Dead fish collected for rendering following an outbreak of IHN at a Vancouver Island salmon farm some years ago. Photo: Dr Paul Hardy-Smith courtesy of Panaquatic Health Solutions / Global Aquaculture Alliance.

European fish farmers on alert for deadly virus

Fish at farms in several European countries are being tested for the serious fish disease infectious haematopoietic necrosis (IHN) virus following its discovery in Denmark and Finland.

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Veterinary authorities in Norway, which has IHN-free status, are concerned the virus may spread to the country via imports of ova, live fish and fresh products of rainbow trout and other susceptible species. The UK has also historically been free of IHN, which is a notifiable disease.

The virus that causes IHN was recently detected in rainbow trout in two fish farms in Jutland, Denmark, according to Norway’s Veterinary Institute. The fish showed no signs of disease.

Test and trace

During 2021, fish were exported to Austria, Germany, the Netherlands, Italy, Finland and Luxembourg from the Danish facilities.

The receiving facilities are now carrying out testing and infection tracing, and infection of rainbow trout has been confirmed at Åland in Finland. Here, signs of disease compatible with IHN were detected. Several Danish fishing facilities are also under investigation for IHN.

North America

The disease was first recognised in the 1950s in sockeye and chinook salmon, spread across North America in the 1970s in rainbow trout, apparently originating from fry or ova shipments from a single source.

Most salmonid species including Atlantic salmon are susceptible to the virus, with fry and small fingerlings becoming infected very readily, and becoming more resistant as they mature. The infection is often lethal, and the mortality rate can be 100% in fry. Fish that survive an outbreak can become carriers of the virus. In addition, infected juveniles will shed IHN virus particles in the faeces, urine and external mucus.

“There is reason to fear that the disease may also be introduced to Norway via imports of ova, live fish and fresh products of rainbow trout and other susceptible species,” said the Veterinary Institute. “It is therefore particularly important that importers are aware of this risk and show caution, among other things through tracing the origin.”

‘Serious consequences’

There are also fears that the virus could spread in connection with angling. To prevent the spread of IHN virus and other viruses between watercourses, it is important to disinfect all fishing equipment before taking it from one watercourse to another.

The Veterinary Institute said the introduction of IHN to Norway would have serious consequences for the aquaculture industry. If there is a suspicion of IHN, the Norwegian Food Safety Authority must be notified immediately. Care must also be taken to prevent further spread of infection.