Bacterial Diseases of Fish: A Clinical Guide to Pathogens, Signs, Diagnosis and Treatment

Reviewed by the Fine PetHealth Editorial Team. Compiled from peer-reviewed veterinary literature, standard fish pathology references and current regulatory sources, all listed at the end. Last updated: September 2026.

Most bacteria that make aquarium fish sick were already in the tank before the fish got sick. They live in the water, on surfaces and in the gut of healthy fish, and they only cause disease when something tips the balance: ammonia after a filter crash, a temperature swing, a rough shipment, a fight that opens the skin. A smaller group of true pathogens arrives with new fish or live food and can spread through a healthy, well-kept system. Telling these situations apart is the whole art of managing bacterial disease, and it starts with knowing which organisms are actually out there.^1,2

This guide is organized by pathogen rather than by symptom, because the same red streak can come from three different bacteria and the same bacterium can produce four different pictures. It covers the Gram-negative rods that cause most outbreaks and the Gram-positive and acid-fast organisms that cause the hardest cases. It then explains how a laboratory confirms a diagnosis, what treatment can and cannot do, which of these bacteria can infect people, and how to keep them out of an established tank. Dosing schedules for individual medications are deliberately left to the linked articles on each drug.

Bacterial pathogens of fish at a glance

The table below is the map for the rest of the article. It groups the organisms that matter in home aquaria, ponds and the ornamental trade by how they stain, where they live and what they do. Species named in bold are the ones an aquarist is most likely to meet.

Sources for the table: Merck Veterinary Manual, Austin and Austin, Noga, Woo and Bruno, and the pathogen-specific references cited in the sections that follow.^1,2,3,5

How bacterial disease develops in fish

Host, pathogen and environment

Fish disease textbooks describe infection as a triangle: the fish, the microbe and the water they share. A healthy fish in clean water can carry motile aeromonads on its skin and in its gut for its entire life without a single lesion. Move the same fish into water with 0.5 mg/L of ammonia, or drop the temperature 6 degrees overnight, and the same bacteria invade through the gills or a scratch and multiply in the blood. The pathogen did not change. The fish's defenses did.^2,6

This is why most bacterial outbreaks in aquaria follow a recognizable event. New fish were added without quarantine. The filter was cleaned too thoroughly. A heater failed. Fish were netted, bagged and shipped. Each of these raises cortisol, suppresses the immune response and damages the mucus layer that is the fish's first barrier.⁶

Opportunistic versus obligate pathogens

It helps to sort fish bacteria into two groups. Opportunists such as Aeromonas, Pseudomonas, Vibrio and the Flavobacterium columnare group are normal inhabitants of water and biofilm. They cause disease when the fish is compromised, and they are the reason "fix the water first" is the single most useful piece of advice in fish medicine. Obligate or near-obligate pathogens such as Edwardsiella ictaluri, Francisella orientalis and Streptococcus iniae do not live indefinitely in a tank on their own. They arrive with an infected fish, spread fish to fish, and can cause losses even in a well-run system. Mycobacteria sit between the groups: they survive in biofilm for long periods and cause a slow, smoldering disease that good husbandry limits but does not eliminate.^2,26

Stressors that trigger outbreaks

The list is short, and it repeats across every published outbreak investigation: ammonia and nitrite, low dissolved oxygen, temperature outside the species' range or changing quickly, overcrowding, poor nutrition, handling and transport, aggression, and skin injury from decor or parasites. Parasites deserve a separate mention, because a heavy load of skin flukes or Ichthyophthirius opens the door for secondary bacterial infection, and treating the bacteria while ignoring the parasites guarantees a relapse.^3,6

Signs of bacterial infection and their look-alikes

No external sign is specific to one bacterium, and several of the most common signs are not bacterial at all. The table pairs what an aquarist sees with the bacterial and non-bacterial explanations, and with the observation or test that separates them. The internal links go to detailed articles on the specific treatments.

