Aquatic life

Fish Tuberculosis


2nd March 2003

Having recently set up an aquarium again after a lapse of several years, I have been unfortunate enough to acquire a fairy severe case of Fish Tuberculosis in the tank. This triggered me to write down here exactly what I do know of the disease and what the various books I possess say about it. There seems to be little on the www other than anecdotal evidence.

I would stress that I am no expert: this is a compilation of information from various sources.

Fish tuberculosis is not the same as the human disease of the same name. However it is possibly the most common disease amongst aquaria fish. It is a high probability than anyone who has kept a range of tropical fish for any length of time will have experienced it. However - it is also quite unlikely that they will have realised this, for the disease is usually not virulent, is not usually highly contagious and does not have sudden and drastic effects.

However: in certain cases the disease can occur epidemically and then great losses will occur. But more usually an infected fish may die years after it initially gets infected. Often the fish shows no external symptoms and the death is usually one of those unexplained death that all aquarists have experienced amongst their pets.

Fish TB, quite apart from being about the most common disease and about the hardest to diagnose, is also about the hardest to cure.

This page is written in part from my own experience and in part from the books I have. I have quoted chunks and information from Diseases of Fishes, by C van Duijn Jr, published by Iliffe Books Ltd. in 1956. The book contains a lot of highly technical information including photographs of specimens and several photomicrographs of the tubercles. Much of the book's information is highly technical and has not been paraphrased here.

Another book which is informative is 'Aquarium Care' by Gunther Sterba, published in Britain in 1967 by Studio Vista Ltd.

These are old books and hopefully more up-to-date information exists. Nevertheless I hope the information here proves useful to other aquarists. If I am in error on anything, or you have information to add to what is here, please contact me.

What is fish TB?

Fish Tuberculosis is a bacterial disease. Sterba says that it is very common and he claims that 25-30 per cent of aquarium fish may be infected. The disease, says Sterba, is most prevalent in overstocked community tanks. This he says is because:

Several different bacteria may cause it, all are members of the Mycobacterium genus:
The above list is not exhaustive! If you search the www, you will find a lot about mycobacterium marinum - which appears to be the most studied cause, but probably not the commonest. Sea fish are commercially more valuable than pet fish and the fish-farm industry can support a lot of research into diseases.

Clearly you need to be a bacteriologist to properly study the infection! But the bacteria that cause fish TB only cause TB in cold-blooded animals. Human tubercle bacteria are not pathogenic for fishes.

What are the symptoms?

Exactly what symptoms appear depend on the affected species (and presumably on the bacterium). In the early stages of the disease, no symptoms whatsoever are apparent and, even in the later stages, often the only way to diagnose the disease is by dissection and microscopic examination of the internal organs (especially the intestines, liver and kidneys), where tuberculous cysts may be found.

All the bacteria of the genus Myobacterium are acid-alcohol fast, this is to say that the bacteria, after having been stained with a phenol solution of fuchsine, retain this stain on treatment with a dilute mineral acid and ethanol. Clearly such tests are not for the usual amateur aquarist!

The tubercles are formed initially in the internal organs and appear as small, dirty grey knots, often containing necrotic tissue of blackish colour. These tubercles may have very different sizes and, especially in small fish, may only be found with the aid of a microscope. Normal tissue is pushed aside by the growing tubercles and if these tubercles grow near or into a blood vessel the circulation can be hindered and sometimes bleeding can occur.

The infection may also spread to the skeleton and then deformities may result, such as crooked spine, deformed cheeks and damaged fin-rays.

Clearly such deformities weaken the fish which will die as soon as the damage gets too severe. However the more delicate species may die an an early stage, before any visible symptoms appear. The more robust specimens, particularly if their general health is good, can withstand the disease much longer and will only die when some of the visible symptoms occur.

There is a huge range of visible symptoms, most of which are not specifically diagnostic of fish TB as they can also occur with other diseases. So there is an extremely good chance of fish TB being incorrectly diagnosed as something else entirely - which would indicate that fish TB is likely to be far commoner than most books (and fish dealers!) would admit. Although Sterba's book was written in the 60s, maybe his pessimistic-seeming "25-30% of fish are infected" is not as pessimistic as it might seem!

Symptoms can include:

How are fish infected?

Sterba says that infection may be by every conceivable route. Bacilli may be in the faeces, scraps of skin or burst abscesses and can be ingested by healthy fish. If infected fish die and are eaten by others (as we have all seen, on occasion, in a poorly attended fish retailer's stock tanks) then an overwhelming infection may result.

What fish are susceptible?

Clearly - this is not an exhaustive list and a list if immune fish is not possible, but it should give an idea of the problems you may face! It is compiled partly from my own experiences and partly from the books I possess. The sheer variety of fishes which can succumb surprised me as I was expecting Cyprinids, Tetras and Anabantids - all 3 genera appear particularly sensitive.

I suppose that, looking at the range of names of the causal bacteria, one might get a clue as to the species of fish involved. However a single outbreak will presumably be one bacterium species only so might be expected not to affect all fish. Van Duijn gives significant detail on the various bacteria and the fish species they infect.

Marine fishes are not immune, neither are frogs, caymans, tortoises and other cold-blooded animals, but there are different strains of bacteria involved.

Logic would indicate that carnivorous fishes are likely to be somewhat immune - after all they are going to eat in the wild, infected fish and would be the first to succumb therefore, so one could expect natural immunity.

