All bacteria are opportunists; some will have invaded the tissues of a fish corpse within half an hour of death. But not all bacteria are pathogenic, that is, capable of causing disease. And not all pathogenic bacterial strains are virulent, that is, capable of killing a host. Bacteria are everywhere; they cannot be kept out of the aquarium.

Here's an excellent post from Mike Duskis (aka MADman) at Aquaria Central:

"The common pathogens in the environment are not sufficient in themselves to cause disease. If they were, every animal would be ill all the time. An animal becomes ill when something changes in its body or in its environment that gives the pathogens an opportunity. Too much of our medicine focuses on eliminating the pathogens while ignoring the environmental factors ...factors such as water and air quality, plumbing, and the health of our neighbors have a great deal of effect on our health. Pathogens will always be with us, but if our environment is healthy, we are much less likely to become ill.

"The same goes for aquarium fish, but perhaps to an even greater degree, because we have so much control over their environment. In the last four years or so, the only disease I've seen in my tanks has been fungal fin rot--— and that has always been brought on by cichlid aggression, clearly an environmental factor. Aggression aside, my fish do not get sick. They do not get sick because their environment is balanced and stable.

"I suppose you could say that I'm on a crusade for thinking about fish wellness rather than fish disease. Yes, we need to treat infections when they arise, but we should also think about the environmental causes of those infections. If we do not, we will be forever dependent on medications which, in my experience, can do great harm to the aquarium ecosystem and result in more disease."

You can be sure that many bacteria that could have a pathogenic effect on a fish with a compromised immune system are already in the aquarium. The main reservoir of latent bacterial infection is not the gravel that you might be vacuuming so diligently, however, but the other fishes. The bacteria involved are ubiquitous, but they aren't ordinarily virulent, that is, not until environmental stresses depress a fishes' immune system. By the time bacterial populations have overwhelmed a fishes' immune response to the extent that it is showing acute symptoms, such as bloating or impaired balance, the sick fish has become a major source of infection for its tankmates.

Bacterial infections aren't "curable." The fishes' own immune system may kick in and swamp the infection. In the meantime, the best any anti-bacterial medication can do is slow down the replication of the invaders.

Products marketed for the aquarium hobby may be called "bacteriostatic" or "bactericidal." A bacteriostatic ingredient stops bacteria from dividing and multiplying. A bactericidal ingredient kills them. An ingredient labelled "anti-bacterial" need only slow the division of bacteria, no more. Microbiologist Dr. Beverly A. Dixon gave an introductory survey of these functions of antibiotics in an article "Antibiotics: how do they work?" in Aquarium Fish, Feb 1992.

Questionable effectiveness doesn't keep various preparations that contain "anti-bacterial" ingredients off the shelves at your LFS. Tetracycline is the classic example. It is routinely added to animal feed as a growth enhancer, over the ineffectual objections of the F.D.A. (It seems to depress certain gut bacteria that destroy the vitamin B12 created by other bacteria.) Does routine use of antibacterial drugs promote growth in fishes, too? I don't know the answer to this, but the annual use of antibiotics in American agribusiness represents some eight times the amount used annually to fight infection in humans. The exact figures are obscured because the chicken/pork/beef industries join ranks with veterinary drug companies to form the Animal Health Institute, to protect as "business secrets" actual figures of drug production and use. Tetracycline is still being sold to you as a medication for tropical fish. But most any bacteria you'll meet with have evolved resistance to it.

It's not easy to establish, even in a double-blind clinical trial, whether a medication is genuinely effective. Even more difficult to prove that it is not effective. Unproven and even ineffective "anti-bacterial" remedies will continue to find a ready market. I'm skeptical, as ever. You'll have to decide for yourself whether you can have any effect in helping a fish recover from a bacterial invasion. Do remember that the purpose of antibiotics is to stall the reproduction of the bacteria while the host's immune system assembles the appropriate ammunition. It's the immune system that actually cures the fish, not the medication.

