“Ultrastructural Analysis of Amoebae and Gill Lesions in Nodular Gill Disease of Rainbow Trout (6973)”

Ultrastructural Analysis of Amoebae and Gill Lesions in Nodular Gill Disease of Rainbow Trout

In Swiss rainbow trout (Oncorhynchus mykiss) farms, Nodular Gill Disease (NGD) remains a major health challenge. In recent years, several studies have been conducted to better understand the disease. In 2023, six amoeba species were identified on the gills of NGD-affected rainbow trout, and a gill score (GS) system was developed. In 2024, Vannella mustalahtiana was detected in NGD-affected fish, with its presence increasing alongside the severity of gill lesions, suggesting a possible association with clinical NGD.

Macroscopy and histopathology of NGD-affected gills (poster presentation) was characterized by extensive proliferation of the gill epithelium, forming nodular structures at the filament tips. Affected fish experience respiratory distress, often resulting in mortality.

In previous studies, presence of amoebae was observed in native smears or using light microscopy.

The aims of this study were to (1) visualize the three-dimensional structure of involved amoebae using scanning electron microscopy (SEM), (2) examine internal organelles, and (3) characterize neighbouring gill cells using transmission electron microscopy (TEM).

Amoebae from ten different cultures, originating from six Swiss rainbow trout farms, were analyzed. The amoeba species present included Vannella sp., Ripella sp., Saccamoeba sp., and an unidentified amoeba. In addition, gill samples from eight healthy and fifteen NGD-affected rainbow trout were collected and preserved in isopropanol. Amoebae cultures were examined with SEM, while gill tissues were analyzed with TEM.

In SEM analysis, amoebae were often covered by filamentous bacteria and organic debris, complicating the visualization of their surface ultrastructure. TEM imaging successfully captured the ultrastructure of amoeba and rainbow trout gills, allowing comparisons between healthy and diseased tissues.

Preliminary observations suggest distinct ultrastructural differences between healthy and NGD-affected gills. Further analyses are ongoing to characterize amoebae at ultrastructural level.

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, Author
3. Kässmeyer, Sabine, Veterinary Anatomy, Vetsuisse faculty, University of Bern, Bern, Switzerland, Author
4. Gaschen, Véronique, Veterinary Anatomy, Vetsuisse faculty, University of Bern, Bern, Switzerland, Author
5. Llorda, Jaime, Microscopy Imaging Center, electron microscopy lab, Vetsuisse faculty, University of Bern, Bern, Switzerland, Author
6. Irobalieva, Rossitza, Veterinary Anatomy, Vetsuisse faculty, University of Bern, Bern, Switzerland, Author
7. Schmidt-Posthaus, Heike, Institute for Fish and Wildlife Health, Department of Infectious Diseases and Pathobiology, Vetsuisse Faculty, University Bern,, Author