“Epidemiological Monitoring of Infectious Hematopoietic Necrosis Virus in Rainbow Trout Fry and Broodstock in a Facility for 5 Years (7096)”

EPIDEMIOLOGICAL MONITORING OF INFECTIOUS HEMATOPOIETIC NECROSIS VIRUS IN RAINBOW TROUT FRY AND BROODSTOCK IN A FACILITY FOR 5 YEARS

Infectious hematopoietic necrosis (IHN) poses a significant threat to salmonid aquaculture operations worldwide. In Japan, IHN remains prevalent among fry and juvenile rainbow trout in aquaculture since its invasion into the country in 70’s and the causative virus, IHNV has evolved to occur in three unique genetic lineages: JRt Nagano (JN), Shizuoka (JS), and North Kanto (JnK) from the original U genogroup. Although information on virus dynamics in the fish production cycle is needed to understand how virus genetic diversity occurs, there is little data so far. This study focuses on epidemiological monitoring of IHNV in rainbow trout Oncorhynchus mykiss at the Nagano Prefectural Fisheries Experimental Station, which received supply water contaminated with the wastewater of two aquaculture facilities upstream. IHNV was isolated in EPC cells from the ovarian fluid of broodstock, and dead and surviving fry between 2019 and 2023 were sequenced for a part of the nucleocapsid (N: 542 nt) gene and glycoprotein (G: 558 nt) genes. The phylogenetic trees were constructed from both partial gene sequences (N+G: 1,100 nt) to investigate the relationships of the virus isolates from different samples year by year. Furthermore, as an experimental epidemiology, fry artificially infected with a known virus strain were raised for approximately 18 months, after which ovarian fluids were collected during their first spawning period to examine the isolated viruses. The results revealed two dominant lineages corresponding to JN and JnK, with distinct genetic clustering of isolates across the five-year period. The isolates from dead and surviving fry, and broodstock differed genetically from each other. Moreover, the same broodstocks produced ovarian fluid containing different isolates each year. During the spawning season, an experimental epidemiology with artificial infection showed no inoculated virus in the ovarian fluid of fish, which had been experimentally infected with the virus at the fry stage, whereas the isolated virus resembled those detected in other spawning fish in the same year. These results indicate that broodfish are re-infected with the emerged viruses, which are genetically different from those in fry, at the maturation period, suggesting that virus selection occurred at the broodfish stage, and consequently, virus evolution may mostly occur in persistently infected fish at the grow-up stage.

Keywords: Infectious hematopoietic necrosis, Rainbow trout, Molecular epidemiology, Genetic diversity

1. SU THIN, CHAW, Tokyo University of Marine Science and Technology, Tokyo, Japan, Presenter
2. NONAKA, AOI, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
3. TSUBOI, GOSUKE, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
4. HIRABE, SUGURU, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
5. SHIMIZU, RYOTA, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
6. SHIROTORI, FUMIAKI, Nagano Prefectural Fisheries Experimental Station, Nagano, Japan, Author
7. TAKEUCHI, TOMOHIRO, Nagano Prefectural Fisheries Experimental Station, Nagano, Japan, Author
8. TAKEHANA, KOTA, Nagano Prefectural Fisheries Experimental Station, Nagano, Japan, Author
9. FURIHATA, MITSURU, Nagano Prefectural Fisheries Experimental Station, Nagano, Japan, Author
10. TAKIKAWA, TOMOHITO, Fuji Trout Hatchery, Shizuoka Prefectural Research Institute of Fishery and Ocean, Shizuoka, Japan, Author
11. NAKAMURA, EISUKE, Fuji Trout Hatchery, Shizuoka Prefectural Research Institute of Fishery and Ocean, Shizuoka, Japan, Author
12. MATSUMOTO, MEGUMI, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
13. KATO, GOSHI, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
14. SANO, MOTOHIKO, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author