Fish Farms Making Diseases More Concentrated and Active
November 18, 2009
By Barbara Axt Reporter
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 Jyväskylä 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 oxytetracyclin, 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.