Pharmacology in Aquatic Animals

Pharmacology in Aquatic Animals

Ron A. Miller


Antimicrobials have long been used to relieve pain and suffering and control infections in food-producing animals, including fish. The safe and prudent prescription of effective antimicrobials by veterinarians to treat aquatic animals has contributed immensely to the increased food production capacity of worldwide aquaculture. Use of antimicrobials in aquaculture is, however, not without risk. The American Veterinary Medical Association has published educational materials for veterinarians that describe prudent and judicious use guidelines for antimicrobials in aquaculture (AVMA, 2006). Antimicrobial-resistant bacteria, pathogenic to animals and humans, have been found in and near fish and shellfish farms where medicated feed has been administered (Damir et al., 2013; Miranda et al., 2013; Huys et al., 2001; Guardabassi et al., 2000; Sathiyamurthy et al., 1997; Husevag and Lunestad, 1995). In addition, fish have been implicated as potential reservoirs of zoonotic pathogens (Haenen et al., 2013), some of which may carry resistance genes, including extended-spectrum beta-lactamases (Sousa et al., 2011). Cabello (2006) suggests the unrestricted use of antimicrobials in aquaculture in any country has the potential to affect human and animal health on a global scale, and further suggests that this problem should be dealt with through unified local and global preventive strategies.

Three traditional antimicrobials are currently approved by the US Food and Drug Administration (FDA) for use in finfish and lobsters (Table 53.1). Continued use of the same antimicrobial(s) in the fish-rearing environment will likely increase the potential for emergence and selection of antimicrobial-resistant bacteria in the rearing facility and may diminish therapeutic effectiveness. A Canadian report stated there has been a significant reduction in antimicrobial usage in salmon farming since 2005 (Morrison and Saksida, 2013). Citing improvements in production, health management, and livestock selection, the authors noted few vaccines and limited chemotherapeutic options remain a concern.

Table 53.1 Antimicrobials approved for use in US aquaculture in poikilothermic food species. Source: US Food and Drug Administration.

Antimicrobial (manufacturer) Species Indication Dosage regimen Limitations/ Comments
Oxytetracycline dihydrate (Terramycin® 200 For Fish, Phibro Animal Health) Pacific salmon Mark skeletal tissue 250 mg/kg/day for 4 days

Salmon <30 g

In feed as sole ration

7 day withdrawal time

            Salmonids Control of ulcer disease (Hemophilus piscium), furunculosis (Aeromonas salmonicida), bacterial hemorrhagic septicemia (A. liquefaciens), and pseudomonas disease 2.5–3.75 g/ 100 lb/day for 10 days

In mixed ration

No temperature restrictions

21-day withdrawal time

            Freshwater-reared salmonids Control of mortality due to coldwater disease (Flavobacterium psychrophilum) 3.75 g/100 lb/day for 10 days

In mixed ration

No temperature restrictions

21-day withdrawal time

            All freshwater- reared rainbow trout Control of mortality due to columnaris disease (F. columnare) 3.75 g/100 lb/day for 10 days

In mixed ration

No temperature restrictions

21-day withdrawal time

            Catfish Control of bacterial hemorrhagic septicemia (A. liquefaciens) and pseudomonas disease 2.5–3.75 g/ 100 lb/day for 10 days

In mixed ration

Water temperature not below 62°F (16.7°C)

21-day withdrawal time

            Lobster Control of gaffkemia (Aerococcus viridans) 1 g/lb medicated feed for 5 days

In feed as sole ration

30-day withdrawal time

Oxytetracycline HCl (oxytetracycline HCl Soluble Powder-343®, Phoenix Scientific; Pennox 343®, PennField Oil, Terramycin-343®soluble powder, Pfizer; OxyMarine®, Alpharma; and Tetroxy® Aquatic, Cross Vetpharm Group) Finfish fry and fingerlings Mark skeletal tissues 200 to 700 mg oxytetracycline HCl (buffered)/L of water for 2 to 6 h


Sulfadimethoxine–ormetoprim (Romet-30®, Pharmaq AS) Salmonids Control of furunculosis (A. salmonicida) 50 mg/kg/day for 5 days