Gram-negative pathogens

The majority of bacterial disease in fish comes from Gram-negative rods, and the majority of those are opportunists.¹ Gram-negative bacteria have an outer membrane that excludes some antibiotics and carries the endotoxin responsible for the shock-like collapse seen in septicemic fish. That outer membrane is also why penicillin-class drugs are unreliable against many of them.

Motile Aeromonas septicemia and Pseudomonas

Motile aeromonads are the most frequently isolated bacteria from diseased freshwater fish worldwide.^7,8 The group is a taxonomic thicket: what hobby literature calls "Aeromonas hydrophila" is a complex of more than a dozen species including A. veronii, A. caviae, A. sobria and A. dhakensis, most of which cannot be separated without molecular methods.⁷ For practical purposes they behave alike.

The disease they cause is motile Aeromonas septicemia (MAS). Early signs are small hemorrhages at the base of the fins and along the flanks, reddening of the vent, and loss of appetite. As bacteria multiply in the blood the fish develops fluid in the abdomen, pale gills from anemia, bulging eyes and skin ulcers that start as raised red areas and slough to expose muscle. Internally the kidney and spleen are swollen and dark.⁸ Pseudomonas species produce an almost identical picture and are handled the same way.¹

Two facts about this group shape treatment. First, aeromonads are everywhere, so wiping them out of a tank is not a realistic goal. Control means removing the predisposing stress and treating affected fish.^1,8 Second, most Aeromonas species carry chromosomal beta-lactamases and are intrinsically resistant to ampicillin and amoxicillin, and surveys of ornamental fish isolates have found high resistance rates to tetracyclines as well.^7,39 A culture and susceptibility test is worth the cost when an MAS outbreak fails to respond to the first drug.

Aeromonas salmonicida: ulcer disease of koi and goldfish

Aeromonas salmonicida is the odd one out in its genus: non-motile, slow growing, and happiest at 20 to 22 °C.⁹ The "typical" subspecies causes furunculosis in salmon and trout. "Atypical" strains cause goldfish ulcer disease and carp erythrodermatitis, and they are a major concern in the koi trade.^1,9

The classic lesion is a deep, crater-shaped ulcer with a red margin on the flank or near the dorsal fin, sometimes several on one fish. In acute cases fish die with hemorrhages in the fins, gills and organs before ulcers form. Because the organism grows slowly and dislikes warm incubators, laboratories that culture koi ulcers at 37 °C can miss it entirely; a good aquatic lab incubates at 20 to 25 °C for up to a week.⁹ Blood culture is a non-lethal way to confirm the diagnosis in a valuable koi.¹ Commercial vaccines exist for salmonids and have been tried in koi with limited data on efficacy.¹

Vibrio and Photobacterium in marine fish

In saltwater the role played by Aeromonas in freshwater is taken over by the family Vibrionaceae: Vibrio anguillarum, V. harveyi, V. alginolyticus, V. vulnificus and Photobacterium damselae.^1,2,5 They cause the same hemorrhagic septicemia, the same fin and skin ulcers and the same sudden deaths, usually after transport or a temperature or salinity shock. Marine aquarists see them most often in newly imported fish and after fights.

Photobacterium damselae subsp. damselae deserves its own line because it is a genuine two-way pathogen: it kills damselfish, sharks, dolphins and farmed sea bass, and it causes wound infections in people who handle fish or swim with open cuts, occasionally progressing to life-threatening necrotizing infection.¹⁰ Culturing vibrios requires media with added salt, and identification to species now relies on MALDI-TOF mass spectrometry or gene sequencing because the biochemical profiles overlap.² Vaccines against V. anguillarum are routine in salmon farming but have no role in the aquarium.¹

Edwardsiella ictaluri, E. piscicida and E. tarda

The genus Edwardsiella was rewritten in 2013 when genetic work showed that most fish isolates filed under E. tarda were actually a separate species, now named Edwardsiella piscicida.¹² The two cannot be told apart by routine biochemistry, so any laboratory report of "E. tarda" from a fish should be read as "Edwardsiella, species not determined" unless sequencing was done.^1,12

E. ictaluri is the cause of enteric septicemia of catfish, the most costly disease in American catfish farming. It matters to aquarists because it also causes epizootics in zebrafish and has been recovered from other ornamental danios and tetras.¹³ Infected fish are lethargic, hang at the surface or spin, and show skin hemorrhages and pale gills; internally the spleen is enlarged, and the bacteria pack the kidney and brain.^1,13 It is an obligate pathogen that spreads through water and feces, which makes it a quarantine problem rather than a water quality problem.

