“Italian IHNV low and highly pathogenic to rainbow trout displays different infection strategies (7178)”

Italian IHNV low and highly pathogenic to rainbow trout displays different infection strategies

Introduction

In recent years, Infectious Hematopoietic Necrosis has been displaying an increase in its distribution and incidence in Europe. In Italy, IHNV is characterized by a fast evolutionary rate and displacement phenomena have been observed with highly pathogenic IHNV strain replacing low pathogenic ones in some areas. The replacements resulted in severe and persistent IHN outbreaks with relevant impact on rainbow trout productions.

Methodology

A low pathogenic IHNV and a highly pathogenic IHNV were characterized through in vitro and in vivo assays to investigate the mechanisms at the base of the different virulence profile. Viral replication steps on RTG-2 cells were compared through both quantitative RT-qPCR and end-point titration methods. In vivo challenges were performed to identify the ID50 and the LD50 of both IHNV isolates. In addition, disease progression was investigated at histological level coupled with immunohistochemistry assays.

Results

The highly pathogenic IHNV isolate displayed a lower adsorption efficiency coupled with a higher total viral production compared to the low pathogenic IHNV isolate. The low pathogenic isolate showed a lower ID50 compared to the highly pathogenic one but conversely it had been not possible to identify the LD50 due to low mortality level observed. At histology, low pathogenic isolate induced mild lesions in target organs analysed at earlier timepoints but the same were not present in later ones. Conversely, in fish exposed to the highly pathogenic isolate severe histological lesions appeared later but were persistent. A similar behaviour was observed through immunohistochemistry.

Conclusions

Differences in the virulence profile of the tested IHNV isolates corresponded to different infection strategies. The highly pathogenic isolate induced slower and long lasting infection with severe histological lesions that could explain the virulence observed. Conversely, the low pathogenic isolate induced a quick and transient infection with a mild effect on the target organs. The characterization of more IHNV isolates and the investigation of the host immune response to infection will be required to shed light on the mechanisms responsible for the different behaviour observed.

Funded by the Italian Ministry of Health project IZSVe RC 13/19.

1. MARSELLA, ANDREA, IZSVE, Presenter
2. PASCOLI, FRANCESCO, IZSVE, Author
3. LUCON XICCATO, ROMY, IZSVE, Author
4. ABBADI, MIRIAM, IZSVE, Author
5. BURATIN, ALESSANDRA, IZSVE, Author
6. BIASINI, LORENA, IZSVE, Author
7. BERTO, PAOLA, IZSVE, Author
8. PRETTO, TOBIA, IZSVE, Author
9. TOSON, MARICA, IZSVE, Author
10. TOFFAN, ANNA, IZSVE, Author