In feed

42-day withdrawal time

            Catfish Control of enteric septicemia (Edwardsiella ictaluri) 50 mg/kg/day for 5 days

In feed

3-day withdrawal time

Florfenicol (Aquaflor®, Intervet) Catfish Control of mortality due to enteric septicemia (E. ictaluri) 10–15 mg/kg/day for 10 days

VFD antimicrobial

12-day withdrawal time

            Freshwater-reared finfish Control of mortality due to columnaris disease (F. columnare) 10–15 mg/kg/day for 10 days

VFD antimicrobial

12-day withdrawal time

            Freshwater-reared salmonids Control of mortality due to coldwater disease (F. psychrophilum) and furunculosis (A. salmonicida) 10–15 mg/kg/day for 10 consecutive days

VFD antimicrobial

15-day withdrawal time

            Freshwater-reared warmwater finfish Control of mortality due to streptococcal septicemia (S. iniae) 10–15 mg/kg/day for 10 consecutive days

VFD antimicrobial

15-day withdrawal time

Sulfamerazine, Zoetis Rainbow, brook, and brown trout Control of furunculosis 10 g/100 lb/day for up to 14 days

In feed

21-day withdrawal time

Not currently marketed

Approval applies only to the specific antimicrobial that is the subject of a new animal drug application (NADA) or abbreviated new animal drug application (ANADA); active ingredients from other sources (e.g., bulk antimicrobial from a chemical company or similar compounds made by companies other than those specified in the NADA) are not approved new animal antimicrobials.

Approval applies only to use of the antimicrobial for the indications and manner specified on the label.

VFD, Veterinary Feed Directive.

In this chapter, many factors will be discussed that should be considered as aquatic animal veterinarians decide whether to prescribe the use of an antimicrobial in a finfish (population). The reader should consult earlier chapters in this text on antimicrobial drugs for further drug-specific discussions.

Legal Considerations when Selecting an Antimicrobial for Use in Aquatic Animals

Prior to making a decision to prescribe treatment for a fish or fish population, careful thought should be taken in selecting the most appropriate antimicrobial, dosage, and route of administration. Aquatic animal veterinarians can be faced with difficult decisions on which approved animal antimicrobial to use or whether to recommend an extralabel (ELU) use of an approved animal or human antimicrobial. In 1994, the Animal Medicinal Drug Use Clarification Act (AMDUCA) provided veterinarians in the US greater flexibility by allowing them to prescribe ELUs of certain approved animal and human drugs for animals under certain conditions. In 1996, the FDA published a regulation that established the conditions under which veterinarians may prescribe ELU of certain approved animal and human drugs for animals (Federal Register, 1996). Discussed here are four of the many key points outlined in the ELU regulations.

A Valid Veterinarian–Client–Patient Relationship

AMDUCA allows ELU only on the order of a licensed veterinarian in the context of a valid veterinarian–client–patient relationship (VCPR). A VCPR can only exist if the veterinarian: (i) assumes the responsibility for making medical judgments about the health of the animal(s) and the need for medical treatment (and the client has agreed to follow the veterinarian’s instructions); (ii) has sufficient knowledge of the animal(s) to develop a diagnosis of the medical condition; and (iii) is readily available for follow-up in case adverse reactions or treatment failure is encountered. Such a relationship can only exist when the veterinarian has recently seen and is personally acquainted with the care of the animal(s) by virtue of examination and/or by visits to the premises where the animal(s) are kept.

General Conditions for ELU

ELU is limited to situations where an animal’s health is threatened or where the animal may suffer or die without treatment. Before a veterinarian can legally prescribe an approved animal or human drug one of these general conditions must also be met: (i) no animal drug is approved for the intended use; (ii) an animal drug is approved for the intended use, but the approved drug does not contain the active ingredient you require; (iii) an animal drug is approved for the intended use, but the approved drug is not in the required dosage form you require; (iv) an animal drug is approved for the intended use, but the approved drug is not in the required concentration; and (v) an animal drug is approved for the intended use, but the veterinarian has found, in the context of a valid VCPR, that the approved drug is clinically ineffective when used as labeled.