E. piscicida has been found in freshwater and marine fish on every continent and often produces granulomas rather than acute septicemia.^1,12 E. tarda in the strict sense is the one with human relevance: it is an occasional cause of gastroenteritis and wound infection in people, usually after contact with fish or contaminated water.¹¹

Francisella orientalis

Piscine francisellosis is one of the diseases the Merck chapter does not cover, and it is increasingly relevant to cichlid keepers. The organism, renamed Francisella orientalis in 2020 from the earlier F. noatunensis subsp. orientalis, is a small intracellular coccobacillus that causes mass mortality in tilapia and infects ornamental cichlids and imported Indo-Pacific reef fish.^14,15 Surveys in Hawaiian tilapia found infection levels rose sharply in cooler water and in smaller fish.¹⁶

Affected fish waste away, breathe hard, lose color and develop pale gills from anemia. At necropsy the spleen and kidney are enlarged and studded with white nodules, each a granuloma packed with bacteria. The gross picture mimics mycobacteriosis, and the two are separated in the laboratory: Francisella is not acid-fast, will not grow on ordinary agar, and needs cysteine-enriched media or, more practically, PCR.^14,16 Because the bacteria live inside host cells, drugs that do not penetrate cells perform poorly, and control rests on quarantine and on keeping susceptible species at the warm end of their range.

Flavobacterium columnare and related species

Columnaris is the most common bacterial disease in the freshwater ornamental trade and the one most often mistaken for a fungus. Its cause was reclassified in 2022: the organism long called Flavobacterium columnare is four separate species, F. columnare, F. covae, F. davisii and F. oreochromis, which differ in host preference and virulence.¹⁷ All are long, thin, gliding rods that colonize skin and gills and secrete enzymes that digest tissue.¹⁸

Lesions begin as a dull gray patch, often on the back behind the dorsal fin ("saddleback"), around the mouth ("cotton mouth") or on the gills, and expand within hours to a yellow-white erosion with a red rim. Gill infection kills quickly by suffocation. Disease accelerates with temperature, high organic load and crowding, and virulent strains can kill a tank of livebearers in one to two days, before the aquarist has decided what they are looking at.^1,18

The diagnosis is one of the few that can be made at home with a microscope. A scraping from the lesion edge on a wet mount shows dense masses of slender bacteria stacked into columns or "haystacks", often gliding at the edge of the mass.^1,18 Water mold, the main look-alike, shows broad branching hyphae instead. Because the organism lives on the surface, bath treatments reach it, and lowering the temperature toward 24 °C slows it while medication works.¹⁸

Flavobacterium branchiophilum: bacterial gill disease

Bacterial gill disease is a problem of crowded, cool freshwater systems, best known in trout hatcheries but described in other species under heavy stocking.^19,20 The bacteria colonize the gill surface and provoke the gill epithelium to swell and fuse, so the fish cannot extract oxygen. Fish gasp, line up at the inflow and stop eating; deaths follow any further drop in oxygen. Unlike columnaris there is little tissue destruction, and unlike parasitic gill disease there are no flukes on the wet mount, just filamentous bacteria along thickened filaments.¹⁹ Reducing stocking density and organic load is the treatment; medication alone rarely holds.

