Mycobacterium fortuitum and M. marinum are bacterial disease-causing organisms of freshwater and marine fishes. These organisms are probably relatively common in the natural environment. An aquarium of pet fish infected with a mycobacterial organism may exhibit chronic illness and death. Affected fish frequently have ulcers somewhere on their body and infected fish with open sores appear to spread the disease from one fish to another and subclinical carriers may exist, shedding bacteria in their feces.

There are zoonotic (spread of a disease from animals to humans) concerns with these bacteria since human beings can become infected. In people, lesions are usually restricted to the appendages (hands and fingers) and appear as persistent ulcers or abscesses. Some people refer to this disease as aquarium finger. In humans, antibiotic therapy is usually successful but the infection may take weeks or months to control. Aquarists with open cuts or sores should not place their hands in an aquarium or handle fish with suspect lesions without protective gloves.

Other grossly visible signs, or those signs obvious to the naked eye, include pop-eye (exophthalmia), weight loss, subcutaneous masses and small masses on the internal organs. This disease appears to be more common in older fish, which have been in an aquarium for a number of years, although younger fish can be infected.

In the ulcerative form of the disease, necrosis (cell death) and bleeding of the skin occurs. It is not uncommon for underlying bone and muscle to be exposed. Such lesions make the fish vulnerable to invasion by secondary bacterial, fungal and protozoal pathogens. Despite attempts at treatment, these fish usually die due to the presence of the primary and secondary pathogens, as well as fluid imbalance that develops because of the severe skin damage. Even if the ulcers are not severe, the internal granulomas can compromise the function of vital organs, leading to morbidity and eventual death.

Veterinary Care

If a case of mycobacteriosis is suspected in your fish, you should contact a veterinarian immediately. Diseases that resemble mycobacteriosis include: Aeromonas disease, cancer, trauma, gastrointestinal parasites and starvation.

Diagnosis is based on clinical signs, the presence of visceral granulomas and the confirmation of acid fast (a special type of staining technique) bacterial rods in tissue sections, especially in the kidney, liver and spleen. Your veterinarian may also elect to culture your fish and attempt to grow and identify the mycobacterial organism with the assistance of a specialized diagnostic laboratory.

Infected fish have, at best, a guarded prognosis. While some veterinarians and others have tried treating this disease with a variety of antibiotics, most cases are unrewarding.

Home Care and Prevention

You should not attempt to diagnose or treat this disease without the assistance of a licensed veterinarian. If a diagnosis of mycobacteriosis had been made in your aquarium or pond, you should never transfer these fish to another aquarium or pond containing uninfected fish. While some hobbyists may elect to depopulate an infected aquarium, this drastic measure is not recommended in the majority of cases.

A population of fish that have been identified to contain mycobacteriosis should be isolated and never mixed with uninfected fish. Such fish should not be bred since the infection is probably passed on to offspring. Any potential fomites (disease-spreading utensils like nets) must be sterilized with chlorine (100 ppm for at least 1 hour) or another suitable disinfectant.