Cures and treatment

Effectively, there is no sensible cure. The only way to cure the infected aquarium is to immediately remove infected fish as soon as they show any symptoms. Ideally all other susceptible fish should also be removed.

Van Duijn cites cases where treatment of the open wounds with penicillin ointment have effected a cure. As the disease is bacterial, antibiotics should work, but in general these are only available on veterinary prescription and injection of infected fish may be required, so such cures are hardly useful to the average aquarium keeper.

Sterba says that the spread of the disease can be hindered by:

Can Fish TB affect humans?

Most of the info you will find on the www seems to indicate that fish TB can infect humans. However - most of this info refers to the marine disease, caused by Myobacterium marinum.

In Freshwater fish, the disease has different causal bacteria and both of the books I have indicate that these are not dangerous to humans and that these bacteria are generally only dangerous in cold-blooded organisms, so trouble in humans would seem to be less likely with the freshwater varieties.

However - there are lots of different organisms that cause fish t.b. and I have heard reports of some people being infected by freshwater varieties, so it may be rare but not, it seems, impossible!

It seems that human infection comes via cuts and skin abrasions.

Tropical fish-keeping is so common, and fish TB so common, that surely if there were high danger, freshwater fish TB would be much better investigated and not the cause of mystery deaths that it quite clearly is! However human affairs don't always follow that logic! Via this site, I have heard of a very few cases where nasty results have followed human infection via cuts, but this is anecdotal only.

How can I avoid such infections in my fish?

Buy fish only from reputable dealers. Carefully inspect all the tanks of the dealers - if there are any significant quantity dead or sick looking fish, avoid the dealer entirely (unless the sick fish are clearly quarantined and being looked after). A good dealer will remove any sick or dead specimens immediately. If left in the show tank, it's a clear indication of bad husbandry, for dead bodies get eaten by other inhabitants, and this is a sure way of spreading infection. I recently (28th March 2003) visited one fish dealer and saw several of their 90 odd tanks where there were dead fish. Some corpses were fresh: others had decayed/been eaten and some were simply skeletons. Several tanks had 4 or five such corpses. Clearly such a dealer should be avoided. This same dealer later cleaned up their act - their tanls loked good. But then even later, many dead fish were to be seen.

I did engage this dealer in a long discussion about their tanks and TB and he said that he hadn't cleaned the tanks that morning (how many days does it take a dead fish to decompose to become a mere skeleton?) and that you had to multiply the normal fish death-rate by 90 as he has that number of tanks. Are his young fish in such a poor state that he expects any significant death rate? If so - he should change his supplier!

Talk to the dealer about diseases, particularly TB, and about their suppliers. Is the dealer interested and caring and aware of the problems? You have in this page a lot of background information. If your dealer is interested - you should now be able to teach him some things. If he is not interested - go elsewhere.

Unfortunately quarantining new fish is likely to be useless: infected fish may die many months after first being infected, so a quarantine of a few weeks will show nothing.

Can I introduce TB with live food.

Clearly, lacking any statistics, the obvious safe answer has to be 'yes'. However, my feeling is that it is not at all likely.

Consider the following:

  1. A pond full of daphnia is not likely to be populated with fish. Daphnia and fish mix well, but the fish always win!
  2. There are very many TB bacteria: types which infect crustacea (specifically daphnia) are unlikely to survive in fish. Daphnia is, after all, a natural fish food and evolution will have sorted out any tendency for dangerous organisms to be transmitted via the food chain
  3. Natural waters are far less densely populated than any fish dealer or aquarists tank, so conditions for spreading the disease are less favourable.
  4. TB bacteria will grow faster in a tropical tank at 25° than in native waters which are usually much colder.
  5. Fish fed on live food are likely to be in peak condition, so will be generally more resistant to disease.
  6. A healthy aquarium, like any closed ecosystem, must have lots of different organisms in balance for best health. Introducing live food from a local pond will introduce new bacteria, animals, worms etc which will all assist the biodiversity of your tank (as well as adding to its interest). Yes, try and use pond water from a source that does not contain fish: if there are no fish, there is virtually no chance that you will introduce disease or parasites.

But learn about pond life: if you, for instance, add dragon-fly larvae to your tank - they are carnivorous and will catch and eat very small fish. But if you have big fish - they are simply more and tasty fish-food. There are many small pond animals like this which can damage small fish but will be good food for larger fish!

Can I introduce TB with dried food. I do not know the answer to this. However, look at the ingredients on a typical flake food: it contains all sorts of ingredients - many of which are fish-derived. If a sick fish did get used as an ingredient - well, bacteria are known to ne able to survive all sorts of harsh conditions and I dare say that processing as fish food might not kill them all.

Manufactured fish foods are after all wide-range: they are designed to feed a wide variety of fish, from vegetarian ones to carnivores. Not all fish eat the same diet naturally and food should ideally be matched to what the particular fish, in nature, would normally eat.

So transmission via flake food would, I anticipate, be a subject worth study! But it is far less likely than is infection from a fish shop: as we know TB can stay dormant in a fish for a long time.

Can you give me any further advice?

Sorry, but the short answer is no: this page is compiled from my own experiences, from the sources quoted and from any reliable feedback I get from others. If you have information to give - I would be pleased to extend the information here but I have little more to give.

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