Can we feel it in our waters? Antimicrobials in aquaculture

No antimicrobials are registered in Australia for use in fish or other aquatic species. In late 2009, the Australian Pesticides and Veterinary Medicines Authority (APVMA) issued minor-use permits to allow the use of florfenicol in medicated feed for salmon, trout and barramundi, and oxytetracycline by injection in finfish. A report prepared for the Fisheries Research and Development Corporation in 20071 indicated that antimicrobial use has declined since the 1980s because of vaccine development and improvements in management. The authors stated that “significant amounts of antibiotics” have been used to control some infections in Tasmanian salmon and noted that amoxicillin, oxytetracycline, oxolinic acid (a quinolone), chlortetracycline, trimethoprim, erythromycin and florfenicol (all Schedule 4 prescription-only products) had been used. They noted that the salmon industry had voluntarily stopped using amoxicillin, oxolinic acid and chlortetracycline. However, according to one newspaper report,2 8 tonnes of antibiotics were used in salmon and trout farms in the first 3 months of 2007. Under state legislation, registered veterinarians can prescribe an antimicrobial registered for use in one food animal species “off-label” to another food-producing species. However, off-label use is restricted to treatment of a single animal, and mass medication is specifically prohibited. For example, an outbreak of Streptococcus iniae in barramundi was treated with “erythromycin administered in the feed for seven days ... at 75 mg/kg fish daily”.3 This is not in accord with legitimate off-label use.

There is no surveillance of antimicrobial resistance or antimicrobial use in animals in Australia. However, significant resistance has been reported to a range of antimicrobials including ampicillin/amoxicillin, erythromycin, tetracyclines and nalidixic acid in bacterial isolates from aquaculture in Australia.4 The Department of Agriculture, Fisheries and Forestry conducts an annual survey of food products for residues of antimicrobials, heavy metals and pesticides (National Residue Survey) but only small numbers of aquaculture samples are included. For example in 2010–2011, one freshwater crayfish, seven barramundi and up to seven samples of abalone were tested for antimicrobial residues; all were negative. The testing regimen seems totally inadequate. In the case of imported products, it has been the press that has drawn attention to antimicrobial resistance.5 Low levels of residues of a number of antimicrobials including enrofloxacin were detected in a number of consignments of products from Vietnam. The detection of residues could be considered a surrogate marker for presence of resistant bacteria. However, Australia does not carry out any surveillance testing for antimicrobial resistance in local animal isolates; thus, under World Trade Organization rules, it cannot test bacteria from imported food products (including aquaculture) for antimicrobial resistance. This places the Australian consumer at some risk, as antimicrobial use is less controlled in many of the countries from which aquaculture products are sourced than it is in Australia. Further, the risk from consuming local product is unknown because of the lack of surveillance.

It is time for aquaculture industries in Australia to be more transparent about their antimicrobial use. They need to work with regulatory authorities to ensure that appropriate permits for use are obtained and that there is a comprehensive system of antimicrobial resistance surveillance and antimicrobial use monitoring. Australian aquaculture industries should be encouraged to continue to develop vaccines and alternatives to antimicrobial use.