Aquaculture: new viruses affecting sea bass
The Fish Virology, Immunology and Ecotoxicology (VIMEP) Unit at ANSES’s Ploufragan-Plouzané-Niort Laboratory was contacted following an unexplained disease affecting juvenile fish (fingerlings) at a hatchery rearing sea bass (Dicentrarchus labrax). Although mortality remained low, the young fish developed pustules and distended bellies, which made them unfit for sale. After taking samples and carrying out genetic analyses, the laboratory discovered not one but two new viruses.
One of them belongs to the Flaviviridae family and has been named Dicentrarchus labrax pesti-like virus (DLPLV). The other, from the Astroviridae family, has been named Dicentrarchus labrax astro-like virus (DLALV).
“The discovery of new viruses in fish is quite common today, due to the development of metagenomic analyses that enable them to be identified”, explained Lénaïg Louboutin, a scientist in the VIMEP Unit and lead author of the study.
However, this is the first time that viruses from these two families have been discovered in sea bass. The discovery was published in the journal Veterinary Research in May 2026.
Understanding virus transmission
To gain a better understanding of how these viruses are transmitted and their pathogenicity, the scientists carried out a number of experiments. In particular, they placed fingerlings with clinical signs from the hatchery in contact with healthy fingerlings. This contact was either direct, with the fish kept in the same tank, or indirect, with the healthy fingerlings kept in a basket placed inside the tank, which allowed water but not fish to pass through.
Pustules were observed briefly in a quarter of the fingerlings placed in the basket. DLALV was detected in these fish, and may therefore be the cause of these symptoms. On the other hand, DLPLV was only detected in fingerlings in direct contact with fish carrying the virus, which seems to suggest that it is transmitted through close contact rather than via water. No mortality was observed.
Fingerlings were also experimentally infected either by immersion in solutions containing the viruses or by injection. These experiments did not reproduce the clinical signs observed at the hatchery. However, infection with the DLPLV virus via immersion led to a mortality rate of between 8 and 16%.
The results of the experiments cannot be used to draw any definitive conclusions about the level of risk posed by these viruses to the health of sea bass and the aquaculture sector. In particular, the differences observed between the hatchery and the experimental infections could be due to the fact that the clinical signs resulted from a combination of several factors.
Development of tests to identify new cases
The scientists developed tests to detect these new viruses by PCR, and to detect specific antibodies directed against DLPLV. Antibodies are produced by the immune system in response to exposure to a pathogen. Their presence indicates that an individual has been in contact with a virus, even if it is no longer present. These tests will enable new cases of contamination to be quickly identified in order to prevent the spread of these viruses.
Testing was able to identify the DLPLV virus at a second hatchery, unrelated to the first. The viruses were eliminated from the affected hatcheries by disinfecting the tanks, and no new cases have been reported since.
This discovery highlights the wide diversity of viruses affecting fish and the need for surveillance in order to identify unknown viruses and study the potential associated risk.