Proliferative Kidney Disease and Proliferative Darkening Syndrome are Linked with Brown Trout (Salmo trutta fario) Mortalities in the Pre-Alpine Isar River

Abstract

For many years, brown trout (Salmo trutta fario) mortalities within the pre-alpine Isar River in Germany were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers during August and September. Moribund fish seemed to be affected by proliferative darkening syndrome (PDS). In addition, proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae was discussed. To investigate this phenomenon, the present field study monitored brown trout mortalities by daily river inspection in 2017 and 2018. Moribund brown trout (n = 31) were collected and examined using histology, immunohistochemistry, qPCR, and quantitative stereology. Our investigations identified 29 (93.5%) brown trout affected by PKD. Four brown trout (12.9%) displayed combined hepatic and splenic lesions fitting the pathology of PDS. The piscine orthoreovirus 3, suspected as causative agent of PDS, was not detectable in any of the samples. Quantitative stereological analysis of the kidneys revealed a significant increase of the renal tissue volumes with interstitial inflammation and hematopoietic hyperplasia in PKD-affected fish as compared to healthy brown trout. The identified T. bryosalmonae strain was classified as part of the North American clade by phylogenetical analysis. This study highlights PKD and PDS as contributing factors to recurrent autumnal brown trout mortalities.

Health Surveillance of Wild Brown Trout (Salmo trutta fario) in the Czech Republic Revealed a Coexistence of Proliferative Kidney Disease and Piscine Orthoreovirus-3 Infection

“…This study shows a widespread presence of the parasite in Czech rivers because at least some of the fish from all eight locations were tested positive for the presence of T. bryosalmonae in the kidney tissue during ten separate sampling events. This prevalence was confirmed by both diagnostic methods (IHC and qPCR) used, with the immunohistochemistry method showing significantly lower percentages of positive fish from each location compared with the results obtained by real-time PCR, which is a finding consistent with other published studies employing both methods of parasite detection [ 24 , 40 ]. This finding puts previous studies relying on histopathology or IHC for the confirmation of T. bryosalmonae infection in perspective and suggests that the percentages of positive samples, significant as they are, could be even higher when utilizing molecular techniques.…”

“…It is, however, possible that the cases presented in this publication did not manifest clinical signs of infection due to the season, when the water temperature was not providing the best conditions for PDS development [ 21 ] or the environment of the Czech rivers was not suitable for PDS development. Further research of the possible relationship between PRV-3 and PDS is required [ 23 , 24 ].…”

“…The first cases of proliferative kidney disease in Czech aquaculture were reported during the 1980s [ 39 ], but no targeted surveillance of the distribution of this parasite in Czech wild salmonids has ever been conducted, despite the growing evidence of adverse effects of the infection on the brown trout populations in neighboring countries [ 24 , 25 ]. This study shows a widespread presence of the parasite in Czech rivers because at least some of the fish from all eight locations were tested positive for the presence of T. bryosalmonae in the kidney tissue during ten separate sampling events.…”

“…PDS manifests itself with pathoanatomical signs such as skin darkening, hemorrhages and necrosis of liver, spleen and kidney resulting in massive species-specific die-offs of brown trout during the late summer months [ 22 ]. On the contrary, the presence of PRV-3 was described in clinically healthy brown trout [ 23 ], and brown trout showing PDS signs also tested PRV-3-negative in other cases [ 24 ]. Arndt et al [ 24 ] connected the mortalities of brown trout in Austrian rivers with proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae .…”

“…On the contrary, the presence of PRV-3 was described in clinically healthy brown trout [ 23 ], and brown trout showing PDS signs also tested PRV-3-negative in other cases [ 24 ]. Arndt et al [ 24 ] connected the mortalities of brown trout in Austrian rivers with proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae . Additionally, Lewisch et al [ 25 ] confirmed the presence of the parasite in wild fish from the majority of examined Austrian rivers, along with the clinical and pathoanatomical signs typical for PKD, such as darkened skin, abdominal distention and enlargement of the kidney and spleen [ 26 ].…”