MYSTERIOUS SYNDROME CAUSING HIGH MORTALITY IN WILD BROWN TROUT IN EASTERN SWITZERLAND, SIMILAR TO PROLIFERATIVE DARKENING SYNDROME, PATHOLOGY AND POSSIBLE CAUSES

Schmidt-Posthaus, Heike; Birrer, Christoph; Mehr, Christoph; Leib, Vera; Kugler, Michael (2020). MYSTERIOUS SYNDROME CAUSING HIGH MORTALITY IN WILD BROWN TROUT IN EASTERN SWITZERLAND, SIMILAR TO PROLIFERATIVE DARKENING SYNDROME, PATHOLOGY AND POSSIBLE CAUSES. Journal of comparative pathology, 174, p. 158. Elsevier

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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.

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