Tenacibaculum maritimum

Marine aquarists meet a columnaris-like disease that is caused by a different filamentous gliding bacterium, Tenacibaculum maritimum, once classed as Flexibacter.²¹ It erodes the mouth, frays fins and tail and opens ulcers on the flank, and it is the leading cause of skin disease in farmed sea bass and turbot. In the aquarium it follows shipping stress and skin abrasion in new arrivals. It needs seawater-based media to grow, and the wet mount appearance is the same as freshwater columnaris.²¹

Gram-positive and acid-fast pathogens

Gram-positive infections are less common in fish than Gram-negative ones, but they include the two hardest problems in the hobby: streptococcal meningitis, which kills fast, and mycobacteriosis, which cannot be cured.¹

Streptococcus and Lactococcus

Streptococcus iniae, S. agalactiae and Lactococcus garvieae cause outbreaks in warm water with mortality that can pass 50 percent in a week, or grind on for weeks with a few deaths every day.^1,22,24 Their target organ is the brain. The sign that separates strep from everything else is neurologic: fish spin, spiral, swim in tight circles, hang head down or lie on the bottom. Bulging eyes with cloudy corneas, bleeding at the vent and around the mouth, and a rigid curved body fill in the picture.²²

Susceptible aquarium species include rainbow sharks, red-tailed black sharks, rosy barbs, danios, tetras and cichlids, and the Florida extension guidance is blunt that all fish should be considered susceptible.^1,22 Sources of infection are previously infected fish, contaminated live foods, and in some cases amphibians or wild fish sharing the water.¹ Diagnosis is by culturing brain and kidney on blood agar; the colonies are tiny and the Gram stain shows chains of cocci.^1,22 Macrolides such as erythromycin are often the first choice on susceptibility data, though no drug is approved for this use in fish.¹

S. iniae is also a proven human pathogen. A 1997 cluster in Toronto involved nine people, eight with hand cellulitis and one with endocarditis, all of whom had handled fresh whole fish and most of whom had punctured their skin doing so.²³

Erysipelothrix

Ulcers on the snout and inside the mouth of tiger barbs, rosy barbs and tetras are frequently caused by Erysipelothrix, a Gram-positive rod better known from pigs and poultry.¹ Affected fish stop eating and die slowly over days. The fish-associated species was formally described in 2020 as Erysipelothrix piscisicarius, with tiger barbs as the experimental model.²⁵ Fish with limited lesions respond to erythromycin.¹ The related E. rhusiopathiae causes erysipeloid, the classic "fish-handler's disease" of fishermen and fish market workers, which is why gloves are sensible when handling fish with mouth lesions.⁴¹

Mycobacteriosis: fish tuberculosis

Mycobacteriosis is the disease behind most "my fish just wastes away and dies one at a time" stories. Mycobacterium marinum, M. fortuitum, M. chelonae and several other species cause it; all are acid-fast rods that grow slowly, survive for months in biofilm and organic debris, and enter fish mainly by ingestion of contaminated material or through skin damage.^26,27 Bettas, gouramis, danios, seahorses and cichlids are frequently reported hosts, but any fish can be infected.^1,27

Signs are vague and chronic: weight loss despite eating, faded color, a curved spine, scale loss, non-healing skin ulcers, bulging eyes, and in some fish fluid in the abdomen.^1,26 Reproductive failure is common in breeding colonies.²⁷ At necropsy the spleen, kidney and liver contain gray-white nodules, sometimes fused into tumor-like masses; a squash of one of these under the microscope shows granulomas, and an acid-fast stain shows the red rods inside them.^1,26 Culture on Lowenstein-Jensen or Middlebrook media at 25 to 30 °C takes three to four weeks, so PCR on tissue is increasingly used instead.^1,28

There is no treatment that eliminates mycobacteria from an infected fish.^1,27 Antibiotic courses may slow the disease in a valuable individual but do not cure it, and the fish remains a source of infection for tankmates. The practical decisions are about the population: euthanize clinically affected fish, decide whether to keep the remaining fish as a closed group, and disinfect properly before restocking. Bleach removes organic matter but is not reliably mycobactericidal; it should be followed by 70 percent ethanol or a phenolic disinfectant.¹ Ultraviolet sterilizers reduce bacterial counts in the water column and help limit spread in display systems.¹

Nocardia

Nocardiosis produces granulomas that look exactly like mycobacteriosis and is separated from it only in the laboratory, where Nocardia appears as branching filaments that are only partially acid-fast.³¹ Nocardia seriolae is a serious problem in farmed yellowtail and sea bass in Asia; sporadic cases occur in aquarium fish.³¹ Management follows the mycobacteriosis model.

