Piscine Orthoreovirus 3 Is Not the Causative Pathogen of Proliferative Darkening Syndrome (PDS) of Brown Trout (Salmo trutta fario)

Abstract

The proliferative darkening syndrome (PDS) is a lethal disease of brown trout (Salmo trutta fario) which occurs in several alpine Bavarian limestone rivers. Because mortality can reach 100%, PDS is a serious threat for affected fish populations. Recently, Kuehn and colleagues reported that a high throughput RNA sequencing approach identified a piscine orthoreovirus (PRV) as a causative agent of PDS. We investigated samples from PDS-affected fish obtained from two exposure experiments performed at the river Iller in 2008 and 2009. Using a RT-qPCR and a well-established next-generation RNA sequencing pipeline for pathogen detection, PRV-specific RNA was not detectable in PDS fish from 2009. In contrast, PRV RNA was readily detectable in several organs from diseased fish in 2008. However, similar virus loads were detectable in the control fish which were not exposed to Iller water and did not show any signs of the disease. Therefore, we conclude that PRV is not the causative agent of PDS of brown trout in the rhithral region of alpine Bavarian limestone rivers. The abovementioned study by Kuehn used only samples from the exposure experiment from 2008 and detected a subclinical PRV bystander infection. Work is ongoing to identify the causative agent of PDS.

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

“…Although a piscine orthoreovirus (PRV-3) was recently suggested as the causative agent of PDS [8], a follow-up study could not confirm a direct association between PRV-3 infection and PDS. The qPCR experiments verified viral nucleic acid not only in healthy control brown trout but also failed to detect it in some diseased animals with histologically confirmed PDS [9]. Thus, the potential impact of reovirus infections on the health status of brown trout populations remains unclear.…”

“…In the present study, we chose clinical and histological features to define the presence of PDS: Brown trout had to show skin darkening and combined hepatic necrosis and splenic lymphocytic depletion. Using this definition, four fish were identified showing the same distribution and quality of lesions when compared with the observations in an established controlled PDS-exposure experiment [9]. We are aware, that in our study only few brown trout met the criteria for PDS.…”

“…PDS leads to high mortalities in affected brown trout in the summer months [6]. Investigations to identify the cause of PDS have been ongoing for decades, however, up to this date the postulated causative pathogen remains elusive [9]. So far identification of the syndrome relies on histological examination of the affected brown trout.…”

“…The specific cause of PDS is unknown and potentially multifactorial. Piscine orthoreovirus 3 (PRV-3) has been recently suggested as the potential cause of PDS [8], but this hypothesis could not be confirmed and rather PRV-3 seems to cause a subclinical bystander infection [9]. Despite various investigations within the last decade, a causative agent has not been detected yet [6,8].…”

“…Despite various investigations within the last decade, a causative agent has not been detected yet [6,8]. Therefore, the diagnosis of PDS is solely based on epidemiological data (seasonal occurrence), macroscopic lesions (darkening of skin), and histological lesions (liver necrosis in combination with depletion of splenic white pulp) [9]. PKD has recently been suggested as a potential cofactor in PDS [10].…”