In the ornamental fish industry (nonfood-producing), veterinarians can prescribe an approved human drug for an ELU even if an approved animal drug is available if the other conditions of AMDUCA are met. Whether prescribing drugs for food-producing fish or ornamental fish, thorough recordkeeping is critical. Records of drugs used, condition treated, animal species treated, dosage administered, treatment duration, number of animals, and drug withdrawal parameters must be kept for 2 years or as required by federal or state law, whichever is greater. Thorough labeling of the drug dispensed on the veterinarian’s order is critical, including the veterinarian’s name, the name of the dispensing pharmacy, and directions for the end user similar to that described for recordkeeping.

Conditions for ELU in Food-Producing Animals

Prior to prescription of an approved animal or human drug for an ELU in a food-producing animal, the veterinarian must: (i) make a careful diagnosis and evaluation of the condition to be treated; (ii) have an appropriate medical rationale for using a specific drug; (iii) make sure the client maintains the identity of the treated animal(s) in the record; (iv) establish a substantially extended withdrawal time supported by scientific information, and, if such information is nonexistent, take appropriate measures to assure that the animal(s) and its food products will not enter the human food supply; and (v) ensure that no illegal residues, including antimicrobial residues, occur and that the client follows the established withdrawal time before marketing food products made from treated animals.

Antimicrobials Prohibited from ELU in Animals

Under the AMDUCA provisions, FDA has the right to prohibit ELU of certain drugs in animals. Currently, no approved antimicrobials are prohibited from ELU in companion animals (including aquarium or pet fish). The following drugs (both human and animal), families of drugs, and substances are prohibited for extralabel animal and human uses in food-producing animals:

  • Chloramphenicol
  • Clenbuterol
  • Diethylstilbestrol (DES)
  • Dimetridazole
  • Ipronidazole and other nitroimidazoles
  • Furazolidone and nitrofurazone
  • Sulfonamide drugs in lactating dairy cattle (except approved use of sulfadimethoxine, sulfabromomethazine, and sulfaethoxypyridazine);
  • Fluoroquinolones
  • Glycopeptides
  • Phenylbutazone in female dairy cattle 20 months of age or older
  • Cephalosporins (not including cephapirin) in cattle, swine, chickens, or turkeys:

    • For disease prevention purposes;
    • At unapproved doses, frequencies, durations, or routes of administration; or
    • If the drug is not approved for that species and production class.

The following drugs, or classes of drugs, which are approved for treating or preventing influenza A, are prohibited from ELU in chickens, turkeys, and ducks:

  • Adamantanes
  • Neuraminidase inhibitors.

Routes of Antimicrobial Administration in Fish

Table 53.2 shows antimicrobial dosages by various routes of administrations that have been reported in the literature (Reimschuessel et al., 2012). Many of these dosages are unapproved in many countries, and were used in many of these studies for the purposes of investigating the antimicrobial’s pharmacokinetics (PK). Researchers and clinicians should consult their regulatory authorities for the approved antimicrobials and dosages in their respective countries.

Table 53.2 Antimicrobial dosages used in fish. Source: Reimschuessel 2012. Reproduced with permission of John Wiley & Sons.

