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Mycobacteria (Fish Tuberculosis)

Mycobacteria (Fish Tuberculosis)

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Fish tuberculosis is caused by a particular group of bacteria known as mycobacteria. This bacteria can be found as free-living environmental organisms, often in the mud and mulm that collects at the bottom of a pond. The Mycobacteria are the smallest organisms that are capable of self-replication. Unlike other bacteria, they lack a cell wall. Three species can be found in Koi; Mycobacterium marinum, M. fortuitum, and M. smegmatus.

The usual means of infection is via the mouth, either by the fish rubbing around and feeding from the substrate, or by eating infected prey or cadavers. The mycobacteria then invade the guy wall, triggering the host's inflammatory response. Alternatively, cuts and abrasions can act as a doorway for infection. They can live in both fresh and salt water, and can survive in the pond environment for 2-4 days outside of the host, but only for approximately 5 hours out of water.

The best temperature for the growth of fish mycobacteria is around 74° F, with an average incubation period before the onset of clinical signs of around six weeks. In the face of an infection, the immune system of the fish will attempt to contain and eliminate the mycobacteria by surrounding it with white blood cells bent on destruction of the invader. These aggregates of cells can be visible to the naked eye on postmortem fish and are called granulomas. They usually appear as gray-white nodules and can be found in a variety of organs. Unfortunately, this defense is often only partially successful. If one such granuloma erodes into a blood vessel, then mycobacteria can be carried to any structure or organ of the body.

Organs affected by Mycobacteria

The liver and kidneys are commonly infected because these are very active organs with a large blood supply. Granulomas develop in these organs. Infective material can then be passed out from the kidneys in the urine, while damage to the kidney itself may affect the ability of the fish to osmoregulate, so these fish develop a buildup of fluid in the body cavity (ascites or dropsy). If the liver is infected, mycobacteria can invade the gall bladder and bile ducts and can then be passed down into the gut along with bile to be eliminated in the feces.

Granulomas in the muscles of the body wall can erode outward to form skin ulcers. Infections in the brain may trigger abnormal behavior, while granulomas and fluid buildup behind the eyes can force them out of the sockets so that they noticeably protrude. Occasionally some mycobacteria will travel in the bloodstream to eventually lodge in one or more bones of the back. These vertebrae are eventually eroded away by the infection to such an extent that they collapse, causing obvious spinal deformities.

Serious damage to the guts and related organs such as the liver and pancreas will affect how the fish is able to digest and utilize its food, thereby causing a gradual wasting of bodily condition. The actual clinical signs that infected fish exhibit will reflect which organs are infected.

The presence of mycobacteria in the environment is an obvious predisposing factor, and they are probably very common. Infected fish will shed large numbers of infective bacteria into their surrounding water and onto the substrate, increasing the likelihood of any in-contact fish developing the disease. Stressors or concurrent disease processes that suppress the fishes' natural immune resistance will also increase the likelihood of infection.

Mycobacteriosis is one of the main reasons why predatory fish should not be fed live feeder fish or "trash" fish. In fact, in addition to the scavenging of corpses, these are ideal ways of transmitting the infection. High stocking densities will increase the risk of infection. In platys (Xiphophorus spp.), the transfer of mycobacteria from a mother's infected ovaries into her unborn young has been demonstrated.

Signs to watch for

All species of fish are potentially susceptible, although this apparent susceptibility can vary. Cyprinids such as goldfish and tench (Tinca tinca), plus anabantids including the fighting fish (Betta spp.), paradise fish (Macropodus spp.), and gouramis (Trichogaster spp.) seem to be particularly susceptible. Aquatic reptiles, amphibians, and crustaceans can also harbor the disease.

Obvious signs of disease can be very variable depending upon where the infection has become established. Some fish will just waste away, while others will develop ascites.

Ulcers may form on the flanks and head. There may be an obvious exophthalmus (protrusion of the eye out of the socket) that can be one-sided, or both eyes may be affected. Sudden deformities of the spin may be seen. On postmortem fish, granulomas may be visible, often in several different organs. Always wear gloves if you are dealing with postmortem fish. Mycobacteriosis is potentially infectious to people and can enter through cuts in the skin.


Mycobacteriosis is highly resistant to the usual antimycobacterial medications. Certain antibiotics can be effective, but administering these treatments at an appropriate dose can be difficult and expensive. In addition, the aquarium or pond must be stripped out and thoroughly cleaned, which may include having to dispose of any gravel, plants, and decorations that cannot be reasonably cleaned and sterilized. The fact that it is potentially infectious to people does beg the question of whether we should treat it at all or whether humane euthanasia is the most appropriate action.

There is a wide range of clinical signs linked to this disease, and many parasitic, bacterial, and fungal diseases can mimic mycobacteriosis. In mollies (Poecilia spp.), Flavobacteria can cause multiple internal granulomas. Aeromonas or Vibrio bacterial infections can produce ulceration and signs of septicemia. Ichthyophonus is an internal fungal infection that can produce a progressive wasting condition, as can heavy internal parasite burdens such as with intestinal worms.

Spinal deformities have been linked to a variety of conditions such as vitamin C deficiency in live bearers, such as swordtails (Xiphophorus helleri).

To prevent this infection, quarantining of all new arrivals is a must. Prompt isolation of sick fish, along with the treatment of any problems, will help to eliminate other possibilities. Consider euthanasia of strongly suspected fish. In groups of susceptible fish, mycobacteria can thrive in aquaria, making its presence an important conservation issue with some captive breeding projects.

To avoid Fish Tuberculosis becoming a problem in your pond use KoiZyme.

First reported in France in 1897 by Bataillon, in carp living in a pond that received tuberculous effluent containing sputum and discharges of human tuberculosis. There are numerous Mycobacteria that have been isolated it seems that every scientist wants his or her name on a strain, there is a great body of evidence that these many strains may all be three or four bacterial species all by different names.

In general, Mycobacteria are a gram-positive acid-fast rod.

Two of the Mycobacteria that infect our fish also infect man, that’s Mycobacterium fortuitum and Mycobacterium marinum. In fish, it’s a wasting disease, but granulomas may also be found internally, and sometimes, slow moving, non-hemorrhagic ulcers may form in the dermis, but especially on the gill cover. It's not a fast killer, and it is spread by cannibalism.

Mycobacteriosis can be transmitted to your fish by improperly pasteurized fishmeal.

Resources: Koi Health & Disease Reloaded by Dr. Erik Johnson

Resources: Common Fish Diseases by: Lance Jepson

You are here: Home > Koi Wellness Center > Koi Diseases > Mycobacteria

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