Intracellular bacteria: epitheliocystis

Epitheliocystis is an infection of gill cells, and occasionally skin cells, by bacteria of the order Chlamydiales that cannot be grown in culture at all.^32,33 Infected cells swell enormously and appear as whitish cysts on the gill filaments. Light infections cause no signs and are found incidentally on gill biopsies; heavy infections cause labored breathing and lethargy and can kill young fish.^1,32 Discus and oscars are the aquarium species most often reported.¹ Diagnosis is by gill biopsy, where the cysts are distinctive, and PCR can name the agent.³³ Oral oxytetracycline has been used to control heavy infections, and improving water quality and reducing stocking density is usually enough for light ones.^1,32

Diagnosis: from tank-side observation to the laboratory

Every reference on this subject makes the same point in different words: antibiotics should be chosen after a diagnosis, not instead of one.^1,3,36 The steps below go from what any aquarist can do in ten minutes to what a diagnostic laboratory does over a week.

Water testing first

A liquid test kit for ammonia, nitrite, nitrate and pH, plus a thermometer, answers the first question in any sick tank: is the environment the disease? Ammonia above 0.25 mg/L or any detectable nitrite in a stocked tank explains red gills, red streaks, clamped fins and gasping without any bacteria at all, and it makes bacterial infection likely within days if not corrected.⁶ Dissolved oxygen is harder for hobbyists to measure but is the first thing an aquaculture pathologist checks when fish crowd at the surface.

Non-lethal sampling

Skin scrapes, gill biopsies and fin clips are standard, low-risk procedures in fish medicine and are described step by step in Noga's and Roberts' textbooks.^3,4 A scrape is taken by drawing a coverslip along the lesion edge, in the direction of the scales, and mounting the mucus in a drop of tank water. A gill biopsy removes the tips of a few filaments with fine scissors. Large or valuable fish should be sedated first. The point of sampling the lesion edge rather than its center is that the center is dead tissue colonized by whatever happened to be in the water.

Wet mount, Gram stain and acid-fast stain

Under a 100x to 400x microscope, a wet mount rules the big categories in or out within minutes. It shows protozoa such as Ichthyophthirius and Trichodina, flukes on the gills, the broad hyphae of water mold, and the column-forming rods of columnaris.^1,18 A Gram stain of a lesion or of a squash of kidney tissue separates the Gram-negative rods that dominate fish disease from the Gram-positive cocci of Streptococcus.²² A Ziehl-Neelsen acid-fast stain of granuloma material is the presumptive test for mycobacteriosis and is available in any veterinary laboratory.²⁶

Culture and susceptibility testing

Culture is where most home diagnosis stops and laboratory diagnosis begins. Fish bacteria grow at lower temperatures and often more slowly than the human pathogens most laboratories are built around, which is why aquatic isolates are cultured at 22 to 28 °C and why a laboratory unfamiliar with fish can report "no growth" on a fish that was dying of septicemia.³⁴ The kidney is the preferred organ for systemic infection; the lesion margin for skin disease; brain for neurologic signs.^1,22 Special media are needed for mycobacteria, Francisella and marine vibrios.

Once an organism grows, susceptibility testing tells you which antibiotics inhibit it. The Clinical and Laboratory Standards Institute publishes methods specifically for bacteria from aquatic animals (VET03 for the procedures, VET04 for the interpretive standards), and a laboratory that uses them will give results that mean something for a fish rather than for a dog.³⁴ The World Organisation for Animal Health's Aquatic Manual sets the reference methods for the internationally listed fish diseases.³⁵

PCR, MALDI-TOF and sequencing

Several of the organisms in this article cannot be named by growth and biochemistry alone. Edwardsiella piscicida and E. tarda are indistinguishable without sequencing; Francisella barely grows; mycobacteria take a month.^1,12,28 Diagnostic laboratories now use PCR on tissue for these, and MALDI-TOF mass spectrometry to identify cultured isolates to species within minutes. For the aquarist the practical message is to ask which method was used before acting on a laboratory name.

