Although Yersiniosis is a freshwater bacterium that thrives best at relatively high temperatures, fish farmers experience outbreaks of the disease in salmon after it has been released into the sea. Photo: Linn Røkenes / Norwegian Seafood Council.
The disease, also known as enteric redmouth disease, can lead to significant losses with a mortality rate of up to 30 percent.
Now, researchers and fish farmers are working together to identify the key risk factors in the production chain.
"By using environmental DNA, we can get a picture of the infection pressure in the water. This allows us to monitor the entire group of fish instead of testing individual fish," says Hans Arne Berg Frantzen.
Frantzen is employed by ACD Pharma and a master’s student at Nord University’s Faculty of Biosciences and Aquaculture. He will help fish farmers tackle the persistent bacterium Yersinia ruckeri, a growing problem for Norwegian salmon farmers.
"2023 was a peak year for yersiniosis, with the highest number of outbreaks since 2012. All indications point to 2024 being even worse," says Frantzen.
Stress and environmental changes
Although Y. ruckeri is a freshwater bacterium that thrives in relatively high temperatures, farmers have experienced that the disease can still break out in salmon after they are released into the sea.
One reason may be that the bacteria remain latent in the fish from the smolt phase and emerge when the fish experiences stress and environmental changes.
"The problem typically arises when the fish is transferred from freshwater to the sea. The fish becomes stressed, and those carrying the bacteria may develop the disease, especially if this happens during the summer," explains associate professor Hetron M. Munang'andu at Nord University.
Another critical point is during the delousing process. The Norwegian Veterinary Institute has recently discovered that salmon populations with latent infections of Y. ruckeri can release large amounts of bacteria during delousing treatment in warm water, so called thermal delousing.
The bacteria become concentrated in the treatment chamber, while the fish are subjected to both stress and physical damage.
"Delousing involves significant risk, but much depends on what has happened before the delousing," says Frantzen.
He highlights how the fish are exposed to several risk factors before entering the sea, such as sorting, transport, vaccination, and vehicle transport. Biofilters in the hatchery can also pose a risk as they can harbor bacteria released by infected fish.
"The disease risk can be managed with various measures, and we are exploring different approaches. So far, environmental DNA looks promising. The next question is how we handle it in real life," says Frantzen.
Testing vaccinated fish
The laboratory company PatoGen is a partner in the project. Sales manager Stian Lernes says they are pleased to contribute to what they consider important research.
“Environmental DNA is a method where one finds genetic material that organisms shed into the environment through urine, feces, and skin cells. By collecting samples from the surroundings, which in this case is water, and concentrating the material using filters, one can analyze the DNA to monitor the presence of specific organisms such as bacteria”, Lernes explains.
Environmental DNA involves monitoring infection pressure by detecting DNA in the water where the fish live. Microorganisms such as bacteria can be detected through water samples without stressing or sacrificing the fish.
"We need to improve biosecurity, and we need to improve fast. We are dealing with increasing salmon lice, Today, half of all injection vaccines are against Yersinia, a clear indication that it is becoming an increasing problem. One thing I want to test is whether vaccinated fish continue to shed bacteria when stressed. This could be significant if the farmer groups fish from different smolt batches, such as one group vaccinated and another not," says Frantzen.
Non-lethal diagnostic method
His work will be the first study to use environmental DNA to identify critical infection points, which is necessary for managing salmon diseases in the aquaculture industry.
"Using environmental DNA to monitor diseases in aquaculture is a completely new approach," says Frantzen, who also highlights vaccination and the use of bacteriophages (host-specific viruses that attack bacteria) as important weapons against the disease.
The project is a collaboration between Nord University and biotech companies ACD Pharma and Patogen. Øyvind Kileng, a senior researcher at ACD Pharma, says the project is important for expanding knowledge about biosecurity and disease transmission in the aquaculture industry.
“Yersinia ruckeri is a disease that tends to fly a bit under the radar for most, but it is a disease that reduces fish welfare and increases mortality in farmed salmon. It is also a good example of how collaboration between Nord University and the industry helps to enhance the expertise of both parties. A nice bonus of the project is that we are validating a time saving, non-lethal diagnostic method”, says Kileng.
The researchers are pleased that as many as 20 hatcheries are participating in the project.
"We at Nord University are very happy to be able to contribute to solving problems in the industry, both through education and by adopting new technology," says Munang'Andu.
