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COLUMNARIS DISEASE, FISH - USA
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A ProMED-mail post
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International Society for Infectious Diseases
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Date: Wed 4 Nov 2009
Source: Animal Pharm News [edited]
<http://www.animalpharmnews.com/news_stories/october_09/fish_farming_can_make_diseases_more_virulent,_say_researchers>
Fish farming can make diseases more virulent, say researchers
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The conditions in which fish are farmed may be the reason infections
such as columnaris disease are becoming increasingly virulent, as
aquaculture creates selective pressures that encourage the most lethal
strains of disease to thrive. That is the conclusion of a 23-year
study conducted at a fish farm in Finland.
According to the research, led by Dr Katja Pulkkinen of the University
of Jyvaskyla and published in the journal Proceedings of the Royal
Society, the high density of fish, the stress of the animals, and even
the treatment administered to them have acted as selective pressures
favouring the more virulent strains of the pathogens.
Columnaris disease, caused by the bacterium _Flavobacterium
columnare_, leads to skin lesions, fin erosion, and gill necrosis and
has become a serious problem in aquaculture. In the US, it is one of
the biggest causes of death in farmed catfish, costing the industry
millions of dollars a year.
The study shows how the conditions in which fish are farmed create
selective pressures, which make the bacteria more virulent than in
nature. In a natural environment, bacteria that cause severe symptoms
(leading to the predation of the weakened animal) or that kill the
host rapidly are usually removed from the gene pool, as extreme
virulence harms their own ability to spread.
In fish tanks, however, these are precisely the strains of bacteria
which thrive, as the bacteria continue to transmit to other animals
from dead fish. In addition, healthy fish usually have direct contact
with dead or severely weakened animals, something that would not
happen in a natural environment.
The researchers also argue that the use of the antibiotic
oxytetracycline, which started in 1992 in Finland, favoured the _F.
columnare_ strains with more severe symptoms. The drug is effective in
animals with weaker symptoms, which can feed on and make use of the
medication. It is less effective on the stronger strains of the
pathogen infecting extremely debilitated animals, or upon the dead
fish. In this way, these strains continue to spread while the milder
ones are removed from the gene pool.
[Byline: Barbara Axt]
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[_Flavobacterium columnare_ is a gram-negative rod that has the
appearance of a fungus but is actually a bacterium. It is sometimes
known as "cotton wool disease". The common signs are fuzzy or
stringy-looking patches on the fins or the body and may vary in color
from white to gray or beige. Infected fish are often lethargic,
anorexic, and may suffer a loss in coloration. If left untreated, the
bacteria will likely spread and may lead to death.
Linking farmed fish disease to aquarium diseases is an interesting
comparison and somewhat surprising that this comparison has not
previously been recognized. - Mod.TG]
[Photo of columnaris in a catfish:
<http://microgen.ouhsc.edu/f_columnare/f_columnare_fig2.jpg>
- Mod.JW]
[see also:
2007
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Catfish die-off - USA: (MN, ND), bacterial etiology 20071027.3495
Undiagnosed die-off, crappie - USA (MN) 20070915.3072
Undiagnosed die-off, carp - Canada (ON) (02) 20070826.2803
2004
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Carp die-off - USA (SC)(02) 20040610.1560
1999
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Columnaris, white bass - USA (Kansas) 19990713.1178]
........................................arn/tg/mj/jw
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