Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Fish TB Mycobacterium marinum

Mycobacterium marinum infection in a human
Mycobacterium marinum infection in a human


Fish TB unlike all other fish diseases can be passed on to humans. Although it is commonly called fish TB it isn't actually tuberculosis. It is caused by an infection of Mycobacterium marinum whereas TB in humans is caused by an infection of the closely related Mycobacterium tuberculosis, it is the two bacteria being so closely related which has given rise to the common name of "fish TB".

Mycobacterium marinum is a very serious infection both in fish and in humans. Almost all cases where a person has become infected with Mycobacterium marinum the disease has come from fish, usually after infected water has entered through broken skin when carrying out tank maintenance.

Mycobacterium marinum

Mycobacterium marinum is a free swimming bacteria which is found in both fresh and salt water and is one of the ever present background bacteria which normally don't cause a problem. It is found mostly living in the mulm along the bottom and fish looking for food there perhaps with an small injury to its mouth or if a predatory fish eats an infected fish they can easily become infected and once established in a host the number of pathogens will rise dramatically making further infection more likely especially in an aquarium.

It is one of the least virulent Mycobacterium's and is far less contagious than its close relatives - TB and leprosy, there is no known case of human to human transmission.

Infections in mammals are quite rare because the maximum temperature at which Mycobacterium marinum can survive 37°C infections in humans are normally restricted to the cooler parts of the body such as hands, finger tips, knees and feet.

Although Mycobacterium marinum is quite resistant to aquarium antibacterial products Mycobacterium marinum is treatable with antibiotics, the disease is not known to be resistant to any antibiotics.

Fish TB in the aquarium and pond

Mycobacterium marinum is a slow growing bacterium. Once infected by some means it takes around six weeks for symptoms to begin to show in fish at 23°C.

The fish's defence response is to surround the pathogens with white blood cells in an effort to kill them. These cells are in such high numbers that they can easily be seen on the skin and if a postmortem is carried out they will be seen on the internal organs. They appear as grey-white nodules and are called granuloma.

Some of the granuloma will break though in to blood vessels and the disease will spread to all parts of the body.

Once the liver and kidneys are infected the fish's osmoregulation will be compromised and the fish will succumb to dropsy. Granulomas which are present in the muscle below the skin will erupt out to form ulcers, they may form in the brain of the fish causing abnormal behaviour and they even form in the bones of the fish causing deformities. When present in the digestive system the granulomas prevent the fish from digesting its food causing the fish to quickly loose condition.

Common symptoms of fish TB

1, Wasting, sometimes the only symptom is fish becoming ever thinner. The disease is sometimes called wasting disease because of this.

2, Ulcers, which can develop on the flank and-or head of the fish.

3, Grey-white nodules on the fish.

4, Dropsy and pop eye which might affect one or both eyes.

Treatment of fish TB

Ordinary antibacterial products which can be purchased over the counter are unlikely to have any affect on this disease. Treatment will require a long course of antibiotics prescribed by a Vet. The whole aquarium/pond will need stripping and sterilising and things like plants and substrate will have to be replaced.

Due to the cost and risks to people with this disease a better option might be euthanasia of the fish rather than treatment because once the symptoms show a great deal of damage to the fish will already have taken place so treatment with antibiotics won't be effective in all cases.

Safety First

Fish TB in a human
Mycobacterium marinum infection in a human.

The risk might be small but the consequences are anything but small. An established infection like this will take many months of antibiotic treatment before it is eventually cured. A few simple precautions will remove all risk!!!

In truth the overwhelming majority of fish keepers will never come across this disease and of those that do, only a tiny minority will actually become infected with it. In a typical year only 27 people per 100,000,000 will become infected (considerably less than win the lotto!!!). So it is important to keep it in perspective.

Fish keepers are more at risk than most because the pathogen is present in aquaria and ponds. It can only enter through broken skin so by simply wearing some water proof gloves when handling fish or carrying out fish keeping maintenance the risk is completely avoided. This is particularly important if you have some small cuts or abrasions and you are dealing with sick fish.

Although there are no reported cases from foot spas where Garra .sp are used to peck away at dead skin ect, the practice does seem to carry a higher than normal risk. Feet can easily have small areas of broken skin and they are one of the easier areas for the bacterium to infect.


Mycobacterium Marinum Water - Health Dissertation

Wikipedia - Mycobacterium marinum

Guide to common fish diseases by Lance Jepson