Gourhannic, E. | bioRxiv Science

bioRxiv2026

Epidemic Risks associated with high-temperature replicating infectious hematopoietic necrosis virus (IHNV) field isolates: from molecular characterization to host pathogen interactions

Infectious hematopoietic necrosis virus (IHNV) is a WOAH-notifiable pathogen responsible for severe acute disease in salmonids at temperatures ranging from 4 to 14{degrees}. At higher water temperatures, the disease is generally associated with reduced mortality and lower viral replication. In the context of climate change, rising water temperatures could favor the emergence of thermotolerant strains while representing a major physiological and metabolic stress potentially altering fish's immune responses. This study aims to characterize the impact of increasing water temperatures on IHNV replication and host-pathogen interactions. Following pre-screening, the replication kinetics of the reference strain N61 and two field isolates RtNag0a and SA-22 were characterized in EPC (Epithelioma Papulosum Cyprini) and RTG-2 (Rainbow trout gonad) cell lines at 14, 20 and 22{degrees}. Virulence and host-pathogen interactions were subsequently investigated in vivo in zebrafish (Zf) larvae using complementary microinjection and bath immersion infection models at 21.5 and 23.5{degrees}. In vitro studies revealed distinct thermal replication profiles, distinguishing thermosensitive from thermotolerant isolates, although these phenotypes were influenced by the cellular environment. The zebrafish model enabled the characterization of virulence and antiviral host responses associated with thermotolerant IHNV isolates. Notably, waterborne infection proved effective, contrasting with previous reports describing this exposure route as unsuitable for IHNV studies. These findings expand the growing list of emerging European IHNV genotypes capable of replicating at elevated temperatures (>20{degrees}) and establish zebrafish larvae as a relevant model for virulence assessment of field isolates. Together, these results provide valuable insights into the potential impact of climate warming on IHNV epidemiology and support efforts to predict and anticipate future outbreaks under changing environmental conditions.

pathology↗

bioRxiv2026

Molecular characterization of thermo-adapted Novirhabdovirus variants and virulence in vivo in rainbow trout and zebrafish at different temperatures.

Controlling viral diseases in fish is a challenge for the sustainable development of aquaculture and improving the health of fish farms. Infectious hematopoietic necrosis virus (IHNV) is a major pathogen in aquaculture. It infects cold-water fish, but loses its infectivity at temperatures above 20{degrees}C. Since fish are poikilothermic animals, their body temperature fluctuates according to that of their environment. Faced with current climate change, it is crucial to understand how these pathogens will adapt to rising temperatures. Through a strategy simulating the impact of global warming, this virus has been progressively thermo-adapted to 25{degrees}C. The selected variant, IHNV25, replicates at 25{degrees}C while maintaining efficient replication at low temperatures, like its parental strain. Genome sequencing and a reverse genetics approach revealed that only 9 amino acid substitutions, distributed across 4 of the 6 viral genes, were essential for thermo-adaptation. Although IHNV25 was highly attenuated in its natural host, the rainbow trout, at any temperatures, it proved highly virulent when injected in zebrafish larvae at 20{degrees}C and 25{degrees}C, inducing severe symptoms identical to those observed in trout (hemorrhages, oedema, and exophthalmia). At both temperatures, mortality was preceded by a massive viral replication and a delayed and inefficient interferon and inflammatory response. Unlike the parental strain, IHNV25 was also capable of infecting larvae by bath immersion at 25{degrees}C, entering via the caudal fin, thus demonstrating its ability to cross the species barrier. The identification of viral markers of thermo-adaptation will therefore be of great interest for the epidemiological surveillance of field isolates.

microbiology↗