How to get a laboratory diagnosis

Many state veterinary diagnostic laboratories and university aquatic animal health programs accept fish from hobbyists, and aquatic veterinarians are listed by the American Association of Fish Veterinarians and the World Aquatic Veterinary Medical Association. The ideal submission is two or three live fish showing signs, shipped in tank water with oxygen. The alternative is a fish euthanized immediately before shipping and kept chilled on ice packs, never frozen: freezing destroys the tissue architecture the pathologist needs. Send a water sample and a written history: tank size, stocking, dates of additions and losses, water test results and every treatment already tried.³

Treatment principles

This section is about the logic of treatment, not the numbers. Doses, durations and routes for individual medications are in the linked guides for each drug, and they differ enough between drugs that a general table would be misleading.

Correct the environment first

Every extension bulletin and every textbook puts this first for a reason: fish in bad water do not recover on antibiotics, and fish in good water often recover without them.^3,6,36 A 30 to 50 percent water change, a check of the filter, extra aeration and a temperature back inside the species' range are the first treatment. For columnaris, reducing temperature toward 24 °C slows the bacteria.¹⁸ For Francisella, warming the water toward 28 to 30 °C reduces losses in tilapia and cichlids.¹⁶

Hospital tank and biofilter protection

Treat in a separate hospital tank whenever the number of sick fish allows. It concentrates the drug on the fish that need it, spares the display tank's biological filter, protects invertebrates and plants, and makes it possible to keep the water pristine with daily changes. When the whole population is affected and the display tank must be treated, remove activated carbon and other chemical media, which adsorb most antibiotics, and expect the nitrifying bacteria to take a hit from some drugs; test for ammonia and nitrite daily during and after the course.³⁶

Bath, medicated feed or injection

Antibiotics reach fish by three routes and the choice depends on where the bacteria are. Bath treatment, dissolving the drug in the water, is the standard hobbyist method and works well for external infections such as columnaris and fin rot, less well for systemic infections because uptake across the gills and skin is limited and variable.³⁶ Medicated feed delivers a reliable dose to the blood and organs and is the preferred route for septicemia and internal disease, with the obvious limit that the fish must still be eating.³⁶ Injection is the most precise route and the standard for valuable koi and large fish under veterinary care.^3,36 A fish that has stopped eating and has systemic signs is exactly the fish that bath treatment is least likely to save, which argues for early medicated feeding while appetite lasts.

Antibacterial classes by spectrum

The table summarizes the classes used in ornamental fish and what they generally cover. "Generally" matters: resistance is common, particularly among aeromonads, and the only reliable guide for a specific outbreak is a susceptibility test.^7,39

Spectrum summaries are drawn from Noga, Austin and Austin, and the Florida extension circular on antibiotic use in ornamental fish.^2,3,36 In the United States only a handful of drugs are approved for use in fish, all for food species, and the FDA maintains the current list; the FDA's Phish-Pharm database collects the published pharmacokinetic data behind extra-label use in fish.^37,38 Products sold for ornamental fish are for ornamental fish only and never for animals raised for food or for human use.

When treatment is not an option

Three situations call for a decision other than "which antibiotic". Mycobacteriosis and nocardiosis cannot be cured, and the affected fish is a reservoir; euthanasia of clinical cases is the standard recommendation.^1,27 Fish that have stopped eating, lie on the bottom and have advanced septicemia are rarely saved by any route of treatment, and prolonged suffering is not a kindness. And any fish that fails to respond to two appropriately chosen courses should be cultured or necropsied rather than given a third drug, because the answer is more likely to be a wrong diagnosis than the wrong antibiotic.

