Abstract

Mycobacteriosis is a bacterial disease that can affect many species of freshwater, brackish, and marine fish. The disease is typically chronic and progressive in nature resulting in a wide variety of clinical signs prior to death.

Mycobacteriosis is of particular economic concern in the ornamental fish production industry. Because no effective treatments or standardized ante-mortem diagnostic methods exist at present, diagnosis of the disease in one fish in a group means the entire population must be considered exposed and possibly infected. Offspring of mycobacterial-infected fish populations are also considered potentially infected through vertical or horizontal transmission and should be examined. To prevent spread of the disease throughout a producer's facility or to that of a buyer's, the recommendation when mycobacterial infected fish are identified in ornamental aquaculture production situations is depopulation followed by disinfection of all system components and equipment. Recommendations for public aquaria display populations, by contrast, include assessment of disease prevalence to determine whether depopulation is warranted.

Diagnosis for mycobacteriosis at the University of Florida's Tropical Aquaculture Laboratory's Fish Disease Diagnostic Lab (UF-TAL-FDDL) is based on: 1) clinical history; 2) necropsy findings, which usually include evidence of granuloma formation in kidney, spleen, and/or liver; 3) positive benchtop or histopathological staining for acidfast bacteria; and/or 4) positive mycobacterial cultures.

It is important to note that positive diagnosis is not always assured because mycobacterial infected tissues do not always stain positive and growth of cultures from positively infected tissues is not guaranteed.

Between 1999 and 2005, the UF-TAL-FDDL evaluated an average of 302 cases per year (range of 258 to 337 cases), predominantly freshwater. The majority of these cases were submitted by commercial freshwater and marine ornamental fish producers with a smaller number being submitted by local aquaria. Mycobacteriosis was diagnosed in an average of approximately 8 cases per year. Families with the highest representation included Cichlidae (21%; 12 of 56 cases), Cyprinidae (12.5%; 7 of 56 cases), and Characidae (11%; 6 of 56 cases). Species with the highest representation included Astronotus ocellatus (oscar), Antennarius striatus (long striated frogfish), Amphiprion clarkii (yellowtail clownfish), and Mikrogeophagus ramirezi (blue ram). Broodstock were the affected age class in approximately 52% of the cases in which age of the fish was known.

Of the cases in which system characteristics were known, 55% involved fish housed in recirculating systems while 34.5% involved fish housed in flow-through systems.

A summary of the cases submitted to UF-TAL-FDDL between 1999 and 2005 will be presented including fish species and age, mortality level, system characteristics, clinical signs, necropsy findings, and method of diagnosis (i.e., benchtop acid-fast stain, histopathology, culture and identification).