Detectives of the virus: the secret of brown trout dying discovered

Every summer in southern Germany, Austria and Switzerland, tons of brown trouts perish. An interdisciplinary team from the Technical University of Munich (TUM) has discovered the culprit of the mysterious endangered brown trout. It is triggered by a previously unknown virus, linked to a virus infecting salmon in the North Atlantic and Pacific.

The frame is still in the same sections of the river. And the victims are still brown trout. In a few days, their skin darkens and the animals die. Researchers and authorities have pursued the cause of this mysterious, mbadive death for decades – without success so far.

Using advanced badytical methods, Professor Ralph Kühn and a research team from the Technical University of Munich have now identified the causative agent of the syndrome. Proliferative darkening (PDS) in brown trout.

From the river to the lab

"The biggest challenge was to identify a pathogen that nobody knew.In the beginning, it was not even clear whether it was a bacterium, a virus, a parasite or an environmental toxin, "recalls the biologist.

Ten years of scientific detective work followed. The researchers set up two experimental stations on the Iller River – one upstream near Obersdorf, where fish disease had never been observed; a second in the lower reaches near Kempten, where brown trout die every summer.

At both stations, the researchers built aquariums fed with river water. From May to September, they observed the fish in the aquaria and took tissue samples that were frozen and sent to the TUM laboratory for badysis.

Looking for clues with next-generation technology

The observations indicated that the PDS was moving forward in three phases. Initially, the fish looks healthy. Then appear changes of internal organs such as liver and kidneys. In the third phase, the brown trout skin darkens and the animals die soon after. "The progression of the disease quickly led us to believe that PDS was a viral disease," Kühn said.

To detect the virus in the samples, the researchers deployed a set of modern molecular genetic procedures, called next-generation technologies. Genomes can be badyzed rigorously using these procedures.

First, the progression of the disease in fish has been characterized at the level of the genetic response. Subsequently, the researchers examined animals with a similar immune response to determine the nucleotide sequence of all inherited genetic material – fish and its possible pathogens.

The needle in a genetic haystack

The researchers deployed computer programs to identify the pathogenic virus in this mountain of genetic data. With the aid of in-depth bioinformatic treatment, they successfully identified parts of the pathogen's genetic profile and compared them to known virus profiles.

Extensive virus-specific sequencing has led to the determination of the viral genome. The result: the culprit responsible for the disappearance of brown trout is a pool reovirus. It is linked to the virus that infects salmon in the North Atlantic and the Pacific, where it causes significant economic damage.

In the next step, researchers hope to develop the virus in the laboratory and study why it occurs only in parts of alpine rivers and how far the global fish trade facilitates its spread.

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More information:
Ralph Kuehn et al., Identification of a Pool Reovirus Associated Pathogen in Brown Trout (Salmo trutta fario) Infected with Proliferative Darkening Syndrome (PDS), with the help of A new generation detection pipeline, PLOS ONE (2018). DOI: 10.1371 / journal.pone.0206164