"Pre-emptive" use of antibiotics is always an error. They have recently crept into hand lotion and kitchen towels. Read Dr. L.K. Handin's informed rant in the "Medications" page here; casual misapplication of antibacterial drugs has spurred the evolution of resistant bacterial strains. The aquarium hobby isn't alone to blame here, but when I first wrote this, in 2001, resistant Pseudomonad and Aeromonad bacterial infections were reported to be commonplace in fishes raised in Hong Kong and Taiwan. Singapore and Bangkok, on the other hand, have begun to address the problems raised by hatcheries' routine use of antibiotics as a prophylactic measure. Al Castro wrote an editorial on this subject in the Jan. 2001 issue of Aquarium Fish.

Apart from the issues of resistant strains, drugs that are effective when injected may be less than effective when diluted in a bath. My own disheartening conclusion is that the bacterial diseases of fishes, like their viral infections, are largely untreatable in the home aquarium.

Nevertheless, it's worth looking into the kinds of bacteria most usually held responsible for problems.

Aeromonads. Though Mycobacteria are the bacteria usually considered responsible for fish deaths, by far the most prevalent bacterial infections in freshwater tropical fish are actually caused by several species of Aeromonads, according to an article by Dr Beverly A Dixon in F.A.M.A., July 1998.

Æromonas and its kin, the bacteria called Aeromonads, are a ubiquitous group of motile bacteria in freshwater and brackish water.

Æromonas-type bacteria can cause "blood pimples" and bloody streaks in fins and skin, and they are among the agents that cause hemorrhagic septicemia, the abdominal swelling we still call "bloat" or "dropsy," by inflaming and swelling the liver or spleen so that the abdomen fills with fluid, sometimes with standing "pine-cone" scales, or by creating intestinal blockage. Aeromonads can be responsible for skin ulcers and lesions, red patches, fin and tail rot, or pop-eye.

You can't sterilize the aquarium environment. Aeromonads are already everywhere in the aquarium, in mulm, in the biofilm, even in fish skin mucus, and form part of the normal intestinal bacterial flora of healthy aquarium fishes and even of wild ones. Stressful conditions, heat, ammonia and nitrite, high organic load and low dissolved oxygen, crowding, abrasions, even constant stress from an aggressive tankmate, all increase the chances that a fish will succumb to an Aeromonad infection.

Download a 2001 Dept of the Interior pamphlet "Aeromonas hydrophila and motile aeromonad septicemias of fish" by R.C. Cipriano, and read the Introduction and Pathology and Virulence Factors sections, then scan the Methods of Control. Are any of the antibioticsmentioned available to non-professionals? It doesn't look promising.

The bad news is, that in an inspection during the 1990s, 96% of the Aeromonads isolated from Asian imports were discovered to be resistant to Tetracycline. Worse, most of these bacterial isolates were multiple-resistant to a range of other drugs tested by Dr Dixon. The Sulfa drugs Nitrofurazone and Romet, tested at the same time, also proved ineffective. The better news: "In general, strains isolated from the environment are less pathogenic than those isolated from diseased fish," say the authors of a Texas A&M paper on Aeromonads .

More effective treatments for Æromonas-inflicted problems are indirect and holistic. They involve reducing the load of minor parasites and improving water conditions (eliminating free ammonia and lowering nitrates and phenols, for example) and improved sanitation.

Ruth Francis-Floyd, U. of Florida Cooperative Extension Service, discusses Aeromonas infections at the U.of Florida/IFAS site; she's more optimistic than Dr. Dixon about Romet in fish feed. One of Dr. Francis-Floyd's remarks applies widely to bacterial infections in general:

"Some strains of Aeromonas are more virulent, which means that they possess special properties which enable them to cause more serious disease outbreaks. If these more damaging strains become endemic in a population of fish (which means that they are there all of the time and the fish develop an immunity to them), it becomes difficult to introduce new fish into the water body without suffering major losses of newly-stocked fish."

