Review of Bacterial Diseases of Aquarium Fish
IAAAM Archive
E. de Guzman; E. B. Shotts; J. B. Gratzek
Department of Medical Microbiology, College of Veterinary Medicine, University of Georgia, Athens, GA


A review will be presented on the predominant factors leading to bacterial outbreaks and how to recognize them. The actual role that bacteria play in a disease outbreak may vary from pathogen to pathogen causing disease only when the fish has become weakened by other causes such as parasites, poor water quality and other stresses. The discussion will include methodology for the isolation of bacteria from fish and submission of specimens. Major aquarium fish pathogens such as Aeromonas complex, Flexibacter, sp., Pseudomonas sp., Vibrio sp. and Mycobacteria sp. will be briefly reviewed.

Bacterial diseases can be a cause of high mortality in aquarium fish. There are members of at least 25 bacterial genera which have been implicated as possible causative agents of bacterial disease in fish, (1) most of which are Gram-negative aerobes or facultative anaerobes (2).

Bacteria which produce disease in fish are usually present in the fish's environment, in the surrounding water, on the fish surface, or in their internal organs (2). The total number of bacteria per milliliter of water in an established aquarium varies from 10(4) to 10(6) organisms per milliliter (3). Collins et al. (4) demonstrated that in a tank undergoing the nitrification cycle the number of lactose fermentors per milliliter of water increases to approximately 10(4) organisms per milliliter on day 7 and then decreases to 10(2). Organisms per milliliter by day 28 while the counts of aeromonads initially are at very low levels and increase to approximately 10(4) organisms per milliliter by day 7. These counts are dependent upon the presence of fish in the tank. If fish are removed from the tank, counts decrease greatly but increase rapidly when fish are reintroduced.

Many investigators have demonstrated the presence of bacterial pathogens such as Aeromonas complex, Pseudomonas spp., and Vibrio spp., in the water, on the surface, and in the tissue of healthy fish (5,6,7,8,9). The question arises, 'what causes the fish to become bacterially infected?" Fish usually succumb to bacterial disease when they are submitted to a period of stress or a series of stresses such as poor water quality, parasitic infections, poor nutrition or temperature extremes (10,11). Bacteria are also frequently secondary invaders following disruption of the integument by parasites, rough netting, or rough handling. It has been established that stress causes the immune system to become compromised and can thus lead to decreased resistance to disease (12).

In establishing a diagnosis of bacterial etiology in a fish disease problem, it is necessary to first exclude factors such as water quality problems or environmental problems (toxicities, nutrition, or oxygen depletion). The exclusion of parasitic infections is also required. These factors frequently act as stressors and frequently precede bacteria or act concomitantly with bacterial infections (2,3).

Clinical signs and lesions for most bacterial disease are not pathognomonic and etiologic diagnosis based on clinical signs alone is ill advised if not impossible. The most frequently observed lesions are hemorrhages, ulcers, fin and tail rot, "mouth fungus", "saddle back lesions", ascites, exophthalmia, and color changes. Fish frequently may appear lethargic and inappetent (2).

If a fish is suspected of having a bacterial disease, identification of the etiological agent will require isolation and identification. This process verifies the presence of a bacterial infection and allows the diagnostician to perform an antibiotic sensitivity test to ensure that correct medication is being used. The possibility of culturing is dictated by the type of client or clinical situation. In commercial production where hundreds of dollars may be at stake, culture and sensitivity testing is mandatory; also in large aquariums it may be advisable to keep a record of the persistent bacteria and sensitivity patterns while in home aquaria, where the amount of money invested is less, it is not economically realistic to culture. If the situation warrants culture, the clinician should either attempt to culture the fish, take samples for culture, or send the fish to a qualified laboratory (2).

The preferred method of submitting specimens is to send both moribund and apparently healthy fish to a diagnostic laboratory. This allows the laboratory to culture the fish as well as keep a few fish in an aquaria for observation of disease development. when sending live specimens is not possible, an alternate method of submitting specimens would be to aseptically remove the kidney of a fish, place it in a sterile receptacle such as a syringe barrel or a petri dish, freeze it, and mail it frozen to the laboratory. A third possibility when dealing with large fish is to culture the kidney using a Culturette® (Charion Scientific, Kansas City, MO) swab and mail the swab to the laboratory. It is imperative to culture live or moribund fish and not dead fish because of rapid postmortem migration of normal bacteria into the tissues. Only as a last resort should fish packed in ice be sent to the laboratory (13,14).

