Fish Tuberculosis

by Leslie Keefer

At the ripe old age of nine, I won my first fish at a carnival. My hobby had begun, much to my mother’s dismay. She was convinced that we were all going to catch a disease from my fish somehow. I always told her she was silly and you couldn’t catch any diseases from fish. I have no intentions of admitting to her that she was right. Eighteen years later, I have encountered my first zoonotic disease. Currently I have a ten-gallon tank set up as a hospital tank harboring the piscine equivalent of tuberculosis. Most references actually call this disease Fish TB, but it is not actually TB and it is transmissible to animals other than fish. Fish TB is caused by Mycobacterium marinum, a bacterium closely related to the TB bacteria, Mycobacterium tuberculosis. There are actually over 50 species of bacteria related to tuberculosis that can cause disease. They are typically able to live in any number of environments, in soil, water and animals.

I am a histotechnologist, which, to those of you unfamiliar with health care, is the person who takes tissue and turns it into stained slides for pathologists to use in their diagnoses. It is often difficult to diagnose M. marinum bacteria this way and often requires a culture. A typical lab wouldn’t usually bother with all this for an aquarist, but being able to do my own lab work is one advantage to this profession. When I discovered what I thought to be TB in my tank, I made slides of a sick fish and luckily located the bacteria without having to go through the trouble of culturing. Mycobacteria are acid fast, which means they stain bright pink against a blue background.

For those of you without a histology lab at your disposal, the symptoms of Fish TB are usually wasting, lesions on the body, skeletal deformities (a few of mine developed curved spines), and loss of scales and coloration. This is a relentless disease. I have read that it is not considered treatable; however, I figured my 15-year-old Raphael catfish deserved a chance. Against the advice of my veterinarian, I have not euthanized my afflicted fish (over half died shortly after the disease bloomed anyway). The typical drugs for treating fish are the same as for humans, most often a combination of two drugs administered for at least 3 months. Currently I am trying Kanacyn (Kanamycin). Once the fish became emaciated I had no luck saving them. Traditional tricks for curing diseased fish, such as adding salt and raising the temperature, are ineffective and in the case of the raising temperature may even be detrimental. The bacteria grow better in warmer water; their optimum temperature is 30°C. They have no problem with salt either; they can infect saltwater fish as well as freshwater.

Mycobacterium marinum is considered slow growing, meaning it will take about 2 to 3 weeks for symptoms to develop after initial contact. People do not often become infected, although it is possible. The bacteria usually enter the skin in small abrasions or cuts when you are performing tank maintenance. In humans, the symptoms are usually restricted to skin and soft tissue destruction. Lesions appear, first small and purple, and gradually grow. Treatment is difficult. The bacteria can also infect bones and tendons that can feel like arthritis (Handbook of Dermatology and Venereology, chapter 16, Cutaneous Tuberculosis and Atypical Mycobacterial Infection by Dr. L. Y. Chong). Certain types of fish tend to be more prone to carrying Fish TB, such as labyrinth fishes (bettas, gouramis). The outbreak in my tank occurred after adding 6 female Betta splendens to a community tank.

Prevention is key to avoiding this disease since it is so difficult to cure. The immune system is usually enough to prevent an infection in healthy fish. Stress, which suppresses the body’s immune system, and/or wounds in fish are most likely to allow an infection to take hold. Therefore, eliminating stress is paramount. Although aquarists don’t frequently get this disease, using gloves when cleaning infected tanks is highly recommended. Starting a siphon by mouth is also a good way to expose yourself unnecessarily to the bacteria. If a tank has been infected, it is considered best to bleach it well and dry it out before restocking it.

Information in this article was obtained from several sources, an infectious disease specialist, several pathologists at the facility where I work, my veterinarian, and a medical text chapter written by Barbara Brown and Richard Wallace Jr., as well as my own experiences.

This article first appeared in PVAS’s Delta Tale, Vol 32, # 2