Mycobacteriosis – the slow, silent fish killer. How can you tell if your fish are infected? Can you catch it?

The disease is caused by a bacteria in the genus Mycobacteria. Another name for the disease is fish TB (short for fish tuberculosis). It can affect your fish grown in ponds, your aquarium fishes, or your zebrafish facility. Some of the species I’ve frequently diagnosed mycobacteriosis include livebearers (e.g. guppies), rainbow fishes, Bettas (aka Siamese fighting fish), gouramis, danios (e.g. zebrafish), various cichlids and goldfish.

The key to understanding fish tuberculosis and its control is to understand the epidemiology of the disease. The bacteria is present in soil (fish raised in earthen ponds), may be harboured in the filter, and carried by persistently infected fish. The immune system of infected fish mounts a response to encase the bacteria in layers upon layers of white blood cells known as macrophages. But this is never enough to kill the bacteria. The bacteria is slowly allowed to replicate, unchecked, these infected fishes go on to shed large amounts of bacteria for their entire lives.

The disease itself may be difficult to diagnose or even recognise, and is often mistaken for a multitude of different diseases. Clinical signs may include swimming disorders, buoyancy disorders, skin ulcers, bloat, dropsy, pop-eye, appetite loss, ill-thrift and more. The reason for the variable clinical presentations is because it depends on which organ/s are affected. In the accompanying picture, the bacteria (in red colour) has taken over the brain. This is the reason for the neurological presentation. This fish was swimming haphazardly and sustained secondary damage to its mouth. So, basically, a fish infected with Mycobacteria can look like anything (from completely healthy, to having skin ulcers, dropsy, etc.).

Infection with Mycobacteria can impact on the health, growth and productivity of your fishes. Persistently Infected fishes are the most dangerous for your fish population. Their ability to continually shed the bacteria at massive levels for their entire lives makes them a ticking time bomb. And when infected fish die, it can make it even worse! As their carcass become cannibalised, the disease spreads even more quickly to other fish!

What’s more is that the bacteria may infect humans! In humans, it’s called "fish-fancier finger disease".It causes localised hard swollen lesions that may be painful to touch. These need to be surgically removed and the patient would need a long course of multiple antibiotics. Disease is more severe in immunocompromised people (the very young, the aged, those on chemotherapy, etc.).

So, if you have been losing fish every now and then, for reasons unknown, how can you tell if your fish have mycobacteriosis?

Get your fishes vet-checked.

What are you waiting for?

Enquire here –

The Fish Vet’s laboratory services are open to fish owners Australia-wide.

Just another way of making aquatic veterinary services within reach.

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Yours sincerely,

Dr Richmond Loh
DipProjMgt, BSc, BVMS, MPhil (Pathology), MANZCVS (Aquatics & Pathobiology), CertAqV, CMAVA, NATA Signatory.
THE FISH VET, Perth, Western Australia, AUSTRALIA.
Mobile Aquatic Veterinary Medical & Diagnostic Services.
Ph: +61 (0)421 822 383
Skype: thefishvet

President WAVMA 2014

Adjunct Lecturer Murdoch University | Secretary Aquatic Animal Health Chapter – ANZCVS.

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4 thoughts on “Mycobacteriosis – the slow, silent fish killer. How can you tell if your fish are infected? Can you catch it?

  1. What dus Tb look like I have a gold fish with foul red body and is really sick I will treat with melafix or is it tb


  2. TB is the HUMAN Mycobacterium, not the fish Mycrobacterium. The only thing the 2 bacteria have in common…basically…(Per the Lab Manager I used to diagnose my fish with it)…is the “waxy outer shell” of the cell. This is the easiest way for people to understand the difference. TB is a lung disease. Fish “mycos” is more of a skin/tissue disease….I see it being more related to leprosy. My infected fish had what looked like columnaris and really aggressive fin rot. One fish developed kidney failure (dropsy). I have not yet put the photo’s of my mycos on my website, but I will as soon as I can get logged back in.


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