Introduction
Certainly, one of the most commonly asked questions concerning the practice of ornamental fish veterinary medicine is, “What antibiotic should I use and what is the dose?”—a straight forward and uncomplicated question with a complex and frequently ambiguous answer. Of course, the problem lies in the lack of sound pharmacokinetic data available, and the overwhelming number of species involved. When environmental differences such as temperature, pH, and water hardness are tossed into the equation, selecting a drug and dosing regimen becomes even less objective.
Historically, most treatment of ornamental fish diseases has been performed by non-veterinarians using empirical dosing regimens of a variety of chemotherapeutics. The Food and Drug Administration is currently examining the wide availability of prescription drugs, especially antibiotics, and may possibly enforce the restrictions on their availability to the lay public within the next few years. Such measures will necessitate sound pharmacokinetics studies to support clinical use of antimicrobials in ornamental fishes by licensed veterinarians. Very little research related to pharmacology has been reported in ornamental fishes. What little information exists is based on clinical efficacy and in-vitro trials using a number of different antimicrobials.
The purpose of this review article is to familiarize the reader with the most current pet fish pharmacokinetic information, provide a list of frequently used antibiotics, and discuss the variables that influence antibiotic dosing regimens in ornamental fish. Several general texts discuss antibiotics and their use on ornamental fishes in some detail.3,5,7
Dosing Routes
Due to their aquatic nature, generally small size, and frequently large numbers, a variety of atypical methods are utilized to deliver antibiotics to the ornamental fish. Standard parenteral methods can and commonly are used to dose aquarium fish with antibiotics, but the clinician must also be familiar with the terminology applied to water borne treatments. Table 1 contains these important definitions.
Table 1. Routes of antibiotic administration for ornamental fish
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Bath
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Usually refers to a treatment in which the drug is dissolved in the water in which the fish are swimming. The treatment usually lasts at least 15 minutes and less than 24 hours. Dosage is usually based on volume of water and not on fish biomass.
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Dip
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Refers to a treatment in which the fish is submerged in a particular solution for between 1 second and 15 minutes. Water volumes are usually smaller than those of bath treatments and drug concentrations are frequently higher.
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Flush or flow though
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Requires constant water flow. Most frequently used in raceways or narrow vats. Medicant is added to inflow area and fish are exposed to drug as it passes over them with the water current. Similar to dip procedure, except fish may not have to be removed from their normal holding area.
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Indefinite bath
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Medication is added to the aquarium or pond and usually there is no water change or immediate retreatment.
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Injection
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The antibiotic is given by injection with the aid of a hypodermic needle and syringe. Routes may be subcutaneous, intradermal, intramuscular, intravenous, and intraperitoneal.
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Oral
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Medication is mixed with the food in order to treat the fish. Usually done by incorporating drug into a gelatinized food mixture. For larger fish patients, medication can be placed in a chunk of food and then fed, or force fed to the patient.
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Topical
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The antibiotic is applied directly to the lesion.
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*Discontinue chemical (e.g., carbon) filtration during immersion treatments as this will inactivate the medicant. Adequate aeration is also important during any water treatment.
**When antibiotics are used as bath treatments, they should be used daily for 5–7 days. Water changes (at least 50%) should take place between treatments.
Drugs and Dosages
The majority of the current information on antibiotics used in aquarium fish has been extrapolated from the finfish aquaculture literature. There are a number of reasons for this, most of which revolve around funding for sound pharmacokinetic research. A recently published review article summarizes the entire body of literature on this subject.6 There are currently only two antibiotics approved for use in fish intended for human consumption (Romet-30® and Terramycin for Fish®).2 Most of the literature dealing with antibiotic usage in aquarium fish is empirical and anecdotal. One recent study report on the pharmacokinetics of enrofloxacin in the red pacu, a tropical aquarium fish closely related to such important species as the tetras, hatchetfishes, silver dollars, and headstanders.4 Fortunately, the veterinarian treating aquarium fish can apply current extralabel drug use regulations when selecting and initiating antibiotic therapy. Whenever possible, appropriate microbiologic culture and sensitivity results should be applied when selecting an antibiotic course of treatment. The majority of ornamental fish pathogens are gram-negative bacteria that are susceptible to a number of antibiotics. Efforts to follow a complete treatment course are encouraged to help reduce the potential for producing antibiotic resistant organisms. Indiscriminate use of antibiotics in the aquarium fish industry both in the United States and abroad has made antibiotic resistance a significant problem.1 At times, appropriate microbiologic diagnostics are financially or logistically unavailable. In such cases, selecting an antibiotic that has good activity against gram-negative organisms may be the clinician’s only recourse.