Antimicrobial resistance in ornamental fish

The ornamental trade moves billions of fish a year through farms, wholesalers and shops where antibiotics are used freely, often in shipping water, and resistance is the predictable result. A systematic review of the published research found Aeromonas the most studied genus in ornamental fish, with high reported resistance to amoxicillin, penicillin, tetracycline and oxytetracycline, and a survey of industry professionals found quinolones, tetracyclines, nitrofurans and aminoglycosides in frequent use.³⁹ Resistance genes on plasmids move between fish bacteria and between fish bacteria and human ones.^7,39

For the aquarist this has three consequences. The drug that worked on a forum post in 2015 may not work on fish from a different supplier in 2026. Prophylactic dosing of new arrivals "just in case" selects for resistant bacteria in the tank that will be there when a real infection comes. And a full course at the correct dose, followed by a culture if it fails, is better medicine. It is also the one way to keep the few effective drugs effective.^36,39

Can you catch it? Zoonotic bacteria from aquariums

A critical review of the evidence concluded that of the many bacteria described as fish-borne zoonoses, only Mycobacterium species, Streptococcus iniae, Vibrio vulnificus and Clostridium botulinum are well supported as zoonoses in the strict sense, with others such as Aeromonas, Edwardsiella and Erysipelothrix resting on weaker or circumstantial evidence.⁴⁰ The risk from a home aquarium is real but small, concentrated in people with skin breaks or weakened immunity, and almost entirely preventable.

Mycobacterium marinum is the one aquarists actually encounter. "Fish tank granuloma" begins as a slow-growing red nodule on a finger, hand or forearm weeks after a cut or scrape sustained while working in a tank, and because the organism grows best below body temperature it stays in the cooler skin of the extremities.^28,30 Diagnosis is often delayed because physicians do not think of it, and in roughly a third of reported cases the infection has spread to tendons, joints or bone by the time it is recognized.²⁹ Telling a doctor about the aquarium is the single most useful thing a patient can do.

Streptococcus iniae infects the hands of people who handle fresh whole fish, as the Toronto cluster showed.²³ Vibrio vulnificus and Photobacterium damselae infect wounds exposed to seawater or raw seafood; the CDC notes that in people with liver disease and other underlying conditions Vibrio can cause bloodstream infection, and that about one in five people with a V. vulnificus infection die, sometimes within a day or two.^10,42 Aeromonas and Edwardsiella tarda are occasional causes of diarrhea and wound infection in people with fish and water exposure.^7,11,41

The precautions are the ones a public aquarium requires of its staff. Cover cuts or wear gloves when your hands are in the tank, do not start a siphon by mouth, wash hands and forearms after tank work, and keep aquarium water away from food preparation surfaces. Take extra care if you are immunosuppressed, on chemotherapy, or have liver disease.^30,41

Prevention and biosecurity

Nearly every obligate pathogen in this article enters a tank the same way: with a fish. Quarantine of new arrivals in a separate tank with its own equipment is the single most effective control measure. Two to four weeks is a reasonable minimum, longer for valuable or wild-caught fish; the Florida extension guidance on recirculating systems describes quarantine periods from one to eight weeks depending on species and the diseases of concern.⁴³ Quarantine also gives new fish time to recover from transport before they meet an established population's resident bacteria.

Beyond quarantine, the measures are the ones that prevent stress: stable temperature, zero ammonia and nitrite, nitrate kept low with water changes, stocking within the system's capacity, a varied diet, and avoiding live foods of unknown origin, which have carried Streptococcus and other pathogens into aquaria.^1,6 Nets, siphons and buckets should be dedicated per tank or disinfected between tanks. After a mycobacterial or francisellosis outbreak, disinfection means removing organic material, then a disinfectant that actually kills the organism, then a period empty before restocking.^1,27 Ultraviolet sterilization on the return line lowers the bacterial load in the water column and is standard in public aquaria for that reason.¹

Frequently asked questions

Glossary

Acid-fast

A staining property of mycobacteria and some related organisms, whose waxy cell wall retains red dye after an acid wash. The Ziehl-Neelsen stain is the standard acid-fast stain.