Antimicrobial Dosage Interval Route Comments
Amikacin 5 mg/kg q12h IM            
            5 mg/kg q72h × 3 IM            
Amoxicillin 25 mg/kg q12h PO Rarely used due to few gram-positive pathogens
            40–80 mg/kg q24h 10 days PO            
Ampicillin 10 mg/kg q24h IM Sharks
            10 mg/kg q12h 7–10 days PO Sharks
            50–80 mg/kg q24h 10 days PO            
Aztreonam 100 mg/kg q48h 7 days IM/IP Used by koi hobbyists
Azithromycin 30 mg/kg q24h 14 days PO            
Ceftazidime 22 mg/kg q72–96h × 3–5 IM/IP            
Cefquinome 5–20 mg/kg Single dose IP Dose used for determining PK
Chloramine-T 20 mg/l 1 h 4 days Bath            
            2.5–20 mg/l Flush (various) Bath Disinfectant control bacterial gill disease and parasites
            5–10 mg/l 1 h Bath            
Ciprofloxacin 15 mg/kg single dose IM/IV Dose used for determining PKa
Difloxacin 10 mg/kg single dose PO Dose used for determining PKa
Dihydrostreptomycin 0.125 mg single dose IM/IV Dose used for determining PK
            10 mg single dose PO Dose used for determining PK
            10 mg/kg q24h IM Sharks
Enrofloxacin 2.5–5.0 mg/l 5 h q24h 5–7 days Bath a
            30–50 mg/l 4–24h (various) Bath            
            5–50 mg/kg q24h × 5–10 days PO a
            2.5–10 mg/kg Single dose IM/IP/IV Dose used for determining PK
Erythromycin 10–20 mg/kg Single dose IP For BKD before spawning
            50–100 mg/kg q24h 10–21 days PO            
            2 mg/l 1 h Bath For BKD in eggs
Florfenicol 5–20 mg/kg q24h 10 days PO Salmon
            10–15 mg/kg q24h 10 days PO Dose approved by US FDA for select species
            40–50 mg/kg q12–24h PO, IM, IP Red Pacu
            25–50 mg/kg Single dose IM            
Flumequine 10–500 mg/l 1–72 h BATH Increase dose in saltwatera
            5–50 mg/kg q24h 5–10 days PO a
            30 mg/kg Single dose IM/IP IP (and IM) dose levels remain at effective levels for 10 daysa
            2–25 mg/kg Single dose IV Dose used for determining PK
Fumagillin 30–60 mg/kg Single dose PO Dose used for determining PK
            3–6 mg/kg Single dose IV Dose used for determining PK
Furpyrinol 4–32 mg/L 5 h Bath            
Gentamicin 3 mg/kg q72h IM

Very nephrotoxic to aglomerular fish

Bath exposure does not achieve blood levels

            6 mg/kg Each week IM Sharks
Kanamycin 50–100 mg/l q72h 5 h × 3 BATH

Nephrotoxic some species

Change water 50% between treatments

            50 mg/kg q24h PO Nephrotoxic some species
            10–20 mg/kg q24h PO Sharks
            20 mg/kg q72h × 5 IP Nephrotoxic some species
Lincomycin 40 mg/kg q24h PO Japan
Marbofloxacin 10 mg/kg q24h 1–3 days PO Dose used for determining PK
Miloxacin 60 mg/kg q24h 6 days PO Japan
Nalidixic acid 13 mg/l 1–4 h Bath            
            20 mg/kg q24h PO, IM, IV Other doses used for PK studies
Neomycin 66 mg/l q3 days × 3 Bath Toxic to nitrifying bacteria in filter
            20 mg/kg Single dose PO Sharks to prevent bloat, poorly absorbed from gut
Norfloxacin 30–50 mg/kg q24h 5 days PO            
Oxolinic acid 25 mg/l 0.25 h q12h × 3 Bath            
            0.15–1.5 mg/l 10 days Bath            
            50–200 mg/l 1–72 h Bath            
            10 mg/kg q24h 10 days PO Freshwater species
            25–75 mg/kg q24h 10 days PO Saltwater species
Oxytetracycline 10–50 mg/l 1 h Bath For superficial infections
            20–50 mg/l 5–24 h q24h 5–6 days Bath Change 50–75% water between treatments
            55–83 mg/kg q24h 10 days PO Dose approved by US FDA for select species
            25–50 mg/kg q24h 5–7 days IM/IP Produces high levels for several days when given IM
            3 mg/kg q24h IV Red pacu
Piromidic acid 10 mg/kg q24h 5–10 days PO Japan
Sarafloxacin 10–30 mg/kg q24h 10 days PO a
Sulfadiazine–Trimethoprim 30–50 mg/kg q24h 7–10 days PO            
            125 mg/kg             IP            
Sulfadimethoxine–Ormetoprim 50 mg/kg q24h 5 days PO Dose approved by US FDA for select species
Sulfamerazine 220 mg/kg q24h 14 days PO            
            200 mg/kg q24h 10 days PO            
Sulfamethoxazole–Trimethoprim 20 mg/l 5–12 h q24h 5–7 days Bath Change 50–75% water between treatments
            30 mg/kg q24h 10–14 days PO            
Tetracycline 80 mg/kg Single dose PO            
Thiamphenicol 20 mg/l 1 h Bath            
            50 mg/kg q24h 7–10 days PO Japan
Vetoquinol 25–40 mg/kg Single dose PO            
Virginiamycin 40 mg/kg q24h 15 days PO            

aExtralabel use of fluoroquinolones in food animals is prohibited by US FDA.

