“Treatment Strategies for Nodular Gill Disease: Insights from Swiss Rainbow trout (Oncorhynchus mykiss) Aquaculture (6979)”

Treatment Strategies for Nodular Gill Disease: Insights from Swiss Rainbow trout (Oncorhynchus mykiss) Aquaculture

Nodular gill disease (NGD) is an infectious condition characterized by proliferative gill lesions, leading to respiratory distress, hypoxia, and increased mortality rates. It predominantly affects freshwater salmonids reared in intensive aquaculture systems worldwide. In Switzerland, recent NGD outbreaks have resulted in mortality rates of up to 50% in farmed rainbow trout (Oncorhynchus mykiss), particularly during spring and early summer. Despite persistent challenges, few studies have addressed therapeutic interventions for NGD.

In the present study, two treatment approaches were evaluated under field conditions: formalin and salt baths, each applied at varying concentrations and durations across two Swiss fish farms. Treatments were administered twice, four days apart. Upon the emergence of NGD symptoms, gill swabs for amoeba culture were collected at three time points: prior to, during, and after treatment. Fish were sedated with tricaine, gill scores (GS) were assigned, and gill swabs were obtained. Concurrently, tank-specific mortality rates were documented.

Amoebae were consistently detected in cultures, even in fish presenting a GS of zero, supporting the hypothesis of their ubiquitous nature. In symptomatic fish, GS values decreased following treatment, although amoeba cultures remained positive throughout. While treatments did not achieve complete eradication of amoebae, a marked improvement in fish health, as evidenced by reduced GS, was observed.

Analysis of treatment efficacy revealed that in some cases, a single treatment was sufficient to reduce mortality to zero, whereas in others, two treatments were necessary. Treatment outcomes were categorized into three groups: (1) effective after the first treatment, (2) partially effective after the first treatment, and (3) ineffective after the first but successful after the second treatment.

In conclusion, the gill score proved to be a more reliable diagnostic tool than amoeba culture results. Bath treatments employing salt (1.9%–2.5%) and formalin (200–220 mL/m³) demonstrated effective under field conditions in Swiss rainbow trout aquaculture. These findings may contribute to the development of improved management strategies against NGD in freshwater aquaculture systems.

1. Vannetti, Stefania, Institute for Fish and Wildlife Health, Department of Infectious Diseases and Pathobiology, Vetsuisse Faculty, University Bern,, Presenter
2. Knüsel, Ralph, fishdoc GmbH, Rain, Switzerland, Author
3. Fankhauser, Benjamin, Institut für Informatik, Pattern Recognition Group, University of Bern, Bern, Switzerland, Author
4. Schmidt-Posthaus, Heike, Institute for Fish and Wildlife Health, Department of Infectious Diseases and Pathobiology, Vetsuisse Faculty, University Bern,, Author