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NTB and TB
I've heard from various people including a fish store owner, that Neon Tetras which come into stores are nowadays almost always infected with Neon Tetra disease. I believe this is the same as fish TB isn't it?
That suggests to me that the bacterium or virus is pretty well endemic to our aquaria. Exposure to it is probably much more common than we realize. I always try to treat any tiny scratch with iodine if I notice one when I'm working in the tanks. Still, its going to get under your fingernails and then you spread it yourself into your eyes or nasal passages. Fortunately for us mammals and the majority of our piscine friends, we don't get too sick from all the bacteria that we ingest every day without even knowing it! ;-)
Sorry if someone has already answered this, but I didn't notice a direct reply, so here goes... Neon Tetra Disease (NTD) is caused by the microsporan (a kind of protozoan) Pleistophora hyphessobryconis. This parasite, like other microsporans, inject an infective sporoplasm into a host cell via a coiled polar filament. Once in the cell, the parasite undergoes asexual (merogony) and sexual (sporogony) reproductive cycles, finally releasing mature infective spores to infect another host fish. The disease is distributed worldwide, and infects sixteen species of fish including tetras, barbs, danios, and goldfish. There is no known cure in fish, although one compound, Toltrazuril (not available commericially) has shown some efficacy experimentally. P. hyphessobryconis is completely non-pathogenic to humans.
Karen has already done a good job describing "fish TB," (actually it is not referred to these days as such, just as "fish mycobacteriosis"), so I won't repeat the explanation. I just wanted to add a few things. First, while human infection with M. marinum is highly unlikely on a systemic level, and generally doesn't cause too many problems when in the extremities, anyone who is immunocompromised for any reason should take extra precautions when working in their aquaria (no open cuts, wear rubber gloves). As Karen mentioned, these are incredibly tenacious and resistant bugs, especially when host defenses are down. Second, fish are commonly carriers of mycobacteria, but seldom show signs of disease. Generally, as is true with most asymptomatic fish infections, problems will only start cropping up when the fish are stressed due to things such as overcrowding and bad water quality. So, if your fish start dying with signs of mycobacteriosis (e.g. curved spine, swollen belly, open sores) take care of the problem first by isolating and destroying infected fish, but then make every effort to identify possible husbandry problems that probably led to disease in the first place. This could help reduce further deaths in the tank. I should mention here that there are no clinical signs that are definitively diagnostic for mycobacterial infection. The only way to know for sure is to culture the bacteria using highly specialized techniques, or to directly observe bacteria in tissue sections with special stains. So, if your fish have the signs I described above, it may be any one of a number of possible etiologic agents, including mycobacteria.