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.

A total of 119 fresh faecal samples were collected from graylag geese migrating northwards in April. Also, cloacal swabs were taken from 100 carcasses of graylag geese shot during the hunting season in August. In addition, samples were taken from 200 feral pigeons and five mallards. The cultivation of bacteria detected Campylobacter jejuni jejuni in six of the pigeons, and in one of the mallards. Salmonella diarizona 14:k:z53 was detected in one graylag goose, while all pigeons and mallards were negative for salmonellae. No avian paramyxovirus was found in any of the samples tested. One mallard, from an Oslo river, was influenza A virus positive, confirmed by RT-PCR and by inoculation of embryonated eggs. The isolate termed A/Duck/Norway/1/03 was found to be of H3N8 type based on sequence analyses of the hemagglutinin and neuraminidase segments, and serological tests. This is the first time an avian influenza virus has been isolated in Norway. The study demonstrates that the wild bird species examined may constitute a reservoir for important bird pathogens and zoonotic agents in Norway.

A serologic survey revealed that Norwegian populations of free-ranging reindeer (Rangifer tarandus tarandus), roe deer (Capreolus capreolus), red deer (Cervus elaphus), and moose (Alces alces) have been exposed to alpha-herpesviruses and pestiviruses. A total of 3,796 serum samples collected during the period 1993-2000 were tested in a neutralization test for antibodies against bovine herpesvirus 1 (BHV-1) or cervid herpesvirus 2 (CerHV-2), and 3,897 samples were tested by a neutralization test and/or enzyme-linked immunosorbent assay for antibodies against bovine viral diarrhea virus (BVDV). Antibodies against alpha-herpesvirus were found in 28.5% of reindeer, 3.0% of roe deer, and 0.5% of red deer, while all moose samples were negative. In reindeer, the prevalence of seropositive animals increased with age and was higher in males than females. Antibodies against BVDV were detected in 12.3% of roe deer, 4.2% of reindeer, 2.0% of moose and 1.1% of red deer. The results indicate that both alpha-herpesvirus and pestivirus are endemic in reindeer and pestivirus is endemic in roe deer in Norway. The viruses may be specific cervid strains. Seropositive red deer and moose may have become exposed as a result of contact with other ruminant species.

The sales and prescription of antibacterials for use in Norwegian fish-farming according to diagnosis, fish species and production stage from 2011 to 2016 are analysed. The study is based on antibacterial sales data from wholesalers, pharmacies and feed mills and on prescription data obtained from a register of all prescriptions of antibacterials used in farmed fish. The results show that the fish-farming industry uses very small volumes of antibacterials. In 2016, a total of 212 kg were sold; the only antibacterial substances sold were florfenicol and oxolinic acid. The total amount corresponded to 0.16 mg kg-1 fish slaughtered, or to approximately 0.14% of the fish produced that year. The majority of prescriptions were for non-specific bacterial infections; as most common diseases are under control by vaccination. Most prescriptions for salmonid fish were during early production stages. However, due to higher biomasses of fish, the highest quantities of antibacterials were prescribed during the seawater production phase of Atlantic salmon Salmo salar. An increasing proportion of the prescriptions was for other species, including cleaner fish used for salmon lice control; in 2016 most prescriptions were for this fish category. Due to the negligible use of antibacterials in Norwegian aquaculture, in particular for on-growers, the risk of development of antimicrobial resistance and its transmission to humans through consumption of fish is considered negligible.

Waterborne viral infections represent a major threat to fish health. For many viruses, understanding the interplay between pathogens, host and environment presents a major hurdle for transmission. Salmonid alphavirus (SAV) can infect and cause pancreas disease (PD) in farmed salmonids in seawater. During infection, SAV is excreted from infected fish to the seawater. We evaluated two types of filters and four different eluents, for concentration of SAV3. One L of seawater was spiked with SAV3, followed by filtration and virus elution from membrane filters. For the negatively charged MF hydrophilic membrane filter (MF-) combined with NucliSENS® lysis buffer the SAV3 recovery was 39.5 ± 1.8 % by RT-ddPCR and 25.9 ± 5.7 % by RT-qPCR. The recovery using the positively charged 1 MDS Zeta Plus® Virosorb® membrane filter (MD+), combined with NucliSENS® lysis buffer was 19.0 ± 0.1 % by RT-ddPCR and 13.3 ± 3.8 % by RT-qPCR. The limits of quantification (LOQ) and detection (LOD) were estimated to be 5.18 × 103 and 2.0 × 102 SAV3 copies/L of natural seawater, by RT-ddPCR. SAV3 recovery from small volumes of seawater, and the requirement for standard laboratory equipment, suggest the MF-filter combined with NucliSENS® lysis buffer would be a candidate for further validation in experimental trials.

To avoid fish pathogens, potential ports of entry into hatcheries and farms have to be identified and barriers put in place, e.g., restricted admission; personnel locks; and disinfection of biological material, water supply, and vectors. For Atlantic salmon, the water supply is of greatest potential risk for introduction of pathogens, both during the freshwater and seawater phases. Seawater constitutes the highest risk both to smolt farms and sea sites. Live fish material is also high risk. Among vectors, personnel and fomites coming into contact with infectious material pose the greatest risk, i.e., vaccinators, net cleaners, and wellboats.