IPN still poses a challenge

Seemingly healthy and vaccinated Atlantic salmon can still experience outbreak of the fish disease infectious pancreatic necrosis (IPN) after being transferred to seawater. The virus is harmless for humans, but not for salmon. Norwegian salmon farmers lose nearly 20 % of their salmon smolt and IPN must take a significant share of the blame.

IPN cases have increased

The salmon industry’s attitude to IPN has long been that this is “something we must live with”. The removal in 2008 of the obligation to report outbreaks of IPN has led to reduced focus on the disease, but not reduced problems for the salmon farmers.

The Norwegian Seafood Federation reports a strong increase in the number of IPN cases compared to 2008 despite a comprehensive effort to find preventative measures.

When tests of IPN vaccines in challenge experiments show good results, one can only wonder why it does not function in the same way under industrial conditions.

Already have IPN virus

A large proportion of Norwegian farmed salmon are already carriers of the IPN virus at the time of vaccination and transfer to sea cages. Whether this is of significance for the effect of the vaccine has for many years been an unanswered question for the salmon industry and vaccine manufacturers. The vaccines are normally tested on salmon that are not infected with the IPN virus.

IPN in the seawater phase is a complex problem that can be influenced by several factors such as stress, smoltification and the timing of the vaccination before the smolts are transferred to the sea cages. Consequently, the significance of the individual factors needs to be studied under controlled experimental conditions.

Vaccination can help

Nofima, in collaboration with vaccine manufacturer Intervet Norbio AS, has recently concluded a project supported by the Research Council of Norway’s Aquaculture programme. The project aimed to check whether the vaccination of salmon that are already carriers of the IPN virus can fight the virus and help prevent outbreaks that can be attributed to the virus infection flaring up.

The IPN virus has different abilities to cause disease (dangerous/less dangerous). The project has studied what significance it can have for the vaccinated and unvaccinated salmon to carry these different virus types. No IPN virus carriers died during the freshwater phase, but the IPN outbreak came after transfer to seawater. The mortality rate was much higher among the unvaccinated fish.

“The result shows that carrying the IPN virus can actually increase the salmon’s protection somewhat after vaccination, but the effect appears to be dependent on which virus type is dominant in the salmon,” says Project Manager and Senior Scientist Ann-Inger Sommer. “The dangerous type appears to give increased protection, but the less dangerous type does not give the same effect.”

New and useful knowledge

“However, this project shows that the immune defence does not remove the virus completely after vaccination, and that the salmon as such is still a possible source of infection for other exposed individuals,” says Sommer. “The vaccinated carrier of the IPN virus will then constitute a hidden risk even though that fish is very well protected against IPN outbreaks. This documents one of the reasons why mixing different productions of salmon should be avoided even though all have been vaccinated.”

The project has provided new knowledge about the risk IPN virus carriers pose for the farmed salmon.

“Consequently, the work to reduce the extensiveness of IPN virus carriers should be intensified by working on several fronts, such as increasing resistance to the disease through breeding and the development of more effective vaccines and vaccination strategies,” says Sommer. “It is important to realise that the absence of the disease does not always mean the virus in the fish is absent. As long as preventative measures do not remove the virus in the fish more effectively, the possibility of developing new and more dangerous virus types will also exist.”

Fish health  

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