The following are brief descriptions on how to obtain bacterial cultures from fish. For identification of the isolated bacteria the diagnostician is referred to laboratory procedures published by Shotts and Bullock or the Fish Health Blue Book (15)

For Large Fish (2):

A. To facilitate handling, wrap fish in a paper towel.

B. Examine the fish and make smears of gross external lesions and gram stain the smears.

C. Kill the animal.

D. Sear area immediately behind the dorsal fin using a glowing spatula. Make sure that both sides as well as the dorsal fin are seared.

E. Dip scissors in 95% alcohol and flame.

F. With the cooled scissors cut into the middle of the seared area and push downward until the spinal column is exposed and sever it.

G. Bend the tail and head together, keeping dorsal fin uppermost.

H. With the spinal column severed, the surrounding tissue will tear and the exposed dark red mass vertical to spinal column is the kidney.

I. Insert loop down into the kidney tissue and streak the culture plate.

For Small-Fish (2):

A. Kill the fish.

B. Place fish in a receptacle containing a surface disinfectant (chlorine, 1:5000 Roccal).

C. Transfer fish to a small flask of 95% alcohol. Use forceps which have been previously sterilized by flaming to transfer the fish.

D. Cut the fish behind the dorsal fin and proceed as described in (F) for large fish.

As previously discussed, there are many genera of bacterial fish pathogens. Five of the most commonly isolated are: Aeromonas complex, Flexibacter spp., Pseudomonas ssp., Vibrio ssp., and Mycrobacteria ssp., Edwardsiella ictaluri is another species which is gaining importance. The methods of transmission of these bacteria are similar; these include the oral route, skin abrasions, gill damage, and latent carriers. The following is a table containing some of the salient features of the diseases caused by the above mentioned bacteria.

Table 1. Common Bacterial Pathogens of Fish


Common Name

Signs and Pathology

Aeromonas hydrophila complex

Bacterial hemorrhagic septicemia;
fresh water; can affect salt & brackish

hemorrhages in skin

salmonicida with A

Ulcer disease of goldfish

injected fins

Edwardsiella ictaluri

Enteric septecemia of Catfish; "Hole in the Head Disease"* Danio; green knife fish; walking catfish freshwater

hole in the head
petechial hemorrhages
brain abscesses
aberrant swimming

Flexibacter Columnaris

Columnaris disease
cottonmouth disease
tail rot; Mouth fungus; freshwater.
marine occasionally

skin lesions
saddle back
fin & tail rot

Pseudomonas fluorescens

Pseudomonas septicemia fresh;
brackish and marine water

hemorrhagic septecemia
ascites; hemorrhages
exophthalmia; ulcers

Vibrio Anguillarum

vibriosis; worldwide
mostly marine and estuarine env.

hemorrhagic septicemia
exophthalmia; ulcers

Mycobacterium fortuitum & M. marinum,

Mycobacteriosis worldwide

skin lesions;
wasting exophthalmia; weakness
color changes

*Limited to channel catfish
Note: Definitive diagnosis for these diseases is achieved by isolation and identification of the organism

Considerations when medicating fish include how the antibiotic is administered, legality of the antibiotic, effectiveness, dose, toxicity, and adsorbability.

Many antibiotics may be administered by inclusion in food; others as bath treatments or by injection. Antibiotics in food are used in the aquarium trade prior to a stress period such as transportation. Administration of antibiotics in water is popular in the aquarium industry. Problems encountered include adsorbability, effect on the biologic flora of the filter bed, and with indiscriminate use possible development of resistant strains (2,3,16,17,18).

Effectiveness of therapy is always a consideration. Most bacterial fish pathogens are gram negative. Thus the use of antimicrobials with a gram positive spectrum is questionable. Legal aspects are also a consideration in food fish therapy. Presently, only tetracycline, Romet (Hoffman-LaRoche, Nutley, NJ) and sulfas are allowable for food fish (2,15).

In the aquarium trade a variety of antibiotics are used as additives to the water. The success of this type of treatment depends on the adsorbability of the antibiotic, a sufficient dose, and lack of toxicity. In general, it is a good practice to use an adequate dose (see below) and limit a treatment to 5 hrs. Overtreatment with nitrofurans and sulfonamides can cause nephrotoxicity (2,19,20).

Antibiotics may be injected which assures a therapeutic blood level. This method is particularly useful in larger fish and can be used prior to or immediately after a stress period, or in the event of a suspected bacterial disease often associated with frank ulcerations. A preferred treatment is an intraperitoneal injection of chloramphenicol. Use of a tuberculin syringe with a 26 gauge needle works well. Fish can be netted with an injection being given through the mesh (16).

On the following page is a list of some antibiotics and dosages which are commonly used in aquaria therapy.