Mycobacteria are also among the common, generally non-pathogenic bacteria found everywhere in the aquarium. They arrive by every route, even in house dust that settles on the water's surface. Among aerobic environmental bacteria, mycobacterial strains are commonly considered the likeliest to become pathogenic. The strain that has attracted attention, because it can be transmitted to humans, is Mycobacteria marinum. Several strains of normally non-pathogenic environmental mycobacteria can produce a range of morbid conditions in fish. "Fish tuberculosis" is a dramatic but misleading name for a variety of symptoms, any of which can be reproduced by a whole range of environmental mycobacteria. With equal injustice, you could refer to various symptoms as "fish leprosy," leprosy also being a mycobacterial infection, though caused by a different specific strain of Mycobacter. The phrase "fish leprosy" has the added virtue of making the faint-hearted squeak with terror and loathing!

Some symptoms of mycobacteriosis: fish refuse feed, they waste away, develop pale "bleached" patches; their translucent musculature can turn milky; their scales may be raised; they can develop skin lesions, sunken patches, ulcers that may bleed, bloody streaks on the body or fins similar to Aeromonad infections, pop-eye, or fin rot; in later stages the liver, kidneys, or spleen may stop functioning, resulting in "dropsy" or "bloat." I'm getting these descriptions from biologists' reports, not my own observations. But you see that these dire symptoms aren't specific to mycobacteriosis, or even to bacterial infections. Personally, I couldn't even distinguish a bacterial infection from an infestation of sporideans, such as Pleistophora, without lab support, the range of shared symptoms are so easily confused.

"The infection is usually very difficult to eradicate in fish culture systems with antibiotics," the authors of the ZFIN website warn lab technicians. Though they mention some antibiotics, including tetracycline, the best recourse is to avoid infections in the first place, with strict quarantine and avoiding cross-contamination from tank to tank.

Mycobacteriosis is endemic in aquarium populations. You might assume that the only fish you have that aren't carriers are ones you have raised yourself and kept separate, isolated from a community aquarium. Mycobacteriosis is perhaps the commonest cause of fish deaths. And some of the characteristics I'd generally associate with an "old" fish, its gently arched spine, darkened colors and enlarged head, its slowed metabolism, may be premature ageing brought on by mycobacteriosis.

Mycobacteria are especially resistant to medications because they are cased in a triple cell wall uniquely rich in waxy lipids, which repel water-soluble medications. But the endemic environmental mycobacterial strains in the aquarium are not highly virulent. Factors that induce the usual chronic low-key mycobacterial infections to mutate into virulent outbreaks with mortality are not clearly understood. Once established in a fish and under the stress of a host's immune system, however, bacteria are under new survival pressures. A selection process is under way, in which they may mutate into more virulent forms. The most dangerous bacteria, therefore, in an enclosed system, are likely to be the mutated strains causing symptoms in a "sick" fish. Within a few generations released back into the aquarium's general environment or being cultured in a Petri dish, the same strains tend to lose their virulence.

Fish that have come down with clinical symptoms and then have been "cured" are likely to be sub-clinical carriers of virulent strains of mycobacteria. I would never return them to a community aquarium.

I'm not the only skeptic when it comes to patent medicines that are supposed to treat bacterial infections of fish: Dr. C. Walker, writing about the laboratory care of Zebrafish (Danio rerio) for biologists--— who are now keeping Zebrafish for genetic studies--— tells these advanced biologists-turned-aquarists--— the same ones, remember, who are able to keep clusters of fish cells alive in a petri dish, "There is no known effective antibiotic to cure mycobacteriosis." Check this at the Zfish site.

Untreatable bacterial infections? This "defeatist attitude" just isn't what anyone wants to hear, and what's worse, it sells no products! Though most of us have firmly-rooted illusions about the effects of our pet "anti-bacterial" patent medications, you might want to keep Dr. Walker's words in mind, as you cruise the shelves at the LFS to see just what "cures" for bacterial diseases corporations are offering you.

Links. Rainbowfish seem to be especially susceptible to mycobacteria. There's a particularly good discussion of mycobacteriosis and the methods used for disinfecting tanks and equipment, at Adrian Tappin's site "Home of the Rainbowfish".

