate this concentration for several hours or days. Permanently raising salinity to 2–3 ppt in a freshwater system can minimize parasitic protozoa. Salt is not generally practical for use in production ponds, except for control or prevention of nitrite toxicity, because of their large volume, but it can be used in ornamental ponds that are not more than several thousand gal. Less information is available on lowering salinity for marine fish; however, freshwater dips that are adjusted for temperature and pH are often recommended when moving animals. Lowering salinity to 16–18 ppt can be very useful when treating some parasitic diseases, particularly Cryptocaryon.

Copper sulfate (CuSO4) is not yet approved by the FDA; however, a number of compounds containing CuSO4 have been approved by the US Environmental Protection Agency (EPA) as algicides for use in aquatic sites. It is currently designated as “of moderate regulatory concern” and is used in food fish practice. CuSO4 has been used for many years as a parasiticide and is particularly useful in large production ponds because of its relatively low cost. Copper is highly toxic to fish, and safe use depends on knowing both the volume of water to be treated and the total alkalinity. In freshwater systems, the concentration of CuSO4 applied should be based on the total alkalinity (TA) of the water. If TA is <50 mg/L, copper sulfate cannot be used safely. If TA is 50–250 mg/L, a safe concentration of CuSO4 can be determined by dividing the TA by 100. For example, if TA = 100 mg/L, a safe concentration of CuSO4 would be 1 mg/L. If TA is >250 mg/L, the concentration of CuSO4 should not exceed 2.5 mg/L. CuSO4 also has algicidal activity. Rapid death of an algal bloom can precipitate catastrophic oxygen depletion. Use of CuSO4 in ponds not equipped with supplemental aeration is risky. Use of CuSO4 is hazardous if a pond has a heavy algal bloom (secchi disk ≤18 in.) or if the water is already deficient in oxygen due to other factors, (eg, cloudy weather or high water temperature).

In saltwater systems, copper is sometimes applied in a chelated form because it stays in concentration longer. Chelated compounds may be difficult to use safely and require careful monitoring. CuSO4 can be used to treat marine fish, but the concentration of active copper (Cu2+) must be closely monitored (test kits are available) and should be maintained at 0.2 mg/L for up to 3 wk. When using over-the-counter products in marine aquaria, label directions should be followed. Cu2+ concentrations should be tested at least once a day.

Copper is extremely toxic to invertebrates and plants; these must be removed before the water is treated. Finally, copper will impact bacteria in biofilters, and a transient increase in ammonia should be expected for several days following treatment. Monitoring ammonia until measurable concentrations subside is recommended.

Potassium permanganate (KMnO4) is also currently designated “of moderate regulatory concern” by the FDA. KMnO4 is used as an external parasiticide, fungicide, and bactericide. It is a strong oxidizing agent and “burns” organic material off the external surface of fish. Overuse, particularly multiple uses within a short period of time (more than once a week), will kill fish. The concentration of KMnO4 used varies with the permanganate demand of the water. In aquaria or ornamental ponds with very clear water, a concentration of 1–2 mg/L is usually safe and effective. Permanganate demand is greater in water with a high organic load. To determine the permanganate demand, a bioassay can be performed: the water to be treated is placed in small containers and KMnO4 is added in incremental concentrations of 2 mg/L. The correct concentration for therapeutic use will be the lowest concentration that maintains a pink color for 4–8 hr. If the concentration of KMnO4 required is >6 mg/L, then the organic load is excessive, and sanitation practices should be evaluated. KMnO4 has little impact on biofilters when applied at 2 mg/L or less. Potassium permanganate is more toxic as salinity of the water increases and use in marine systems is not recommended.

Organophosphates have been used in nonfood fish practice for decades to control monogenea, parasitic crustaceans, and leeches. Organophosphates can be used in freshwater systems, for ornamental fish only, at concentrations of 0.25 mg/L as an indefinite bath. Most compounds are sold as 85% active ingredient wettable powder. Toxicity and efficacy are affected by pH, with more acidic pH resulting in increased toxicity. For this reason, an increased dosage may be necessary in marine systems (pH 8.0–8.3), with concentrations up to 1.0 mg/L used by some facilities. Some veterinarians add atropine to the food of marine exhibit fish prior to treatment with organophosphates. Due to environmental concerns, organophosphates should not be used in outdoor ponds, unless specific provisions for such use exist in state law and are followed by the veterinarian. Following treatment with organophosphates, must facilities hold water for up to 96 hr before allowing any discharge, and divers are usually restricted from entering exhibit tanks for at least 48 hr. Organophosphates may not be used in food fish in the USA.

Difluorobenzuron is a chitin synthesis inhibitor with effectiveness against anchor worms (Laernea), fish lice (Argulus), and other copepodid parasites in aquarium fish only. The drug is used as a prolonged bath at concentrations of 0.03 mg/L. It has a fairly long half-life (>1 wk), and treated water should be run through a carbon filter prior to discharge.