BKD, bacterial kidney disease.

Data obtained from many authors.


Injectable treatments using an antimicrobial are typically used in higher-valued fish (i.e., brood stock, some ornamental species, etc.) due to the labor and care involved in minimizing the potentially significant stress to the animal due to sedation and physical handling. This procedure can be a massive undertaking for commercial producers. Despite these challenges, advantages of administering antimicrobials by injection include assuring that all animals receive the desired dose. Oftentimes the route is intramuscular (IM), but can be intraperitoneal, intravascular, or intradorsal (caudal to the dorsal fin). IM injections are usually administered in epaxial muscles, above the lateral line and near the caudal fin. If an aminoglycoside (e.g., gentamicin) is to be used, care should be taken to inject cranial to the dorsal fin to avoid large doses entering the kidney.

Immersion Baths

Immersion or waterborne treatments are most useful when a large number of animals need to be treated, when limited stress to the animal(s) is important, and of course when it is likely that the pathogen will be exposed directly to the medicated solution (e.g., on surface of fish or gills). There are three classical ways of administering an antimicrobial via a waterborne treatment. Static baths (and dips) can be used by adding an antimicrobial solution directly to the holding system. Flush treatments are most common in flow-through systems where the entire dose is added and then the water flow is returned to slowly dilute the dose. Constant flow treatments are comprised of a constant dose injected into the water system from a stock solution.

The disadvantages of waterborne treatments include expense, waste, and potential environmental contamination. Biological filters may also be compromised due to killing the filter bacteria. A rapid rise in ammonia has been seen using therapeutic concentrations of erythromycin in a catfish recirculating system, but chloramphenicol, nifurpirinol, oxytetracycline, and sulfamerazine did not affect the filter function (Treves-Brown, 2000).

It is also important to consider the ability of an antimicrobial to be absorbed from the water. Lipophilic compounds under a molecular weight of 100 will more likely diffuse across the gills. Antimicrobials that are absorbed from the water include chloramines, dihydrostreptomycin, enrofloxacin, erythromycin, flumequine, furpyrinol, kanamycin, oxolinic acid, oxytetracycline, nifurpirinol, sulfadimethoxine, sulfadimidine, sulfamonomethoxine, sulfanilamide, sulfapyridine, sulfisomidine, and trimethoprim. Antimicrobials that are absorbed poorly or not at all include chloramphenicol and gentamicin (Treves-Brown, 2000; Reimschuessel et al., 2005).

Dipping Treatments

These are a shorter and more controlled method of administering bath treatments. The advantages of this type of treatment are reduced waste (thus reduced expense) and less environmental contamination. The disadvantage of this type of approach is the increased stress to the animals through handling. Therefore most dip treatments are done when fish are small or in pet/aquarium cases, but commercial aquaculture producers have used tarpaulins to contain the antimicrobial (Vavarigos, 2003), and more recently “well boats” to more effectively contain treatments (Burka et al., 2012). Localized topical treatment, often under light anesthesia, has been recommended for small external lesions in pet fish (Stoskopf, 1993; Noga, 1996).

Some studies have used either hyperosmotic infiltration (first high osmolarity, >1200 mOsm/l, then lower osmolarity containing the antimicrobial) or ultrasound treatments to try to improve permeability across the gills. Antimicrobial absorption and elimination can be affected by salinity under normal conditions, and the effects of hyperosmotic treatments have not been adequately studied. Certain antimicrobials that bind divalent cations (such as the tetracyclines) may have their bioavailability compromised by the addition of salts. Ultrasound treatments to enhance absorption may be feasible in a small aquarium setting but have not been studied extensively. Both hyperosmotic and ultrasound treatments are fairly stressful. They are mainly used for vaccination rather than antimicrobial treatment (Treves-Brown, 2000; Navot et al., 2004).


