Epidemiology of bacterial diseases in Norwegian aquaculture--a description based on antibiotic prescription data for the ten-year period 1991 to 2000.

In Norway, antibacterial drugs for use in farmed fishes have to be prescribed by a veterinarian. Moreover, a national surveillance programme requires that copies of all prescriptions be sent to the Directorate of Fisheries. The prescriptions give information regarding fish farm and locality, weight and species of fish to be medicated, diagnosis, type and amount of drug prescribed, and date. These prescription data for the 10 yr period 1991 to 2000 have been recorded and systematised. A total of 6322 prescriptions issued for 11 fish species were registered; Atlantic salmon represented 87% of the prescriptions, rainbow trout 4.5%, arctic char 0.3%, turbot 3.8%, halibut 2.1%, Atlantic cod 1.2%, and European eel 0.3%. European sea-bass, wolf-fish, coalfish and wrasse represented a total of 0.4% of the prescriptions. Furunculosis was the most frequently given diagnosis in Atlantic salmon, accounting for 79% of all prescriptions for this species. Furunculosis was more frequent during the summer and early autumn, and in the western parts of Norway. Fish weighing more than 1 kg seemed to be the most susceptible. However, furunculosis has almost disappeared in Norwegian aquaculture since 1993. Vibriosis gave cause for antibiotic treatments in almost all fish species, and was the most common diagnosis in rainbow trout, halibut, turbot, cod and European eel. In Atlantic salmon, fish of small and medium size (up to 1 kg) seemed to be more at risk from vibriosis, and outbreaks were more frequent during summer, and in the western counties. Cold-water vibriosis was the second most frequently treated disease in Atlantic salmon, creating severe problems mainly in larger fish, in the northern parts of the country, and during winter and spring. The seasonal distribution was similar for winter ulcer disease, the only disease which seemed to be of increasing importance in Atlantic salmon. Non-specific diagnoses, such as 'bacterial infection' and 'fry disease', were given in a much higher proportion of prescriptions for marine fish species than in prescriptions for salmonids.