Metronidazole is used to control intestinal protozoans and can be delivered in a medicated food or as a bath when fish are anorectic. Although very effective against Spironucleus spp, metronidazole does not seem to be effective against gastric infections with Cryptobia iubilans. A concentration of ∼7 mg/L (∼250 mg metronidazole dissolved in 10 gal. of water) can be administered sid for 5 days. A daily water change a few hours after treatment is recommended. Metronidazole can be administered in medicated feed at 50 mg/kg, PO, for 5 days. Anecdotal information suggests that excessive treatment (10× the recommended dosage for 30 days) with metronidazole may be associated with reproductive failure in some fish. Metronidazole may not be used in food fish in the USA.

Fenbendazole has been used to control intestinal helminths in fish. A dosage of 25 mg/kg, delivered in food for 3–5 days, has been commonly recommended, but efficacy has not been evaluated in controlled trials. Levamisole administered in a bath treatment at a concentration of 2 mg/L is also used by some clinicians. These compounds may not be used in food fish in the USA.

Praziquantel is used to control intestinal cestodes and monogenea on the gills and skin. The most common use of praziquantel is as a prolonged bath in large marine aquaria for control of Neobenedenia spp. It is applied at a concentration of 2 mg/L and appears to remain active for several weeks. Treated water should be run through an activated carbon filter prior to discharge. Praziquantel can also be administered PO at a dosage of 35–125 mg/kg for up to 3 days or as a short-term bath treatment at a concentration of 10 mg/L for 3 hr. Recent work demonstrated efficacy in yellowtail kingfish fed praziquantel at a dosage of 50 mg/kg, sid, for 7 days. Use of praziquantel is not permitted in food fish in the USA.

Chloroquine has been used to control Amyloodinium sp in ornamental marine fish. It is applied as a prolonged bath at concentrations of 10 mg/L. Efficacy in recirculating systems seems to be very good; however, there are essentially no data on treatment intervals, effects on biofilters, or other basic husbandry effects. Weekly examination of fish, including biopsy of infected tissue, is recommended to assess treatment efficacy. Some aquarists have used chloroquine (10–15 mg/L for 7 days; followup with 10 mg/L may be required) coupled with decreased salinity (16–18 ppt) as a treatment for Cryptocaryon in marine fish. Results are mixed, but advantages include decreased labor, as intensive water testing is not required (which is the case for Cu2+). Use of chloroquine is not permitted in food fish in the USA. Treated water should be run through a carbon filter prior to discharge.

Tricaine methanesulfonate (MS-222) is a benzocaine derivative, and 2 products, Finquel and Tricaine-S, have been FDA approved for use in finfish. MS-222 has a 21-day withdrawal time when used in food animals. It is very useful for sedation, surgical anesthesia, and euthanasia. MS-222 is acidic; therefore, baking soda should be used as a buffer (measured by weight, 2 g baking soda:1 g MS-222). Sedation can generally be achieved with MS-222 concentrations between 50–100 mg/L, although species-specific sensitivities should be expected. Induction for most species may be near 125 mg/L; however, when working with unfamiliar species it is best to start at a lower concentration (ie, 50 mg/L) and increase the concentration until the desired effect is achieved. Following induction, the concentration may be decreased to 50–100 mg/L to maintain the desired depth of anesthesia. Respiration should be monitored; if opercular movement ceases, fish should be immediately moved to clean, aerated water. Buffered MS-222 can also be used to euthanize fish at concentrations of 1,000 mg/L (1 g/L). MS-222 is light sensitive and should be stored in the dark. Solutions that turn brown should be discarded.

Eugenol and clove oil (an over-the-counter product, usually 84% eugenol) have become popular with pet fish enthusiasts as anesthetics. They are not FDA approved for use in fish in the USA, and the FDA may take regulatory action against veterinarians using eugenol in food animals. Eugenol at concentrations of 50, 100, and 200 mg/L was compared to MS-222 in red pacu. It was found to result in effective immobilization of fish; however, there was concern about analgesia, prolonged recovery, and a narrow margin of safety, especially at higher concentrations. Use of both MS-222 and eugenol resulted in hypoxemia, hypercapnia, respiratory acidosis, and hyperglycemia in red pacu.

Chorionic gonadotropin is FDA approved as a spawning aid for finfish. Veterinarians may work as part of a team for fish hatcheries and may be asked to assist in obtaining spawning hormones. Chorionic gonadotropin is a prescription drug and is restricted to use by or on the order of a licensed veterinarian.

