Mycobacteriosis in Koi Fish: Chronic Wasting Disease and Zoonotic Concerns

Mycobacteriosis is a long-term bacterial infection caused by several Mycobacterium species, including organisms such as Mycobacterium marinum. In koi and other ornamental fish, it is often called fish tuberculosis, although it is not the same disease complex seen in mammals. The infection tends to form granulomas, which are firm inflammatory nodules inside organs and sometimes in the skin. Over time, affected fish may slowly waste away, develop ulcers, or show vague signs that are easy to confuse with other illnesses.

One challenge for pet parents is that this disease is often chronic and nonspecific. A koi may look thin, weak, pale, or less active for weeks to months before the problem becomes obvious. Some fish show spinal curvature, pop-eye, abdominal swelling, or skin sores. Others die with very few outward clues.

This condition also matters because it has a zoonotic component. People can become infected when contaminated water or fish tissues contact broken skin, especially during pond cleaning, netting, necropsy, or handling fish with ulcers. Human infections are usually localized to the skin and are often called fish tank granuloma. That is why your vet may discuss both fish health and household safety at the same visit.

Mycobacteriosis is caused by infection with environmental acid-fast bacteria in the genus Mycobacterium. These organisms can persist in aquatic systems, biofilms, organic debris, and infected fish tissues. Koi may become infected through skin wounds, gills, or ingestion of contaminated material. Once inside the body, the bacteria can spread to organs such as the spleen, kidney, and liver, where they trigger chronic granulomatous inflammation.

Disease risk often rises when fish are under stress. Common contributors include poor water quality, crowding, chronic organic waste buildup, transport stress, temperature swings, poor nutrition, and mixing new fish into an established pond without quarantine. These factors do not create the bacteria, but they can weaken normal defenses and make infection more likely to take hold.

In some ponds, the first clue is not one dramatic illness but a pattern of fish that never fully thrive. A koi may stay smaller than expected, lose condition, or develop recurring skin problems. Because the bacteria can be difficult to eliminate from an environment, your vet may recommend looking at the whole pond system, not only the visibly sick fish.

Diagnosis starts with a careful history and exam. Your vet will usually ask about recent fish additions, quarantine practices, water testing, filtration, temperature, stocking density, diet, and whether other fish have become thin or died. Because the signs overlap with parasites, ulcers, kidney disease, and other chronic infections, mycobacteriosis is rarely confirmed from appearance alone.

A presumptive diagnosis may be made when your vet or a diagnostic lab finds granulomatous inflammation and acid-fast rods on tissue smears or biopsy samples. Samples are often taken from lesions or internal organs such as the kidney and spleen. More definitive testing can include histopathology, bacterial culture on specialized media, and PCR. Culture can take weeks, and some cases are only confirmed after necropsy of a deceased or humanely euthanized fish.

Because this disease can affect people, your vet may also talk with you about safe handling while the workup is underway. Wearing waterproof gloves, covering cuts, avoiding kitchen sinks for pond equipment, and disinfecting work areas are practical steps while waiting for results.

Prevention focuses on biosecurity and stress reduction. Quarantine new koi before adding them to the pond, avoid overcrowding, and keep filtration, oxygenation, and routine water testing consistent. Promptly remove dead fish, clean organic debris, and work with your vet if you notice chronic ulcers, wasting, or repeated unexplained losses. A stable environment does not guarantee prevention, but it lowers the chance that opportunistic infections will spread and become established.

Because mycobacteria can persist in aquatic systems, hygiene matters for both fish and people. Wear waterproof gloves when handling fish, cleaning filters, or scrubbing pond surfaces, especially if you have cuts or cracked skin. Do not wash pond equipment in food-preparation sinks when another option is available. Clean and disinfect work surfaces after use, and let someone else handle pond maintenance if a household member is immunocompromised.

If mycobacteriosis is confirmed or strongly suspected, your vet may recommend a more intensive plan that can include isolation, selective removal of affected fish, and environmental disinfection before restocking. Bleach helps remove organic material, but mycobacteria are relatively resistant, so your vet may advise follow-up disinfection methods that are more appropriate for these organisms.