Etiology
Mycobacteriosis is a chronic or acute, systemic, granulomatous disease that
occurs in aquarium and culture food fish, particularly those reared under intensive
conditions. Mycobacteriosis results from infection by several species of
Mycobacterium, aerobic, Gram-positive, pleomorphic rods which are members of the
order Actinomycetales and family Mycobacteriaceae. Mycobacteria are widespread in
the environment, particularly in aquatic reservoirs. The two most important species
causing mycobacteriosis in fish and humans are Mycobacterium marinum and
Mycobacterium fortuitum. Other species known to cause mycobacterial disease in fish
include M. chelonei, M neoaurum, M simiae, and M scrofulaceum.
Mycobacterium marinum was first recognized in 1926 from the liver, spleen and
kidney of tropical coral fish kept in the Philadelphia Aquarium. M. marinum can grow
prolifically within fibroblast, epithelial cells and macrophages. In the past, human
outbreaks of M. marinum were sporadic and most commonly associated with
contaminated swimming pools. Chlorination practices used today have greatly
minimized to frequency of outbreaks from these sources. In the last decade, a small but
steady increase in the frequency of Mycobacterium marinum infections in cultured or
hatchery confined fish and human cases associated with fish aquaria has been noted.