Comparative virulence and evolution analysis of infectious hematopoietic necrosis virus (IHNV) isolated from cultured Oncorhynchus mykiss in three different regions of China.

During the period of 2021-2023, infectious hematopoietic necrosis virus (IHNV) was investigated on Oncorhynchus mykiss in the Shandong (SD), Chongqing (CQ) and Yunnan (YN) regions of China. Comparatively, there were more serious clinical signs in the diseased fishes of SD than these of YN and CQ. On the whole, tissue distribution showed that kidney and spleen have high virus loads in the diseased fishes of three regions. In SD, the golden trouts were more susceptible to IHNV than rainbow trouts. Ultrastructural analysis of YN sample showed that the bullet-shaped virions located in the intercellular space or around the nucleus of renal cells. In vivo and in vitro studies showed that IHNV-SD was more virulent than IHNV-CQ and -YN. According to the survey, IHNV-SD and -CQ were potentially derived from the purchased eyed eggs and parent fishes carrying with IHNV, while IHNV-YN were derived from the external factors, such as the cycling water, commercial fish and vehicle. In addition, a step-wise transmission model of SD-CQ-YN was inferred, and next further researched. Furthermore, the phylogenetic analysis showed that IHNV-CQ and -YN were clustered into a branch and close with -SD, and all belong to the J (JRt) genogroup.