'Flavobacterium psychrophilum' on rainbow trout. (Photo: Stockfile)
Antibiotic resistance in fish poses threat to public health
Wednesday, November 21, 2012, 04:20 (GMT + 9)
Scientists believe that the increase in bacteria resistant to antibiotics and the spread of this phenomenon exist in part because the aquaculture industry has paid little attention to the use of antibiotics. This is particularly worrisome because the antibiotics used in veterinary medicine and in aquaculture belong to the same group of antibiotics for humans.
As part of his doctoral research project at the Norwegian School of Veterinary Science, Syed Qaswar Ali Shah has studied the genetic basis for antibiotic resistance in bacteria isolated from Norwegian salmon. He has gathered quinolone-resistant isolates of the bacterium Flavobacterium psychrophilum from rainbow trout and equivalent isolates of Yersinia ruckeri from Atlantic salmon – the bacteria that cause bacterial coldwater disease in rainbow trout and redmouth disease in salmon.
The resistance to quinolones -- one of only a limited number of antibiotic agents permitted for use -- must be managed with restrictions, increased focus on infection prevention and the invention of a vaccine against Flavobacterium psychrophilum to prevent infection.
Shah has also studied bacteria isolates -- from freshwater carp aquafarms in Pakistan, tilapia farms in Tanzania and fish farms in Chile and Norway -- to discover genes as codes for resistance to antibiotics used in veterinary medicine. He found that many isolates are resistant to several different types of antibiotics.
The isolates from freshwater farms contained more resistant bacteria -- and Shah discovered resistance to antibiotics in isolates from freshwater farms which had not used antibiotics. This occurs when fertilizer from farm animals is used as feed for farmed fish.
He found more bacteria resistant to antibiotics in the seawater isolates from Chile than in those from Norway, likely explainable by Chile’s excessive use of antibiotics: approximately 840 g of antibiotics per 1000 kg fish in 2008, versus approximately 1 g per 1000 kg of fish in Norway.
When antibiotics are given to farmed fish, the surrounding environment is directly exposed as well when fish faeces fall to the sediment. Many of these antibiotic substances are broken down slowly, so they form an accumulation of antibiotics in the surrounding environment, often resulting in local bacteria selecting genes resistant to antibiotics.
This development can itself lead to a reservoir of resistant bacteria that spread this resistance via horizontal gene transfer. Numerous aquatic bacteria and bacteria that affect humans are part of the same group, such as Aeromonas, Acinetobacter, Kluyvera, Vibrio and Yersinia, and this facilitates the transfer of the bacteria’s resistant genes to other bacteria in the group.
“The presence of identical factors in bacteria found in aquatic environments and in clinical pathogens is clear proof that resistant genes from bacteria in aquaculture have spread to human pathogens,” wrote the Norwegian School of Veterinary Science. “The main reservoir of genes resistant to antibiotics is thought to exist in aquatic environments, and fish pathogenic bacteria represent a possible intermediate stage in the transfer of resistance from aquatic environments to resistant bacteria on land.”
By Natalia Real