Fish mycobacteriosis (fish TB) — diagnosis, incurability and risk to humans

Mycobacteriosis is a chronic bacterial infection caused by various Mycobacterium species (most often marinum, fortuitum, chelonae). In the hobby it's called 'fish TB', though the agent differs from human tuberculosis. The disease is slow: a fish can carry it for months and infect tankmates before showing symptoms.

Symptoms develop gradually: progressive wasting (the fish loses weight despite eating normally), spinal curvature, bulging eyes, scale loss, non-healing open ulcers, apathy and refusal of food in the final stage. Often several fish in the same tank are affected. A confirmed diagnosis is only possible by histology in a veterinary lab — the characteristic granulomas in organs.

Treatment and human risk

In a home aquarium mycobacteriosis is essentially untreatable. Previously used kanamycin, erythromycin and rifampicin only temporarily suppress growth without eradicating the bacterium — it forms persistent granulomas inside tissues. Most ichthyologists recommend humane euthanasia of affected fish with clove oil, full disinfection of the tank (3% sodium hypochlorite, a week of drying) and replacement of the decor.

Zoonosis: Mycobacterium marinum infects humans through small skin wounds on the hands — producing firm, slow-healing nodules known as 'aquarist's granuloma' or 'fish-tank granuloma'. Treatment is by an infectious-disease specialist — a long course (several months) of clarithromycin or an antibiotic combination. Prevention: NEVER work in the tank with bare hands if you have any skin breaks; use long aquarium gloves. Don't drain a suspect tank into a kitchen sink — only into a sewerage line.