Fish diseases: atypical mycobacteriosis or tuberculosis in aquarium fish

Mycobacterial infections, sometimes referred to as atypical mycobacteriosis or fish tuberculosis, present a significant challenge both in commercial farming and for hobby aquarists.

Mycobacteriosis alias fish tuberculosis

Mycobacterial diseases are chronic and usually manifest in aquarium fish as very debilitating illnesses. All fish are susceptible to it. Especially certain types of farms, such as recirculating aquaculture systems (characterized by high fish production using a very small footprint and low need for inflow water), show higher sensitivity and susceptibility to the occurrence of fish tuberculosis. Typical symptoms of fish tuberculosis include loss of scales, ulcers on the skin, and in severe cases serious infections that can lead to fish mortality.

Diagnosis

The diagnosis of mycobacteriosis is quite complex because it mimics other diseases. It is usually done as follows:

• Clinical signs: Chronic exhaustion, loss of scales, ulcers on the skin.
• Histological examination: The presence of granulomas—clusters of immune cells formed in response to infection—is indicative but not a definitive sign.
• Acid-fast staining: Used on tissue samples to detect mycobacteria, which stain differently from other bacteria due to the waxy composition of their cell walls.

Mycobacterial infections in fish manifest with nonspecific symptoms, which often include loss of appetite, progressive weight loss, emaciation, water retention in the body cavity, and changes in behavior. Fish are often skittish, not typically allowed near feeders or at the surface where food is offered by other individuals. The characteristic clinical symptoms of a generalized process include secondary skin lesions, which involve changes in skin pigmentation, ulcers, or loss of scales. Changes in the skeleton often occur, especially in older infected fish, where lordosis, scoliosis, and other defects that hinder smooth swimming are observed. Rarely, an acute form of the disease is encountered, resulting in the death of fish without any external symptoms (1).

Preventive measures against the occurrence of fish tuberculosis

• Quarantine new fish: Newly added fish should be placed in quarantine for at least 30 days to observe any signs of disease before they are introduced into stabilized tanks.
• Maintaining water quality: Optimal water conditions reduce stress in fish, making them less prone to infections. Regular water changes and monitoring of water parameters are essential.
• Environmental hygiene: Regular cleaning of tanks and substrate and possible use of UV sterilization can help reduce the amount of mycobacteria in the environment.

Control and treatment of tuberculosis in aquarium fish

• No effective treatment: Fish mycobacteriosis is not treated due to the need for long-term therapy, poor penetration of drugs to affected sites, and high resistance of isolated strains to antitubercular drugs. Therefore, prevention and early detection are the only tools that an aquarist has.
• Depopulation and disinfection: In severe cases, it may be necessary to euthanize the entire fish population and thoroughly disinfect tanks and systems to completely eliminate the pathogen.
• Biological safety: Involves the use of dedicated equipment for infected tanks and strict disinfection protocols to prevent spreading to other tanks.

Handling of infected systems

• Personal protection: When handling infected tanks, it is essential that aquarists wear gloves and protective clothing to prevent transmission, as mycobacterial infections can also affect humans (however, treatment is possible in humans).
• Disinfection: Cleaning and disinfecting tanks that contained infected fish should involve mycobactericidal agents, as mycobacteria have high resistance to standard disinfectants.

Recommendations for hobby aquarists

• Regular monitoring of symptoms of fish tuberculosis: Hobby aquarists should monitor their fish and look for any signs of disease, especially in species known to be more sensitive to mycobacteriosis.

Dead fish should be removed from aquariums in a timely manner using tweezers or a net; never handle them with bare hands, and be careful not to get injured by their teeth or fins (1)

• Awareness: Understanding the risks and symptoms of mycobacterial infections can help with early detection and elimination, thereby preventing spread within the tank and potentially to the owner.

Humans are infected through contact with infected fish, water, or aquatic environment. The prerequisite for infection is skin injury. The literature describes the occurrence of disease caused by M. marinum associated with swimming in pools and bathing places, hence this disease was named "swimming-pool granuloma." Later, patient histories began to include contact with fish, and the name "fish-tank granuloma" started being used. However, it is important to note that the pathogens of these diseases are identical (1)

Conclusion

Although the recognition and elimination of mycobacterial infections can be challenging, understanding the nature of mycobacterial infections and learning how to handle infected tanks can help aquarists maintain healthy fish populations and prevent significant losses due to this disease.

Mycobacterial Infections of Fish Ruth Francis-Floyd1 VI PR, SRAC Publication No. 4706, November 2011

Obr: A dwarf gourami displaying blister like lesions on its body, a clear sign of fish tuberculosis, and with all cases of fish suffering from TB it is best to euthanise them. (https://www.castledawnaquatics.com/blogs/question-of-the-day/can-aquarium-fish-disease-be-passed-on-to-humans?srsltid=AfmBOooXDWGl36kc0B6sA3nCW1AUe8IrX_cjQHzc1Lah7FA8OFQQ4tvW)

https://mendelu.cz/novy-vyzkum-se-zameri-na-mykobakterie-nebezpecneho-skryteho-nepritele/

(1) https://www.prolekare.cz/casopisy/cesko-slovenska-dermatologie/2008-6/mykobakterioza-ryb-a-rizika-infekce-cloveka-2402