Biofilm

A layer of bacteria and their secretions on submerged surfaces, including filter media, gravel and decor. Mycobacteria and many opportunists persist in it.

Culture and susceptibility (C and S)

Growing the bacteria from a sample on laboratory media, identifying them, and testing which antibiotics inhibit them.

Gram-negative and Gram-positive

The two large groups into which bacteria fall on the Gram stain, reflecting different cell wall structures and, in practice, different antibiotic susceptibilities.

Granuloma

A nodule of immune cells walling off an organism the body cannot destroy. Typical of mycobacteria, Nocardia and Francisella.

Hospital tank

A separate, bare tank with its own filter and heater used to treat sick fish away from the display system.

MALDI-TOF

A mass spectrometry method that identifies a cultured bacterium to species in minutes from its protein signature.

Obligate pathogen

A microbe that causes disease whenever it infects a susceptible host and does not persist indefinitely in the environment without one.

Opportunistic pathogen

A microbe normally present in the environment or on the host that causes disease only when the host's defenses are impaired.

PCR

Polymerase chain reaction, which detects the DNA of a specific organism in tissue without needing to grow it.

Septicemia

Bacteria multiplying in the bloodstream and spreading to organs; in fish usually accompanied by hemorrhages, fluid in the abdomen and organ swelling.

Wet mount

A fresh, unstained sample of mucus, fin or gill tissue in a drop of water on a microscope slide, examined live.

Zoonosis

An infection that can pass from animals to people.

References

1. Merck Veterinary Manual. Bacterial Diseases of Fish. Merck and Co., Inc. merckvetmanual.com
2. Austin B, Austin DA. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish. 6th ed. Springer; 2016. doi:10.1007/978-3-319-32674-0
3. Noga EJ. Fish Disease: Diagnosis and Treatment. 2nd ed. Wiley-Blackwell; 2010. ISBN 978-0-8138-0697-6.
4. Roberts RJ, ed. Fish Pathology. 4th ed. Wiley-Blackwell; 2012. ISBN 978-1-4443-3282-7.
5. Woo PTK, Bruno DW, eds. Fish Diseases and Disorders, Volume 3: Viral, Bacterial and Fungal Infections. 2nd ed. CABI; 2011. ISBN 978-1-84593-554-2.
6. Francis-Floyd R. Stress: Its Role in Fish Disease. UF/IFAS Extension, Circular 919 (FA005), revised 2018. edis.ifas.ufl.edu/publication/FA005
7. Janda JM, Abbott SL. The genus Aeromonas: taxonomy, pathogenicity, and infection. Clin Microbiol Rev. 2010;23(1):35-73. doi:10.1128/CMR.00039-09
8. Hanson LA, Hemstreet WG, Hawke JP. Motile Aeromonas Septicemia (MAS) in Fish. Southern Regional Aquaculture Center, SRAC Publication No. 478; 2019. srac.msstate.edu
9. Dallaire-Dufresne S, Tanaka KH, Trudel MV, Lafaille A, Charette SJ. Virulence, genomic features, and plasticity of Aeromonas salmonicida subsp. salmonicida, the causative agent of fish furunculosis. Vet Microbiol. 2014;169(1-2):1-7. doi:10.1016/j.vetmic.2013.06.025
10. Rivas AJ, Lemos ML, Osorio CR. Photobacterium damselae subsp. damselae, a bacterium pathogenic for marine animals and humans. Front Microbiol. 2013;4:283. doi:10.3389/fmicb.2013.00283
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This article is for general educational purposes only and is not a substitute for examination and advice from a veterinarian. Products sold for ornamental fish and pet birds are for those animals only, not for animals raised for food and not for human use. This page was prepared by the Fine PetHealth Editorial Team with AI assistance, checked against the sources listed above, and reviewed by a person before publishing.