Prescription of antimicrobial drugs in Norwegian aquaculture with an emphasis on “new” fish species

The usage of antimicrobial (AM) drugs in farmed fish in Norwegian aquaculture for the period 2000–2005 was investigated by using prescription data. These data were validated against national sales data of AM drugs sold for use in farmed fish and were found to be highly valid. The defined course dose (DCD) was applied as the unit of measurement to correct for the variations in the dosages between different AM drugs. The DCDkg was the amount of an AM drug recommended for the treatment of a 1-kg fish. The calculated number of prescribed DCDkgs is an estimate of the biomass of farmed fish that can be treated with a certain amount AM drug. In the present study, the number of prescriptions issued (i.e., numbers of initiated treatments), weight of active substance prescribed and biomass treated were applied to describe the usage. An increase, although modest, in the AM drug usage in Norwegian aquaculture was observed from 2002 to 2005. This increase was accounted for by new-farmed fish species (other than Atlantic salmon and rainbow trout), especially Atlantic cod. The increased usage of AM drugs in cod in the study period was significantly positively correlated to the biomass produced; even so from 2001 to 2005 the number of prescriptions for cod relative to the produced biomass declined. The AM drug usage in Atlantic halibut as well as the production varied during the study period. For other species such as turbot, coalfish and wolffish the usage of AM drugs was found to be negligible. “Mono-therapy” with quinolones may present a selective pressure in regard to development of quinolone resistance.