“Deciphering mechanism of petechial skin rash in Gilthead seabream and European seabass (7148)”

Deciphering mechanism of petechial skin rash in Gilthead seabream and European seabass

Introduction Petechial skin lesions frequently affect gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) during seasonal transitions in the Mediterranean, likely due to environmental stressors such as temperature and water quality fluctuations. These superficial haemorrhagic lesions impact skin and fins, causing economic losses in aquaculture and raise welfare issues. This study aimed to investigate the potential aetiology of the condition and explore preventive measures.

Methods We sampled skin and internal organs from 56 seabream and 20 seabass, both symptomatic and asymptomatic, during outbreaks in November 2023 and January/February 2024. Environmental samples were also collected during the second outbreak. Samples were preserved using various fixatives (formalin, RNAlater, glutaraldehyde, ethanol) and transported to Switzerland for analysis. Bacterial cultures were performed on lesions and swabs. DNA and RNA were extracted from RNAlater- and ethanol-fixed samples for sequencing. Glutaraldehyde-fixed samples were processed for electron microscopy.

Results Lesions showed syncytial formations and occasional lymphocytic vasculitis. Scale resorption appeared secondary to compromised blood supply. Pancreatic histology revealed lymphoplasmacytic vasculitis and tissue degeneration. Transmission electron microscopy (TEM) indicated tissue damage without viral inclusions but with probable bacterial structures. Syncytia appeared to result from epithelial cell fusion. Most bacterial growth from the first outbreak was considered contamination, though Chryseobacterium spp. was consistently isolated. In the second outbreak, 35 bacterial strains were identified, including known fish pathogens such as Tenacibaculum spp., Flavobacterium sp., Vibrio spp., and Shewanella sp. Analyses are ongoing, as metagenomic sequencing was initially limited by high host DNA contamination. Alternative approaches, including 16S rRNA gene amplicon sequencing and RNA-based methods, are now being used.

Conclusions Petechial skin lesions in seabream and seabass during seasonal transitions likely have a multifactorial origin involving stress and opportunistic bacteria. Histological and ultrastructural analyses revealed tissue degeneration and syncytial formations. While various bacteria were cultured, their role remains uncertain. Metagenomic approaches are expected to provide new insights and help validate or refute the relevance of these findings.

1. RUETTEN, MAJA, PATHOVET AG, Presenter
2. Qi, Weihong, FGCZ, Author
3. Cascarano, Maria Chiara, HCMR, Author
4. Katharios, Pantelis, HCMR, Author
5. Mateos Melero, José Maria, ZMB, Author
6. Hotz, Adriana, FGCZ, Author
7. Thiersch, Markus, PATHOVET AG, Author