Mycobacteriosis is a disease in fish caused by acid-fast bacteria in the genus Mycobacterium.    M.marinum and M.fortuitum are the most common species seen in fish.  These bacteria are found in the normal aquatic environment.   

Clinical signs are usually non-specific and can include ulcerative skin lesions, reduced appetite, emaciation, lethargy, exophthalmia ("pop-eye"), swollen abdomen, and fin and tail rot.   This disease is usually slowly progressive in nature and causes low to moderate mortalities.

Internal organs (kidney, spleen, liver, heart muscle) are usually affected with numerous granulomas. The skin and gills may also be affected.   These granulomas may be visible grossly (to the naked eye) or on wet mounts under the microscope.  Once granulomas are seen an acid-fast stain should be performed.  Diagnosis is based on clinical signs, the presence of granulomas and the demonstration of acid fast bacterial rods in tissues.  Culture of these bacteria can be difficult and lengthy. 

Mycobacteriosis is a zoonotic disease that can cause non-healing ulcers (often called "fish tank granuloma") in humans. 

There is no effective treatment for Mycobacteriosis in fish.  Depopulation of infected fish and disinfection of system(s) is recommended. 

Back to Bacterial Infections