Shawn Prescott discussed mycobacteriosis in 1997 in his "Diseases of Fish" series for Aquarium.Net,

The combined effects of mycobacteria, called "mycobacteriosis" are discussed at ZFIN, the Zebrafish Information Network (for biologists keeping zebrafish in the lab).

Mycobacteriosis is a problem in marine fish too. Dr. Lance Ichinotsubo offers a brief notice at the AMDA site. His conclusion is that mycobacteriosis isn't successfully treatable in marine aquaria either.

"Columnaris Disease." Flavobacterium (Flexibacter) columnaris causes headaches for fish farmers raising Channel Catfish under crowded and stressful conditions and prolonged summer heat but doesn't so often affect aquarium fish. Symptoms include "gill rot" and so-called "saddleback" lesions, pale areas across the nape. "Columnaris Disease" can also have insidious internal effects that are hard to cure. There's a good downloadable article on "Columnaris Disease" at the Southern Regional Aquaculture site, and Discus Page Holland also has a good article on "Columnaris Disease" which includes treatments with saltbath dips or potassium permanganate.

These bacteria are hard to isolate in the lab, technicians say, and probably don't ever naturally occur isolated from other bacteria and fungi. The same strains of bacteria and fungi that can be isolated in smears from affected fish can also be found in cultures taken from neutral areas of the aquarium, for example from decaying flake feed. They are often found perfectly harmlessly growing on healthy fish. So whether they are responsible for "mouth fungus" or not is arguable. No species of fish is especially resistant. You've probably noticed that fish are rarely troubled with fin rot or mouth fungus in your balanced aquaria--— it's much more likely to be found among new fishes in Quarantine. Fish with bacterial/fungal conditions have to stay in Quarantine to recover, even if it takes longer than a month.

How does bacterial resistance come about? When you use antibacterial products you don't kill all bacteria, just the susceptible ones. The balanced community of bacterial strains is disrupted at the first use of the "antibacterial" ingredient, but the surviving bacteria swiftly take up the newly-available space and nutrients. The next antibacterial round will be less effective, because you are conducting what amounts to a long-term experiment to select for resistance to the particular anti-bacterial agent you are using. The resistant bacteria were probably already part of the richly-mixed bacterial community as a small potential "founder population," but they were unable to establish themselves until competing microbes were destroyed. Now their neighboring bacteria may even borrow nucleic acids that code for resistance.

Taking the broadest long-term view, it looks as if scattershot antibacterial medications do more harm than good. Now consider an effective antibacterial agent, but which has been diluted in the aquarium water. At this low concentration, an agent is unlikely to have the kind of effect it might have had in a Petri dish in the laboratory. Most successful antibiotic treatments of fish require intramuscular injections: a treatment we're mostly just not prepared to administer, and not practical with small aquarium fish anyway.

Some precautions. If cure is largely impractical, then controlling pathogenic bacteria is all the more important. Prevention of bacterial disease is the strategy behind some key tactics:

You can take some precautions to minimize the transfer of pathogenic bacteria. Avoid unnecessary transfers of fish from tank to tank. Isolate fish that are showing early symptoms of bacterial infection. Don't wait for a fish to develop terminal swim bladder disfunction, edema so severe that the scales stand up, popeyes or ulcerating lesions that trail mucusy slime into the water. Remove a fish with symptoms of bacterial disease at an early stage. Even if you can't bring yourself to euthanize it, at least isolate it. And don't wait for a dying fish to expire in the aquarium. A dying fish is increasingly a reservoir of bacteria and weak parasites, some of which will be released into the water when it dies. The viscera and carcases of dead fish are smorgasbord for their tankmates, creating a most effective vector for disease transmission. In a related note, at salmon hatcheries, a substantial reduction of "tb" infection was achieved simply by eliminating uncooked salmon viscera from the diet of fry. So never allow a dead fish to be picked over by other fishes. I'm unsettled to see nibbled corpses in the tanks at the LFS for just this reason.

