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 favoring 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.