Surgical Considerations

Increasingly, surgery is an option for management of some medical problems in pet or exhibit fish, including neoplastic disease, failure to ovulate (ie, “egg bound” fish), and swim bladder repair for buoyancy problems. Fish skin does not have the subcutaneous tissue that provides flexibility to domestic animal tissue; therefore, wounds are not usually treated by surgical closure but instead allowed to heal by second intention. The clinical evaluation prior to surgery is similar to that in other species, although there may be more emphasis on imaging and less on blood work. Radiology and ultrasound work very well in fish, and use of these techniques prior to an invasive surgical procedure is recommended.

Descriptions in the literature show how a fish may be positioned for a surgical procedure. For smaller animals, such as koi, a foam “bed” is easily constructed and can be covered with something as simple as clear plastic wrap so that the fish does not lose skin, scales, or mucus from direct contact with the foam. The “bed” can be positioned over an aquarium using a plastic tray with holes to allow water drainage. A pump can move water containing an anesthetic solution (usually MS-222 see Anesthetics) from the aquarium through a small tube or catheter, and across the fish's gills. Ideally, dissolved oxygen, ammonia, and pH should be monitored in the anesthetic solution. The fish can be covered with a clear plastic surgical drape (avian drapes work well) followed by fairly minimal preparation of the incision site. Plucking scales along the incision line and carefully cleaning the area with a sterile swab soaked in sterile saline may be all that is required. Very dilute disinfectants such as povidone-iodine or chlorhexidine can be used, but if the fish is clean this is probably not necessary (or recommended) because the normal mucus has significant antibacterial properties.

Absorbable sutures are generally not recommended in fish, as they may persist for significant periods of time (in some cases, >1 yr). Monofilament material and a needle with a cutting edge generally work well. The simple interrupted suture pattern works well to close fish skin, but other techniques can be used successfully. Skin sutures should be removed when the incision site has healed, generally at 3–4 wk. Surgical staples have been used successfully in fish. Results using cyanoacrylate tissue adhesive have been mixed due to a significant inflammatory reaction observed in some species. If postoperative antibiotics are used, enrofloxacin delivered IM at a dose of 5 mg/kg is a good option, but can only be given to non-food fish. Butorphenol given at a dose of 0.1–0.4 mg/kg, IM has been used for postoperative pain control in non-food fish. Increasing the salinity in freshwater systems to 1–3 ppt (g/L) is strongly recommended during the recovery and healing periods. Most freshwater fish should be able to tolerate a salinity of 3 ppt (3 g/L) on an almost indefinite basis.

Reportable Diseases and Regulatory Concerns

With the increased regulation of ornamental and aquacultured fish in the USA, there are increased requirements for professional veterinary services, including USDA health certification for movement of animals. A national aquatic animal health program is being developed that should more clearly define the role of the veterinary practitioner over the next few years. USDA-APHIS provides voluntary training to practitioners who wish to develop clientele in this market, and is currently developing a special aquatic certificate for accredited veterinarians. Information on federal regulations pertaining to fish medicine is available at State law may be more restrictive than federal law, and there is significant variation in state regulations regarding importation of aquatic animals. The state veterinarian is the ultimate resource for information on state animal health regulations.

The OIE monitors diseases of animals, including fish, molluscs, and crustaceans and publishes an online aquatic animal health code and manual of diagnostic tests for aquatic animals ( Practitioners should stay informed of changes in the status of diseases of regulatory concern, as they are liable if they have a case which is not reported. Regulated fish diseases of greatest concern to aquatic animal veterinarians in the USA include koi herpesvirus, spring viremia of carp, and viral hemorrhagic septicemia (See Table: Fish Diseases of Regulatory Concern in the USATables). Notification of reportable aquatic diseases should be made directly to the state veterinarian and to the USDA Area Veterinarian in Charge.


Quarantine is strongly recommended for pet fish and certainly for animals intended for display in public aquaria. A 30-day period is the minimum time for quarantine, but longer periods may be necessary. Quarantined animals should only be handled when contact with all other animals is finished for the day. Quarantined animals should have their own designated equipment (ie, nets, buckets, siphons, etc) and be kept separate from other animals. Disinfectant should be used on equipment and in footbaths at entry points to the quarantine area.

When receiving fish, a thorough history should be obtained regarding previous treatment or disease outbreaks. Fish should be examined early in the quarantined period; a visual examination may suffice but for valuable specimens a full clinical examination including recording the weight of the animal and completing gill, skin, and fin biopsies is recommended. Prophylactic use of antibiotics may be warranted in some shipments, especially recently imported marine specimens. Treatment with praziquantel for monogeneans is often prudent with marine fish as well. Goldfish commonly have significant monogenean infestations and treatment with formalin or praziquantel may be appropriate. Koi should be quarantined to prevent introduction of koi herpesvirus (KHV), a serious and reportable disease, to established populations. Koi should be quarantined for a minimum of 30 days at a temperature of 75°F (24°C). Fish that become ill during quarantine should be tested for KHV (p 1636).

Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM

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