Sea trout can pass deadly parasite to farmed salmon
Sea trout can transmit a deadly parasite to farmed salmon but do not get sick themselves, new research in Norway shows.
“Sea trout can be infected with the parasite Spironucleus salmonicida, but do not become ill. The trout can still transmit the parasite to salmon, where it causes illness and death. The trout themselves get rid of the parasite quite quickly,” researcher Bjørn Olav Kvamme says about the findings in a new report from Norway’s Institute of Marine Research.
The background to the research is a large outbreak of the parasitic disease in fish farms in Finnmark in 2022. Outbreaks of spironucleosis do not occur often, but when they do, the consequences are major in the form of suffering for the salmon and financial losses for the farmers.
The costs of the 2022 outbreak are estimated at around NOK 1 billion (£73 million at today’s exchange rate).
Spreads inside the fish
The spiro parasite is harmless to humans, but salmon can become seriously ill. The parasite often invades the fish’s intestines, and if it gets from there into the blood, the path to systemic spironucleosis is short.
“From the blood, the parasite can move on to any organ; it has been found in the eye, the gallbladder, the heart and the brain. It creates abscesses and lesions in both internal organs and muscles, and there is currently no treatment for the disease,” says Kvamme.
Little known about infection routes
It is not known how the parasite is transmitted from fish to fish. It is spread freely through the water, but other routes of infection are also possible.
“The parasite is found naturally in some wild fish species, and has been detected in wild trout and rudd, among others. Therefore, it has been important to find out more about the risk that these species can be intermediate hosts in the spread of infection. We are also concerned about the spread to other species,” says Kvamme.
Trial method
First, both salmon and trout were bathed together with parasites that had been cultured from the outbreak in Finnmark.
Then, salmon and trout from the infection bath were placed in new tanks together with salmon and trout that had not been exposed to parasites, respectively. The researchers took blood samples from the fish after four and nine weeks, and when the forced coexistence ended after 17 weeks, the fish were euthanised and autopsied.
Trout got rid of the parasite
After four weeks, between 70% and 90% of the salmon from the infection pool had parasites in their blood.
“In comparison, very few of the trout were infected, and none of them became ill. After 17 weeks, none of the trout from the infection pool had the parasite anymore. This indicates that spironucleosis is not a threat to this species, and that it is quite resistant,” says Kvamme.
After 17 weeks, however, several of the salmon had both boils and lesions on various organs.
“The experiment also showed that trout both acquire parasites from salmon and themselves transfer parasites to salmon that are not previously infected. This means that trout can function as carriers of the infection in the sea,” explains the researcher.
‘More research is needed’
Spironucleosis was first detected in Norway in 1989, and since then there have been several outbreaks, but with quite a long time between them.
“It is still important that we do more research on the parasite. It is about having a basic knowledge preparedness in place for the next time it appears, because we expect it to do so at irregular intervals. In any case, we should get a better overview of where it is found and how it is transmitted,” says Kvamme.
The study is part of the SpiroFri project, which is funded by the Fisheries and Aquaculture Industry Research Fund (FHF).
Partners in the project are Nofima, the Norwegian Veterinary Institute, PSK v/Erik Sterud, Grieg Seafood ASA, Uppsala University and HI.