Oral treatments are the most feasible methods for large commercial aquaculture systems because they are the least stressful for the animals; however, sick fish may not eat, resulting in lessened antimicrobial exposure and compromised therapeutic effectiveness. This was shown in a study where the concentration of oxolinic acid was examined in Atlantic salmon (Salmo salar) treated during an outbreak of winter ulcer disease (Moritella viscosa). Oxolinic acid was detected in plasma and tissues of healthy fish, whereas levels were below the limit of detection in moribund and dead fish (Coyne et al., 2004a). The moribund and dead fish also had no food in their gastrointestinal (GI) tracts. These results indicate that the antimicrobial appears to help actively feeding healthy fish fight off the infection, whereas fish with clinical signs are anorexic and therefore not receiving the antimicrobial. Variable intake can also occur if fish vary in size. Larger fish will probably consume more of the medicated feed than their smaller, less vigorous counterparts. Palatability, especially of the sulfonamide products, can also be a problem (Poe and Wilson, 1989).

Absorption from the intestinal tract may vary from species to species. As mentioned, saltwater fish will drink and, therefore, antimicrobials may bind cations in the water in their intestinal tracts, affecting bioavailability. The formulation of the antimicrobial may either enhance or decrease absorption.

Various methods for administering oral medications include commercial medicated feed, custom surface-coated feeds, custom feeds (e.g., gelatin diets), medicated live feeds (e.g., Artemia grown in or fed antimicrobials), injecting food (e.g., small fish used for food) and tube feeding (Noga, 1996; Treves-Brown, 2000). Obviously, some of these techniques are appropriate only for pet/aquarium fish.

Clinical Approaches by Aquatic Animal Specialists

The importance of implementing responsible antimicrobial use guidelines in clinical practice has been advocated by prominent international organizations (AVMA, 2006; World Organization for Animal Health, 2013; FDA, 2014c). A major component to responsible use of an antimicrobial in veterinary medicine is the proper identification of the causative agent, employment of standardized antimicrobial susceptibility testing (AST) procedures (when available), and reporting of such results to a veterinarian. The reporting of susceptibility test results is derived from the interpretation of the laboratory susceptibility test data using clinical breakpoints to denote an isolate as being susceptible, intermediate, or resistant. It is ultimately the responsibility of the veterinarian to make a decision on the appropriate antimicrobial to use therapeutically.

In general, it is not uncommon to encounter the following decision process in an aquatic animal disease veterinary diagnostic laboratory: (i) admit sample to laboratory; (ii) identify causative agent based on gross pathology and/or phenotypic procedures; and (iii) prescribe antimicrobial therapy. However, ideally, the following process would be followed: (i) admit sample to laboratory; (ii) identify causative agent; (iii) conduct standardized AST (CLSI, 2006, 2014a); (iv) consult available interpretive criteria (CLSI, 2014a) or published susceptibility data distributions; and (v) prescribe antimicrobial treatment or alternatively consider improved animal husbandry practices.

Rigos and Smith (2013) published a review of the multitude of factors that should be considered when recommending antimicrobial therapy in an aquaculture setting. They outlined challenges including limited in vitro susceptibility testing procedures (CLSI, 2006, 2014a), limited interpretive criteria to inform the proper dosage to be administered to the animal such that it will be effective and minimize the emergence of antimicrobial resistance (CLSI, 2014b), and differences in the PK dependent upon temperature, salinity, and fish species.

Antimicrobial Susceptibility Testing and Interpretive Criteria

When veterinarians decide whether or not to treat fish with an antimicrobial, they should consider the anticipated PK of the antimicrobial agent in the target fish species under the given conditions (i.e., water temperature, salinity, hardness). They should also choose an antimicrobial that is effective against the disease. A well-controlled and standardized antimicrobial susceptibility test is the best means to obtain this information. A critical component of an AST method is its ability to accurately predict a clinical outcome following treatment and/or detect emerging resistance. In other words, does a likely susceptible in vitro AST result automatically mean therapy will be efficacious? Conversely, does a likely resistant in vitro test result mean therapy will not be efficacious? The purpose of an in vitro antimicrobial susceptibility test is not to mimic in vivo conditions, but rather to provide reproducible results that can be used to predict clinical outcome.