Table 2 lists a number of antibiotics that are available to the zoo veterinarian and private practitioner which are clinically effective for treating bacterial disease in aquarium fish including pond koi. This table must be treated with respect and the clinician must understand that the routes and dosages are only guidelines and that a biotest is in order when dealing with an unfamiliar drug, fish species, particular aquatic environment, or all of the above.
Table 2. Antibiotics for ornamental fish
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Aztreonam
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100 mg/kg IM or IP q 48 h for 15 days.
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Used primarily by koi hobbyists. Effective against Aeromonas salmonicida.a
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Chloramphenicol
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20–40 mg/kg IM or IP q 48 h for 15 days.
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Human health issues, wear gloves when handling.
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Enrofloxacin
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5–10 mg/kg given IM or IP q 48 h for 15 days; injectable or oral form 5–10 mg/kg PO for 10–14 days or 0.1% (10 mg/10 g) can be used in food and feed fish for 10–14 days; 2.5–5 mg/L as a 5-h bath, repeated q 24 h for 5–7 days, 50–75% water change between treatments.
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Erythromycin
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50–100 mg/kg PO q 24 h for 10 days.
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Bath formulations also available.
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Kanamycin sulfate
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50–100 mg/L as a 5-h bath, repeated q 72 h for 3 treatments, 50–75% water change between treatments.
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Available through pet products suppliers.
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Metronidazole
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400 mg/L as a 5–12-h bath, repeated q 24 h for 3 consecutive days; 0.2% (20 mg/10 g) in food and feed for 10 days.
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Good for anaerobes and some flagellates.
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Nitrofurazone
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20 mg/L as a 5-h bath, repeated q 24 h for 5–7 days, 50–75% water change between treatments.
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Water soluble formulations work best.
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Oxytetracycline
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20–50 mg/L as a 5–24-h bath, repeated q 24 h for 5–7 days, 50–75% water change between treatments; 25 mg/kg given IM or IP q 24 h for 5–7 days; 50 mg/kg PO q 24 h for 10 days.
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Many resistant bacteria.
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Silver sulfadiazine cream
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Apply cream directly to wound q 12 h. Allow affected area to remain out or water for 30–60 seconds while medication is absorbed.
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Triple antibiotic ointment
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Apply ointment directly to wound q 12 h. Allow affected area to remain out or water for 30–60 seconds while medication is absorbed.
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Trimethoprim/sulfamethoxazole
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30 mg/kg PO q 24 h for 10–14 days; 0.2% (20 mg/10 g) in food and feed to fish for 10–14 days; 20 mg/L as a 5–12-h bath, repeated q 24 h for 5–7 days, 50–75% water change between treatments.
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Very effective as a bath treatment.
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aDosage information courtesy of Spangenberg and Hedrick, UC Davis.
†Article has been modified from a chapter to be published in: Current Veterinary Therapy XIII, (Kirk and Bonagura eds.), WB Saunders, Philadelphia, PA, 1998.
Literature Cited
1. Dixon, B.A., J. Yanashita, and F. Evelyn. 1990. Antibiotic resistance of Aeromonas spp. isolated from tropical fish imported from Singapore. J Aquat Anim Health. 2:295.
2. Francis-Floyd, R. 1993. Extra-label drug use in aquaculture. J Am Vet Med Assoc. 202:1651–1653.
3. Gratzek, J.B. 1992. Aquariology: The Science of Fish Health Management. Tetra Press, Morris Plains, NJ.
4. Lewbart, G., S. Vaden, J. Deen, D. Whitt, C. Manaugh, A. Doi, T. Smith, and K. Flammer. 1997. Pharmacokinetics of enrofloxacin in the red pacu (Colossoma brachypomum). J Vet Pharmacol Ther. 20:124–128.
5. Noga, E.J. 1996. Fish Disease: Diagnosis and Treatment. Mosby-Yearbook, St. Louis, MO.
6. Stoffregen, D.A, P.R. Bowser, and J.G. Babish. 1996. Antibacterial chemotherapeutants for finfish aquaculture: a synopsis of laboratory and field efficacy and safety studies. J Aquat Anim Health. 8:181.
7. Stoskopf, M.K. 1993. Fish Medicine. WB Saunders, Philadelphia, PA.