Virus resistant salmon
Investigating the genetics of virus resistance in salmon.
Atlantic salmon farming is a £500 million industry in the UK, which is the third largest producer in the world. Infectious Pancreatic Necrosis (IPN) is caused by a virus that can cause major disease outbreaks and has no fully effective vaccine. The disease is often fatal to salmon, and it presents a significant animal welfare, environmental, and economic burden.
Some salmon are naturally resistant to the IPN virus, so Roslin Institute scientists, working with colleagues at the University of Stirling’s Institute of Aquaculture, used DNA sequencing techniques to compare the genetic make-up (genome) of resistant and susceptible salmon. They identified an area of the salmon genome linked to IPN resistance and developed a simple genetic marker test for use in salmon breeding programmes.
Working with Landcatch Natural Selection (now part of Hendrix Genetics), the genetic test was successfully trialled on commercial salmon farms in Scotland and Chile. The test is now being routinely used to breed IPN-resistant salmon, and has prevented millions of salmon mortalities over the past decade. The project is estimated to have contributed £26.4 million to the UK economy which in turn supports rural communities and decreased disease transmission to wild salmon.
The IPN research is an excellent example of the application of molecular genetics to tackle a major production issue in aquaculture. The success has helped spark interest in the application of genetic and breeding technologies to address infectious disease problems. IPN was rather unique in that resistance was largely controlled by one locus. Resistance to most diseases, such as sea lice, are controlled by multiple genes, and therefore the state-of-the-art is to use high density genetic marker information routinely in a procedure known as genomic selection.
What is IPN?
Infectious Pancreatic Necrosis (IPN) in salmon is caused by IPN virus, which affects multiple body systems, including the skin, eyes, and pancreas. It can cause fish to swim in spirals and ultimately leads to the death of most infected fish. Most at risk are newly hatched salmon, and juvenile salmon that have just entered salt water. When an outbreak occurs on a fish farm, mortality rates are 30-90%, making IPN a big threat to the salmon and trout farming industries.
Why do fish survive?
Scientists and fish farm operators observed that some salmon can survive the virus, suggesting that there might be a genetic link to resistance.
In general, there are a few main ways that viral resistance can occur: the virus can be prevented from attaching itself to the animal’s cells, stopped from getting into the cells, or stopped from reproducing once inside the cells.
The salmon that had natural resistance were found to have minor differences in their DNA at a specific genomic location when compared to non-resistant fish. It has been shown that even in resistant fish the virus can enter and replicate in the cells of the salmon, and therefore the main difference between resistant and susceptible fish is in their response to infection with the virus.
It is likely that the genes linked to better survival enable the salmon fight off the disease more efficiently, and are much more resistant to re-infection.
How do we breed IPN resistant fish?
Specific parts of the salmon genome linked to resistance were identified, and used as genetic tests to select only resistant fish for mating. Offspring of fish selected in this way had effectively zero mortality. These tests were applied commercially, contributing millions to the UK economy, and to the economy of other salmon producing countries such as Chile and Norway.
Who will this benefit?
Using similar techniques, we are able to use breeding to improve resistance to other fish diseases too. Salmon farms sometimes face criticism for harbouring infection and transmitting disease back to wild stock. When salmon are able to fight infection, they pose less risk to the environment, and require fewer treatments. Less mortality also means more profit for local economies, and a better response to illness is better for fish welfare.