Three reproducible AST methods are currently available for use in veterinary medicine. Agar and broth dilution susceptibility tests result in a minimal inhibitory concentration (MIC) for a single bacterial isolate and provide the most clinical relevance, where the MIC may be directly related to an achievable tissue or plasma concentration in vivo. Disk diffusion susceptibility tests yield diameters of the zone of inhibition which provide no correlation with achievable concentrations in vivo, but are simple and very affordable tests to run. The E-test has also gained popularity as a simple nonstandardized diffusion-based susceptibility test resulting in an MIC shown to yield virtually identical results as the more traditional broth microdilution tests (Luber et al., 2003).

Prior to 2001, aquatic animal disease researchers commonly used AST methods and clinical breakpoint values (susceptible, intermediate, and resistant) developed in their own laboratories. Different methods and breakpoint values prevented accurate interlaboratory comparisons and correlation to clinical cases. Currently, there are two Clinical and Laboratory Standards Institute (CLSI, formerly National Committee for Clinical Laboratory Standards, NCCLS) guidelines for disk diffusion and broth microdilution AST of bacteria isolated from aquatic animals (CLSI, 2007, 2014a). These guidelines have provided standardized methods and quality control procedures that have allowed testing of nonfastidious aquaculture pathogens (termed Group 1 organisms) on unsupplemented MuellerHinton media, and Flavobacterium columnare and F. psychrophilum (Group 3 organisms) on diluted MuellerHinton media, in such a way that is reproducible and more reliable (Table 53.3). These documents also describe recommended nonstandardized methods for some of the more fastidious pathogens (e.g., halophilic Vibrio spp., streptococci, etc.) (Table 53.4 ).

Table 53.3 Standard methods for broth dilution susceptibility testing of aquatic bacterial pathogens. Source: Reprinted with permission by the Clinical and Laboratory Standards Institute (CLSI) from M42/M49-S1 (Performance Standards for Antimicrobial Susceptibility Testing of Bacteria Isolated from Aquatic Animals; Second Informational Supplement, VET03/VET04-A).

Organism Medium Incubation
Group 1: Nonfastidious bacteria
Enterobacteriaceae CAMHB 22°C (24–28 hours and/or 44–48 hours) or
Vibrionaceae             28°C (24–28 hours)
Aeromonas salmonicida (typical and atypical strains)                        
Aeromonas hydrophila and other mesophilicAeromonads                        
Pseudomonas spp.                        
Plesiomonas shigelloides                        
Group 3: Gliding bacteriaa
Flavobacterium columnare Diluted CAMHB (4 g/l) 28°C (44–48 hours)
Flavobacterium psychrophilum Diluted CAMHB (4 g/l) 18°C (92–96 hours)

CAMHB, cation-adjusted Mueller-Hinton broth; NaCL, sodium chloride.

aSome modification may be necessary for testing Flavobacterium branchiophilum, which may include cations, horse, or fetal calf serum, or NaCl.


Table 53.4 Potential modifications for broth dilution susceptibility testing of aquatic bacterial pathogens. Source: Reprinted with permission by the Clinical and Laboratory Standards Institute (CLSI) from M42/M49-S1 (Performance Standards for Antimicrobial Susceptibility Testing of Bacteria Isolated from Aquatic Animals; Second Informational Supplement, VET03/VET04-A2).

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Feb 8, 2018 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Pharmacology in Aquatic Animals
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Organism Medium Incubation
Group 2: Strictly halophilic Vibrionaceae CAMHB + NaCl (1%) 22°C (24–28 hours and/or 44–48 hours) or
                        28°C (24–28 hours and/or 44–48 hours)
Group 4: Streptococci                        
Lactococcus spp. Vagococcus salmoninarum

CAMHB + LHB (2.5% to 5% v/v)

22°C (44–48 hours + CO2 if necessary for growth)