"Fish tuberculosis" is the common name for a fish infected with Mycobacterium spp. This bacterial genus has a unique outer protective coating that makes it impossible to treat once it is inside your system. Infections will show up differently and may not show any clinical signs at all. This is a significant disease because it is zoonotic, and can be passed to humans through open wounds.
What Is Fish Tuberculosis?
There are several bacterial species within the Mycobacterium genus. They are common environmental contaminants and do not require a fish host. Mycobacterium spp. can infect mammals, birds, reptiles and fish. Some species will cause a localized infection, while others a full-body, systemic disease. This disease is called "fish tuberculosis" since Mycobacterium tuberculosis is the causative agent for tuberculosis in humans.
Symptoms of Fish Tuberculosis
Non-specific, occasional deaths
Granulomas seen on internal organs during necropsy
Increase in secondary infections
The most common symptoms of fish tuberculosis is low, non-specific mortality, making it very hard to detect. Some infections may be present with no clinical signs for long periods of time.
Causes of Fish Tuberculosis
- Addition of non-symptomatic (carrier) fish
- Addition of infected plants or other biological materials
- Transferred on owners hands without proper sanitation
Since many Mycobaterium spp. are common in the environment, not taking the time to properly wash your hands may transfer bacteria to your system.
Treatment of Fish Tuberculosis
There is no effective treatment of fish tuberculosis other than supportive care. Some fish can live for a long time with an infection and show no outward clinical signs of disease. You may see a long-term pattern of random deaths with no links to other diseases, fish additions, diet or other environmental causes, such as water quality.
If any fish test positive for fish tuberculosis in your system, it is assumed all fish are infected. Once a fish in your system has tested positive, you may keep the tank as a closed system, no new fish in or out, and be sure to take proper biosecurity precautions, such as gloves, or elect to depopulate and disinfect your system. Depopulation is recommended in immune-compromised environments.
How to Prevent Fish Tuberculosis
Mycobacterium spp. are one of the hardest pathogens to detect, even with proper quarantine protocols. Even if you sacrifice one fish for testing, you may have another harboring the disease. Maintaining proper quarantine protocols for all new fish and plants is your best chance of catching an infection.
Mycobaterium spp. cannot be treated with a UV sterilizer. The infection lives within the fish, where the UV light cannot penetrate.
Is Fish Tuberculosis Contagious to Humans?
Most humans are infected with Mycobacterium spp. through open wounds in contact with infected fish or water. Fish with pointy spines, such as catfish, may introduce bacteria through a puncture when handling. This disease is also known as "fish handler's disease" or "fish tank granuloma." It commonly presents in humans as a localized rash or pustules. Wear proper protective equipment when handling pointy fish or you have an open wound on your hand.
Most Mycobacterium spp. do not cause serious disease for healthy individuals. Immune compromised individuals are at higher risk for developing a systemic infection. If you are concerned you may have contracted Mycobacterium spp. from a infected system, contact your physician.
Veterinary Diagnostic Procedures for Fish Tuberculosis
Any fish that pass away in quarantine should be immediately processed by your veterinarian. Fish tissue breaks down very rapidly, so diagnostic testing needs to happen quickly. If a fish has passed away hours earlier and been nibbled on by its tank mates, it will not be a good candidate for testing.
For small fish, there is no antemortem diagnosis for fish tuberculosis. Usually, in a suspected infection, one clinically-ill fish will be sacrificed for histopathology testing. Larger fish may be sampled by performing open coelomic surgery or laparoscopic surgery into the coelomic cavity. Granulomas may be visualized and sampled for diagnostic testing.
Specialized acid fast staining is required to confirm the presence of Mycobacterium spp. All fish samples should be sent to a lab familiar with fish tissues since they are very different from other pet species.
Proper Sanitation for Infected Systems
Any porous materials such as branches or moss should be thrown away. There is no method to effectively clean these materials. Small, granular substrate, such as sand, may be cleaned, but larger rocks and stones have too many nooks and crannies to effectively clean.
Since Mycobacterium spp have a specialized outer coating, choosing the correct disinfectant is critical. A one percent Lysol solution is the most effective at eliminating systems previously affected with Mycobaterium spp. Never add Lysol to a system with fish!
Hashish, E., Merwad, A., Elgaml, S. et al. Mycobacterium marinum infection in fish and man: epidemiology, pathophysiology and management; a review. Veterinary Quarterly, vol 38(1), pp.35-46. doi:10.1080/01652176.2018.1447171
Zoonotic Diseases of Birds, Reptiles, and Fish. Veterinary Information Network.
Erlacher-Reid, Claire. and Yanong, Roy P. E. Biosecurity in Aquaculture. Southern Regional Aquacultural Center, No. 4707, 2012, U.S. Department of Agriculture.