Aquariums can be mesmerizingly beautiful…and healthful, too. Watching the colorful fish and rippling water can reduce blood pressure, heart rate, stress and anxiety. But aquariums also can present a hazard that few people—and few doctors—are aware of.

The danger: Some home aquariums harbor a bacterium that can lead to a dreadfully painful, unsightly and long-lasting infection that typically goes undiagnosed for many months, according to a new report from Henry Ford Hospital in Detroit.


Mycobacterium marinum is a bacterium sometimes found in fish and in nonchlorinated water. When skin that has been “traumatized”—even by something as minor as a paper cut, splinter, hangnail or overzealous fingernail clipping—is exposed to M. marinum, the bacterium can find its way inside and set up an infection. Typically this happens when home-aquarium owners clean their freshwater or saltwater tanks and/or handle their fish (though it also can occur through contact with barnacles or infected fish in the wild).

The problem: It takes two to four weeks after the initial infection begins for the telltale symptoms to appear—and by then, any association with the aquarium typically has long been forgotten. The aquarium-lover is left with a growing number of painful, red, swollen bumps and open sores as large as quarters…which progress up his arm from his hand all the way to his armpit. This is highly unpleasant for anyone—but for a person with a compromised immune system (for instance, due to an autoimmune disorder, HIV, chemotherapy or drugs to prevent organ transplant rejection), M. marinum can even lead to a severe bone marrow or blood infection.

The study: Doctors at Henry Ford reviewed 10 years’ worth of records on patients who were treated for M. marinum and discovered that the average length of time it took from symptom onset to correct diagnosis and treatment was a staggering 161 days! By that time, the infection typically had spread from the initial site far up the arm…and patients had already been on multiple ineffective treatments, each with its own potential side effects.

In each of the cases at Henry Ford Hospital, the correct diagnosis was ultimately made after a skin biopsy was performed and the culprit bacterium was finally identified. Only then was the correct treatment initiated, typically using a combination of several different antibiotics that needed to be taken for about two months.

Self-defense: If you have an aquarium, go fishing or clean barnacles, be on the lookout for any redness, swelling or bumps on your hands or forearms. If you develop such symptoms, see your doctor without delay and ask whether you may be infected with M. marinum. Remember, many doctors are unfamiliar with this infection, so bring this article along to your appointment. Treating the problem properly from the start can spare you needless pain and useless medication. Reassuring: Handling fish bought from the grocery store or swimming in open water would not transmit the M. marinum infection, researchers said.