Institute for Fish and Wildlife Health

Nodular gill disease (NGD) in rainbow trout (Oncorhynchus mykiss) in Swiss fish farms

Nodular gill disease (NGD) is an infectious disease, causing proliferative gill lesions leading to respiratory prob-lems, oxygen deficiency and increased mortality. It globally impacts primarily freshwater salmonids in intensive aquaculture systems. In recent years, several outbreaks of severe gill disease have caused up to 50% mortality in farmed rainbow trout (Oncorhynchus mykiss) in Switzerland, mainly in spring and early summer months. This disease resulted in economic losses of more than 250’000 CHF per year.

The suspected disease-causing agents are freshwater amoeba. However, diagnostics is still challenging. His-tology reveals massive proliferation of the gill epithelium, often in a nodular manner at the filament tips. Dis-eased fish harbour different species of amoebae based on one-off case studies in New Zealand, Germany, Czech Republic, Canada and the USA (Tubbs et al. 2010; Dyková et al. 2010; Quaglio et al. 2016). However, taxonomic assessment of involved amoeba species is unreliable because different species often look extremely similar, partly because difficulties in culturing, and partly because of lack of molecular assays. Hence, no mor-phological or genetic information is available so far to accurately identify the amoeba species suspected to be involved in Swiss outbreaks.

The aim of this project was to identify the causative agent(s) of NGD and establish a reliable diagnostic and therapeutic measure.

Contact: Stefania Vannetti; Heike Schmidt-Posthaus; Ralph Knüsel

Kiemenproben sammeln
Taking gill samples
Makroskopische verdickte Kiemen bei einen Regenbogenforelle
Macroscopically thickened gill in a rainbow trout
Amoeben auf den Kiemenoberfläche an Mikroskop
Amoeba on the gill surface under the microscope