Isolate asymptomatic carriers. You may medicate a beloved sick fish if you wish, but don't return to a community aquarium a fish that has "recovered" from symptoms of bacterial disease once its outward symptoms have been alleviated. "Dropsy" is a case in point. Sometimes a fish recovers enough from a bout of severe ascites to lead an outwardly-normal life. Then the "cured" fish is returned to the community aquarium, where it may become a sub-clinical carrier of bacteria, free of visible symptoms. A sub-clinical carrier remains a source of infection for all your other fish. When it dies quietly among the plants, a couple of months after the episode of "dropsy," the two events may not seem connected.

Initial quarantine, even a full four weeks' time, may not be long enough to identify weakened fish that are bacterial carriers before they enter your system. It's quite probable that all your fish have already been exposed to a variety of bacteria that could be pathogenic, given the right circumstances.

Encourage a low-stress environment, to keep immune systems responsive. Keep levels of dissolved organics and metabolic end-products low.

Encourage bacterial diversity in the aquarium. This may be unfamiliar advice. I encourage bacterial diversity in several ways. I add a pint of water drawn from a fish-free plant nursery to each aquarium, from time to time. I also vary water temperatures, keeping within comfortable tolerance ranges of the fish, of course; after a couple of months at 77°F, for instance, I might re-set the heater to 74°F for six weeks or so. Why? Well, our constant thermostatically-controlled temperatures favor the success of whatever strains of bacteria are the most ideally-suited to that particular temperature. Mycobacteria marinum reproduces best in the lab at a steamy 33^°C. Varying the temperature may avoid inadvertent culturing of dominant strains of bacteria. Seasonal variation of temperature--— within the limits tolerated by the fish--— seems to have a protective effect: Dr. H. Reichenbach-Klinke was suggesting in the 1960s (Diseases of Fishes, TFH, 1965, p. 23), "Obviously the adaptability of the fish has to be considered, but attempts to protect them by varying their temperature seem to be well worth while." I think we've ignored this sound old-fashioned advice, which I first read in William T. Innes' Exotic Aquarium Fishes. As far as tropical fishes are concerned, the difference between surface waters in a slow-moving stream at dawn and at mid-afternoon may be as great as any seasonal differences. "Night is the winter of tropical waters," I recently read.

I also encourage a healthy biofilm, to let "good" bacteria crowd out "bad." I encourage healthy intestinal fauna with a varied diet that includes plenty of roughage from cellulose and chitin.

Meanwhile, apart from aquarium situations, the Center for Disease Control, Atlanta GA, recommends a 10% solution of household bleach to sterilize equipment contaminated with mycobacteria.

Possible transfer of mycobacteriosis to aquarists. In the unnaturally close confines of aquarium life, fish with bacterial diseases are likely to infect other fishes. Under certain conditions they may infect you, too.

Fish Diseases That Can Affect Humans (contributed by Erik Cohen)

"How many of you wash your hands after playing in your aquariums? How many of you wear some kind of protective gloves while you're digging around in your tanks? Since I don't usually wash my hands after playing in my tank and never wear gloves, and I know the dangers that can arise, I'm guessing that a lot of you don't wash your hands or use gloves either. In most cases, there's nothing to worry about, but unfortunately, I've seen first hand, how fish diseases can be transferred to and actually affect the health of humans.

"Before taking my current job, I worked for one of the largest indoor aquaculture facilities in the country (and probably the world).

"Aquaculture facilities are prime breeding grounds for infectious aquatic pathogens. Anytime you crowd 16,000 fish ranging in size from 30g to 600g in a 30,000 gallon tank, you're going to have problems. Densities often reached over one pound of fish per gallon of water (imagine a 10 lb fish in a 10gallon tank!). If one fish has a disease, it's in the water and all fish have that disease. We dealt with a lot of nasty ones there. Fish were often plagued with Costia, Mycobacteria, Myxobacteria, Staph infections, Aeromonas, Columnaris, and Trichodina, among others.

