Photo: Institute of Marine Research Havforskningsinstituttet (HI)

Sea trout can transmit dangerous parasite to farmed salmon

NORWAY
Wednesday, December 17, 2025, 02:00 (GMT + 9)

Significant Risk Identified Following 2022 Outbreak Estimated to Cost One Billion Kroner ($98.3 million US$).

Bergen – New research from the Institute of Marine Research (HI) confirms that wild sea trout can transmit a dangerous parasite, Spironucleus salmonicida, to farmed salmon, where it causes the fatal disease spironucleosis. While the trout themselves do not become ill and quickly clear the infection, they can function as carriers, posing a significant risk to the aquaculture industry.

Sea Trout as Silent Carriers

The main findings, summarized by researcher Bjørn Olav Kvamme from HI, stem from a study initiated following a major outbreak of the parasitic disease in fish farms in Finnmark in 2022. This outbreak highlights the severity of the issue, with estimated financial losses reaching around one billion kroner ($98.3 million US$).

“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,” explained Kvamme. The research demonstrated that trout acquire the parasite from salmon and subsequently transfer it to previously uninfected salmon, indicating their role as potential carriers in the marine environment.

The Path of the Parasite

Though harmless to humans, the spiro parasite can cause severe systemic spironucleosis in salmon. It initially invades the fish's intestines and can then enter the bloodstream, allowing it to spread throughout the body.

“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,” Kvamme stated.

Experimental Confirmation and Next Steps

In the study, both salmon and trout were exposed to parasites cultured from the Finnmark outbreak. After four weeks, 70 to 90 percent of the exposed salmon had parasites in their blood, and several showed boils and lesions after 17 weeks. In contrast, very few of the exposed trout were infected, and none became ill. After 17 weeks, all trout had cleared the parasite, confirming their resistance.

Lesions seen in salmon at the final sampling week 17. Either bath-infected salmon or cohabitants with them. A , abscess in dorsal fin, with two continuations along pterygiophores down into the musculature (cohabitant vessel 4). B , lump at base of tail, associated with abscess under skin (small image, where tail is cut off and folded) (cohabitant, vessel 5)).

Despite spironucleosis first being detected in Norway in 1989, and outbreaks occurring irregularly since then, the exact routes of infection remain poorly understood. The parasite is known to spread freely through the water and has been detected in other wild fish species like rudd.

C , lesions inside liver, barely visible as light areas, which on sectioning were found to contain pus (small image)(bath-infected fish, vessel 3), D , more extensive lesions in liver, anterior part (bath-infected fish, vessel 4).

The research is part of the SpiroFri project, funded by the Fisheries and Aquaculture Industry Research Fund (FHF), with partners including Nofima, the Norwegian Veterinary Institute, PSK v/Erik Sterud, Grieg Seafood ASA, Uppsala University, and HI.

E , heavily affected fish, extensive necrotic lesions in liver and behind swim bladder, swollen spleen and bloody ascites (bath-infected fish, vessel 5). F , lesions on surface of kidney, behind swim bladder (bath-infected fish, vessel 3).

“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... we should get a better overview of where it is found and how it is transmitted,” concluded Kvamme.

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