Multiple rivers in Trøndelag, Norway see simultaneous outbreaks of ulcerative dermal necrosis and saprolegniasis in wild salmon, spread to new river reaches

An investigation by the Norwegian Veterinary Institute (Veterinærinstituttet) into diseased wild salmonids in several rivers in the Trøndelag region shows a complex composite disease picture in fish health. The institute had previously issued a warning to the Norwegian Food Safety Authority (Mattilsynet) about findings consistent with ulcerative dermal necrosis (UDN) in four rivers: Orkla, Gaula, Verdalselva and Mossa; on Wednesday, November 4, the Stjørdalselva river was also brought within the scope of the UDN warning.

Veterinary Institute researcher Åse Helen Garseth said that ulcerative dermal necrosis (UDN) and saprolegniasis caused by Saprolegnia parasitica remain the most prominent findings, while a composite disease picture has been observed and reports pointing to significant impacts at the population level have been received, making the situation very serious.

In addition to UDN and saprolegniasis, the investigation also found bacterial septicaemia (blood poisoning), multiple parasitic infections and soft-tissue calcification. Some conditions were already present before the development of UDN and saprolegniasis, while others appeared as secondary infections or secondary symptoms following skin damage, impaired barrier function and a decline in overall condition.

The most common bacterial findings include motile Aeromonas spp., Lactococcus spp., Pseudomonas spp., Yersinia ruckeri and Carnobacterium maltaromaticum. The significance of these findings and their role in the disease complex are still under further investigation. Among parasitic infections, blood anaemia caused by nematodes (Anisakis spp., commonly known as "kveis") is relatively common, and a variety of other parasites are also present, causing tissue damage and health effects of varying severity.

Overall, the investigation results show a significant disease burden at both the individual and population levels. Given that disease arises from interactions between host, infectious agent and environment, and that climate change is altering the rules of this interaction, Garseth expressed concern about the continuation of the situation.

The Veterinary Institute is currently following up reports from several new rivers both inside and outside the Trondheimsfjord. In recent weeks, population assessments have also been completed in several new watercourses. The disease outbreaks have led to substantial fish losses and may have serious long-term consequences for wild Atlantic salmon and sea trout populations.

Knowledge about UDN is largely based on investigations from decades ago, when environmental conditions differed from those of today. At the same time, earlier studies indicated that the disease may be contagious and may appear during the seawater phase. However, there is currently no information indicating that farmed fish are affected. Garseth said UDN is regarded as a problem of wild fish, but still called for increased vigilance at seawater aquaculture facilities and freshwater facilities that use water sources with ascending wild salmon and sea trout.