A fourth case of Infectious Salmon Anaemia (ISA) was this week confirmed at a controlled site off the west coast of Shetland.
Government officials announced they had found evidence of the virus on Wednesday evening.
The site is owned by Hjaltland Seafarms Ltd, which also owns the other farms in which the disease was first discovered earlier this year.
Plans have been made for the fish at the farm to be slaughtered as soon as is practicably possible.
The farm is within the control zone which was set up in January following the first outbreak of the disease.
Statutory movement restrictions have been placed on all sites in the control zone centred on the infected farms and in a wider surveillance zone.
Although it is no threat to humans, ISA poses a severe threat to farmed salmon stocks, with symptoms including high mortality rates, darkening of the liver, severe anaemia and visceral haemorrhage.