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RE: Fish TB (Dr. Yanong responds, this is a long mail but hope it helps)
Allen,
Please forward this email to Mr. Wright. No, Allen, I don't believe you
quoted me incorrectly. I know this email may be a bit lengthy, but hope it
helps shed some light on this disease in fish.
I am not quite what exactly was the situation you are both discussing, but
please see below.
Although it is definitely possible Mr. Wright is doing adequate disinfection
under his specific protocols and conditions (not sure what his concentration
and contact times are), regardless, by his comments, it is clear that he has
some misconceptions about the mycobacteria group and mycobacteriosis in
fish, some of which I would like to address.
Please let Mr. Wright understand that I run the Tropical Aquaculture
Laboratory, University of Florida's Fish Disease Diagnostic Laboratory, am
one of two extension veterinarians working in Florida for the University (my
colleague, Dr. Ruth Francis-Floyd is based in Gainesville), and work with
approximately 200 ornamental fish producers in the state of Florida, as well
as hobbyists, game fish, and food fish producers in the state and throughout
the U.S. I have been working as a fish veterinarian (i.e., fish ONLY) for
almost ten years, and have quite a bit of experience with
mycobacteria-infected fish populations.
Mr. Wright is correct, the mycobacteria are all part of the same genus,
Mycobacterium, because they have similar biological, biochemical, and
structural characateristics. However, those of us who are fish health
specialists are trying to eliminate the term Fish TB, because this term
arouses incorrect suspicions or presuppositions. First, that granulomatous
("tubercular") type lesions are always associated with the disease. This is
incorrect. Certain species of fish do not have typical granulomatous
lesions. Secondly, that the bacteria that cause mycobacteriosis (the word
we prefer to use)in fish can be transmitted to people and cause human TB (a
much more serious disease including, specifically, lung lesions). This is
also quite incorrect. The mycobacteria that cause significant disease, and
are considered reportable, in humans, are known as the tuberculous
mycobacteria. The mycobacteria that cause disease in fish are part of the
atypical mycobacteria, and are considered environmental bacteria. Although
the atypical mycobacteria can cause disease in humans under specific
conditions (and Mr. Wright was correct that having cuts in ones exposed skin
will increase potential for infection), most people, even those that do have
cuts, have a significantly lower risk at actually developing disease than
those with immunosuppressive disorders (such as AIDS, organ transplants
require immunosuppressive drug therapy, etc.). Those with immunosuppression
can develop significant and serious disease, that may involve a more
systemic infection. Those without significant immunosuppression that do
develop disease caused by the atypical mycobacteria generally have a more
localized infection, usually on the skin of the hands or arms. A study done
in cooperation with the U.S. Navy many years ago (50's/60's) demonstrated
that a much larger percentage of US Nay recruits from the Southeastern U.S.
tested positive to certain types of TB skin tests, a result of exposure to
atypical mycobacteria (non-tuberculous mycobacteria). These recruits were
not infected (i.e., did not have active infection and disease), but had been
exposed and subsequently had their immune systems fight off the disease. In
short, it appears that exposure in most healthy people can be fought off
relatively easily. Please use the term fish mycobacteriosis when referring
to the disease in fish.
Although there are many factors, to be sure, that will determine whether or
not Mr. Wright has disinfected his tank adequately, specifically, contact
time, type of disinfectant, disinfection of all infective materials, and
origin of any new animals and materials going into the tank, in general,
mycobacteria are different than most other types of bacteria, and require
different disinfectant protocols. Most of the common mycobacteria that
infect fish are environmental bacteria, and have been isolated in high
numbers in acid water/ peaty, brown water swamps, as well as swimming pools
and water fountains. Some predisposing conditions include: igh organics,
high soluble zinc, high fulvic or humic acid, low dissolved oxygen, low pH,
and warmer temperatures. Species raised in intensive recirculating systems
(such as tilapia and striped bass) are particularly prone to development of
the disease, due to a probable "amplifying and concentrating effect" of the
system. I have noted a predilection for mycobacteriosis in other groups of
fish including (among many others--we have isolated mycobacteria from a
large host of species) rainbowfish, tetras, dwarf gouramis, and dwarf
cichlids.
Mycobacterial species that cause disease in fish, again, are
environmental--often found in water and soil, are ubiquitous (found
everywhere), and in one study, remained viable for up to 2 years or more in
the soil and water.
We culture myco routinely from mcyobacteria-infected fish, and I have been
working with fish and populations infected with mycobacteriosis, off and on,
for about 10 years, in both hobbyists systems, as well as on commercial
ornamental fish farms, stock enhancement facilities, public aquaria, both
marine and freshwater. Most of our isolates have been identified as
Mycobacterium marinum or Mycobacterium fortuitum, but we have had others.
Bleach may be okay, but for CONTACT disinfection (contact time I believe
less than 1 minute), one study in the literature has demonstrated that up to
10000 ppm was required for one species of Mycobacterium.(Manual of Clinical
Microbiology, 6th Edition, Murray, et al., 1995, published by the American
Society for Microbiology). For much longer periods, bleach may be adequate,
but such factors as sequestration (possibly "hiding" of the bacteria in
microenvironmental areas that are high in organics, such as microscopic
pockets of debris) may prevent adequate disinfection. 65-90% alcohol
increases the likelihood of kill, but, as with chlorine, organics will
diminish this response. We recommend the use of bleach for initial
reduction in organics and kill of most other pathogens, including some of
the weaker strains of myco. However, we also recommend following up with a
65-90% soak/spray with alcohol (after neutralization of chlorine and
drying). Increased contact time will increase likelihood of kill, so, if
possible, we suggest trying to wipe down and/or soak materials for 15-30
minutes, with alcohol.
Also, mycobacteria are not difficult to grow because it is "fragile." There
are some species that may be more difficult to grow because they require
specific media. So although it may not grow on certain commonly used media,
it probably is not necessarily dead, just not growing at that time in that
media. Also, I have grown strains on blood agar, which is the media
commonly used to grow most common strains of fish pathogenic bacteria.
Thus, although some strains are a bit more fastidious when it comes to
growth OUTSIDE OF A NATURAL SYSTEM OR FISH, others grow relatively easily
on a common media. Also, some strains may require 2-4 weeks to grow,
because of relatively slow metabolism, relative to common bacteria. I have,
however, gotten some strains to grow within 3-5 days, relatively quickly,
for mycobacterial species. Hence, for some, longer incubation times are
required.
Mycobacterium sp. are more difficult to kill than most bacteria because they
have a very thick waxy outercoat. These bacteria are designated "acid-fast"
bacteria (using acid fast stains, they retain a pink or fuschia colored
stain) because of this waxy coat. This waxy coat, plus the clumping that
occurs in some strains, make them a bit tougher to penetrate with common
disinfectants.
Antemortem diagnosis (i.e., determination that a fish is infected prior to
its death or without running routine diagnostics on a sacrificed fish) is
very difficult in many cases. Fish may not show overt lesions or signs for
many months, even if they have a significant mycobacterial load. Thus,
shedding can occur within a system without any obvious signs, for some time.
If any additional reference literature is desired, please let me know, and I
can forward a list. Also, if you have any further questions, please let me
know.
The University of Florida has a fact sheet on mycobacteriosis in fish, if
there is interest. Although it may not cover all of the information
discussed above, it is a good summary, geared toward aquaculture producers,
hobbyists, and the general public.
Allen, hope this email has been helpful.
Roy
Roy P. E. Yanong, VMD
Assistant Professor/Veterinarian
Tropical Aquaculture Laboratory
University of Florida
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