"Sometime in 1997, several of my co-workers at the fish farm developed strange lesions on the knuckle area (usually the middle finger knuckle) of one of their hands. It was a red sore that sometimes discharged a white/yellow substance and was painful/tender to the touch. About the same time, I happened to receive an article from the New England Journal of Medicine that described the cases of several Canadians that had gotten sick from cleaning/filleting farm-raised fish in their fish market. As I read the article, it did not describe the symptoms that my co-workers were experiencing, but the information was important to me anyway.

"The Canadians who got sick were all Asian and the likely culprit of the infection was farm-raised Tilapia. Tilapia is one of the most popular farm-raised seafood items in the Asian markets. If you've ever been to Chinatown in Philadelphia, New York, or any large city, you've probably seen restaurants with tanks with live Tilapia, Hybrid Striped Bass, etc swimming in them. The Asian markets prefer to buy and sell live fish rather than fresh fish on ice. They cook the fish whole, and to see a fish swimming in the tank only minutes before eating it, is the ultimate sign of freshness to the Asian community.

"The bacteria afflicting the Canadians, Streptococcus iniae, is a fairly common and very dangerous disease among cultured Tilapia. Outbreaks of S. iniae (often referred to as "Strep") have been responsible for completely wiping out Tilapia farming operations. The Asian fishmongers in Toronto recalled being punctured by fins or spines while cleaning freshly-killed, farm-raised Tilapia. Most of the workers experienced cellulitis (sudden tenderness, swelling, reddening of the skin, often accompanied by fevers or chills, etc) of the hand. After a lengthy treatment with antibiotics, they were cured, although much more cautious about handling Tilapia in the future.

"The Journal article didn't mention the red lesion that I had seen on the hands of a few people at my job. But, there was a page stapled to the Tilapia/Strep article with a picture of someone's hand with the exact lesion as my co-workers. The caption mentioned a laboratory worker who was working with the bacteria Mycobacterium marinum and had cut her hand while working and become infected. Mycobacterium is a genus whose cousins are such pleasant ailments as leprosy (Mycobacterium leprae) and tuberculosis (Mycobacterium tuberculosis). That's why Mycobacterium marinum is often called "Fish Tuberculosis," even though it's a completely different disease and is not nearly as harmful as human TB.

"Besides Tilapia, the company I worked for also raised Hybrid Striped Bass (a cross of the freshwater White Bass, Morone chrysops, and the saltwater Striped Bass, Morone saxatilis). Striped Bass are known to carry the mycobacterium, even in the wild. Now the clues were adding up. But those who were experiencing the symptoms had varying duties along the life cycle of the fish. Some were biologists who handled the fish when they were sampling the fish or grading them by size. Others were workers who harvested the fish for the Asian customers. One of the fish-farm plumbers developed a lesion as well. Interestingly, some workers who had no contact with the fish were getting the lesions too. The construction manager got infected. The woman who managed the hydroponic greenhouse was infected too. And suspiciously, not everyone who worked directly with the fish remembered specifically getting punctured by a Hybrid Striped Bass. This meant that the disease was free swimming in the water. It's likely that those who didn't work with the fish had some kind of cut or opening already in their hand or arm which allowed the bacteria to enter when they placed their hand in the fish culture water.

"Infected employees were given a lengthy series of antibiotics, which eventually cleared up the problem. The reason the mycobacterium infection always showed up on the knuckle was because it likes colder-bodied animals (like fish). The knuckle area's thin skin was the coldest place it could find on the warm human body."

-Erik Cohen

For more information on mycobacteriosis ("fish tuberculosis") and especially its transferal to aquarists, in the form of dark nodules and swellings on the skin of the hands and arms, see Shawn Prescott's illustrated article archived at www.aquarium.net/ Chances of transmission are much higher for persons with HIV/AIDS. There's an important site discussing some simple precautions that aquarists with compromised immune systems should be taking; it's at www.aricinc.org/guapotes/ Click there on "Aquariums and people with HIV/AIDS"