A highly infectious fish disease known as furunculosis has been discovered at a salmon farm in Namsenfjorden in central Norway.
There is also a suspected case at nearby farm belonging to the same company, says the Norwegian Food Safety Authority. Both locations have been sealed off.
The Authority said all the classic signs of the disease are showing up. Furunculosis is highly contagious and affects fish of all ages, causing high mortality.
The disease one of the most commercially significant salmonid diseases, occurring in freshwater and marine salmonid aquaculture in several countries. The mortality rate is high and so far only Australia and New Zealand appear to have escaped.
Outbreaks typically occur at temperatures above 10C, however outbreaks can occur in very young fish and at temperatures as low as 24C.
Horizontal transmission occurs via the water column, but also through direct fish-to-fish contact and animal vectors (birds and invertebrates such as sea lice).
Non-salmonids may become infected by ingesting tissue of infected salmonids. Similarly, transmission to non-salmonids can occur where fish caught for feed are taken from waters near an outbreak.
The other problem is that fish which survive the disease can become carriers without showing any outward signs of infection.
The Food Safety Authority said in a press release that following this new outbreak, breeders and fish health services should remain on the alert as infection pressure in the area has probably increased.
Salmon breeders who use seawater from the area for hatchery production or for transporting fish should be particularly aware, it adds.