Table II. Common Antibiotics Used in Therapy of Aquarium Fish




Adsorbed, resistant bacteria


Adsorbed, minimal resistance bacteria

Nalidixic Acid

Adsorbed, minimal resistance bacteria


Marine fish (swallow H20)


Minimal resistance, expensive, adsorbed?


Pot. sulfa, adsorbed?

Oxolinic acid

Not adsorbed, minimal resistance
Stable in H20
Ulcer disease (U.K.)


Not adsorbed, use for injection
For external problems

* = 12 ppm dose (500 mg. in 10 gal. water)
** = 1.2 ppm dose (50 mg. per 10 gal. water)

Note: Most antibiotics will be adsorbed within 5 hours. During therapy: remove carbon, remove air stems from lift stacks, change water at 5-8 hours. Treatment can be repeated daily for 3-4 days or until signs of infection have disappeared.

Mycobacteria (13,16,19,21,22,)

Mycobacteria are ubiquitous in nature and piscine mycobacteriosis is common in fish in home aquaria. Signs of the disease may include wasting, external ulcerations, lifting of the scales (edema), fin rot, ascites and exophthalmia which may be unilateral or bilateral. In home aquaria, the aquarist notes a fish which will not eat and which is slowly wasting away. The fish eventually dies and is eaten by other fish thus perpetuating the disease.

In routine examination of fish, many have granulomas associated with acid fast bacteria. It is apparent from clinical observation that a mycobacterial infection in a tank need not be a catastrophe, eventually leading to widespread mortalities. This is especially true if the water quality is maintained, fish are not overcrowded, and the level of nutrition is kept high.

Once a diagnosis has been made in an aquarium (by acid-fast staining of impression smears of organs) the question arises whether to treat with antibiotics. Indications are that good water quality and diet is an adequate therapy. However, if treatment is requested or deemed necessary, feeding of food with antibiotics added (Kanamycin 3 mg/100 g food, Rifampin 6 mg/100 g food, Cycloserine 3 mg/100 g food, Minocycline 0.05 mg/100 g food) is recommended. Remissions should be expected.

Bacterial disease in aquaria can be minimized by maintaining good water quality, eliminating parasitic infections, providing good nutrition and minimizing stress.


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2.  Brown, E. E. and Gratzek, J. B. Fish Farming Handbook. AVI Publishing Co., Westport, Connecticut, 391 pp. (1980).

3.  Gratzek, J. B. An overview of ornamental fish diseases and therapy. J. Small Anim. Pract. 22r 345-366 (1981).

4.  Collins, M. T., Gratzek, J. B., Shotts, E. B., Dawe, D. L., Campbell, C. M., and Senn, D. R. Nitrification in an aquatic recirculating system. J. Fish. Res. Ed. Can. 20 2 5-20 3 1 (197 5).

5.  Gratzek, J. B., Shotts, E. B., and Blue, J. L. A survey of parasites, bacteria and viruses associated with tropical fish imported from southeast asia. Aquatic Mammals 1, 1-5 (1976).

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10. Gratzek, J. B. and Reinert, R. Physiological responses of experimental fish to stressful conditions. Natl. Cancer Inst. Monogram 65, 187-193 (1984).

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13. Shotts, E. B. and Bullock, G. L. Bacterial diseases of fishes: diagnostic procedures for gram-negative pathogens. J. Fish. Res. Rd. Can. 22, 1243-1247 (1975).

14. Shotts, E. B. and Bullock, G. L. Rapid diagnostic approach in the identification of Gram-negative bacterial diseases of fish. Fish Pathology 102, 187-190 (1976).

15. Amos, K. H. (Editor). Procedures for the detection and identification of certain fish pathogens. 3rd ed. Fish Health Section, American Fisheries Society, Corvalis, Oregon (1985).

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17. Aoki, T. and Kitao, T. Detection of transferable R plasmids in strains of the fish pathogenic bacterium, Pasteurella piscida. J. Fish Dis. D., 345-350 (1985). Shotts, E. B., Vanderwork, V.L., and Campbell, L. M. Occurrence of R factors associated with Aeromonas hydrophila isolates from aquarium fish and waters. J. Fish. Res. Board Can. 31, 736-740 (1976).

18. Post, G. W. Textbook of Fish Health. TFH Publication, Neptune, NJ (1983).

19. Roberts, R. J. Fish Pathology. Bailliere Tindall, London (1978).

20. Snieszko, S. F. Mycobacteriosis (tuberculosis) of fish. Fish & Wildlife Service. Fish disease leaflet #55 (1978).

21. van Duijn, C. J. Tuberculosis in fishes. J. Small Anim. Pract. 21, 391-414, (1981).

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E. de Guzman