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Mycobacterium infection

Posted By: Tom W <tbwaltzek@ucdavis.edu>
Date: Saturday, 26 February 2005, at 2:19 a.m.

Hi Guys,

I wanted to post a little more info. on Mycobacterium infections in fish. First off I have added a picture of what Myc. looks like under a microscope. The picture shows red colored tissue (spleen) with lots of brown (melanomacrophages) as well as two large granulomas (macrophages trying to wall off the infection). Myc. infections often present with granulomas in the liver, kidney, and spleen of the fish. These organs take on a flecked appearance due to granuloma formation (see photo). As I have previously stated granulomas were not seen in the "skinny" loaches I looked at. However, this certainly doesn't rule Myc. as being a problem. Myc. is very common in fish as well as many other vertebrates.

Unfortunately, Myc. infections appear very difficult to treat. Many bacteria do not invade cells while other bacteria are intracellular parasites as seen in Myc. Intracellular parasites including viruses, protozoans, and bacteria are difficult to treat as the therapeutic agent may not reach the intracellular pathogen. Having said all that Mycobacterium infection can be treated in humans with prolonged antibiotic regimens. As stated in previous posts Myc. seems to infect immunocompromised or severely stressed specimens.

A thought on treatment. Several LOL members are reporting good success in treating "skinny " loaches with antibiotics effective against Myc. If the skinny loaches truly had Myc. infections it would be of considerable interest if the antibiotics were in fact responsible for the recovery. However, a positive diagnosis of Myc. as well as controls are needed to know for sure. How many people are going to find and then buy 20 "skinny" loaches and devote two tanks to them? One tank receives the antibiotics and the other doesn't. At the end of the study euthanize everybody and do pathogen checks. Do the untreated skinny fish have Myc. infections? How about the treated fish? Are they still skinny or have they gained weight? Is Myc. still present in the treated fish? Is the Myc. sensitive to the antibiotic used. O.k. you get the point and I know I won't be doing this experiment in the future. However, if the treatment regimens are working than stick with what is working.

Anyway, I don't think the idea that small Botias come in skinny due to starvation should be abandoned. Consider how much food small fish need and how little they receive from the time they are captured or cultured until the time they reach our tanks. I for one have turned very emaciated fish into vibrant specimens given time and TLC. With all the knowledge and skill the LOL crew has, I wonder if the skinny recoveries have been the result of good husbandry. Once healthy, the immune systems can usually fight off most pathogens from the parasites I posted pictures of to Myc. infections. Just my thoughts on an issue that is clearly complicated and unlikely to be resolved. Cheers- Tom


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