Mycobacterium marinum is the least virulent of the infectious mycobacterium that is well known for creating lesions in human tissue. The M. marinum bacterium is free-living in aquatic environments, and because of its similarity to the tuberculosis bacterium, M. tuberculosis, it is sometimes called “fish tuberculosis.” First seen on dying fish at a zoo aquarium, Mycobacterium marinum produces small red pustules that spread slowly within the body of an infected fish. Attempts to treat such infected fish without protection or attempts to clean an infected tank, may result in human infection. When infected humans unknowingly enter swimming pools that are not adequately chlorinated, other swimmers are at risk.
Infections are rare in mammals, because of their high body temperatures. Mycobacterium marinum survives well at temperatures below 98.6 degrees F (37 degrees C) and so usually invades the cooler parts of the human body. In natural fresh water, salt water, or swimming pool exposure, the elbows, knees, and feet — hands and fingertips in the aquarium hobbyist — are vulnerable. The disease is not known to be fatal in humans.
Symptoms of Mycobacterium marinum infection arise when freshwater tropical fish are stressed by lack of food, unclean environment, or poorly heated tanks. The pustules that form are internal and the bacterium is rarely passed from fish to fish.
Humans contract M. marinum when open wounds are infected in aquatic environments. The wounds exposed to the bacteria do not completely heal, and form nodules along the reddened scar tissue. Mycobacterium marinum can travel from the site of the wound into the blood stream where movable and compressible nodules have been observed on walls of veins in the areas of infection. The strains of Mycobacterium marinum are so weak that the only place the two-to-four-week incubation period has been observed is in immunocompromised patients.
M. marinum is thought to be less virulent than its relatives, tuberculosis and leprosy. Without a means of adapting, the bacterium is only mildly contagious and susceptible to antibiotics. Unlike its tuberculosis and leprosy, Mycobacterium marinum is an atypical mycobacterium because it cannot resist any number of mycobacterial drugs, so an early cure is comparatively easy.
Fish tuberculosis is rare in humans, and diagnosis of the disease takes up to a year because many physicians are not familiar with how the disease manifests. A cocktail of antibiotics is most effective against the disease. Complete healing and disappearance of the nodules may take as long as six months, so prevention is encouraged. One should not swim in or handle fish tank water while having open wounds or drink such water by inadvertently siphoning it into one’s mouth.