Mysterious Syndrome Causing High Mortality in Wild Brown Trout in Eastern Switzerland, Similar to Proliferative Darkening Syndrome – Pathology and Possible Causes

Introduction: In the Thur, a river situated in the Eastern part of Switzerland, massive mortalities have occurred regularly in brown trout since 2016. Dead fish were recorded in a period of a few weeks in July/August and with a sharp demarcation between affected and unaffected river stretches. The majority of affected animals were 1-year old. Materials and Methods: From June to November 2018, brown and rainbow trout were sampled from the Thur. For each species, five fish were sampled every 2 weeks. Parasitology, bacteriology, virology and histology were performed. The water temperature was measured regularly. Water samples were taken every 10 minutes and pooled for daily samples for a period of 6 months. Non-target screening, target screening and suspect screening were performed. Results: Macroscopically, clinically ill fish showed dark colouration and apathy. No consistent pathogenic agent was isolated. By histology, extensive liver necrosis, haemorrhage and severe lymphohistiocytic myocarditis were diagnosed exclusively in brown trout. The first lesions were found in mid-August, while the highest degree of alterations occurred from the end of August to October. Full recovery did not occur until final sampling at the end of November. As alterations were in agreement with those recently described for brown trout proliferative darkening syndrome (PDS) in Germany, severely affected specimens were analysed for piscine reovirus 1 and 3, agents proposed to be involved in PDS. None of the examined samples were found to be positive. Discussion: Up to now, the cause of the described syndrome remains unclear. Results from water analyses are still outstanding.