First report of Cyprinid herpesvirus 2 outbreak in cultured gibel carp, Carassius auratus gibelio at low temperature

Abstract

AbstractCyprinid herpesvirus 2 (CyHV‐2) is the causative agent of herpesviral hematopoietic necrosis (HVHN), an infectious disease with high morbidity and mortality worldwide. Current research indicates that CyHV‐2 only infects fish in the genus Carassius. Outbreaks of fish viral diseases are highly temperature‐dependent, and HVHN outbreaks usually occur at water temperatures of 15–25°C. In November 2018, an infectious disease occurred on cultured gibel carp farms in ZiGong, Sichuan Province, China. The water temperature at the time was approximately 10°C. The clinical signs of the diseased fish included lethargy, inappetence, severe anoxia, hemorrhage on the body surface, and pale gills. Clinical and histopathological examinations, transmission electron microscopy (TEM), and molecular analyses were used to determine the etiology of this disease. Pathological observation revealed severe necrosis of the hematopoietic organs, particularly the gills, kidney, and spleen. Spherical herpesvirus‐like virion particles were observed with TEM in the kidney and spleen tissues from moribund fish. An experimental infection demonstrated a cumulative mortality rate of 50% (10/20) at a water temperature of 13°C, whereas the mortality of the positive control group was 75% (15/20) at a water temperature of 23°C. A specific fragment of the helicase gene of CyHV‐2 was detected with polymerase chain reaction. In a phylogenetic analysis of the helicase gene, our isolate clustered with CyHV‐2 isolates in the database. Together, these results show that the putative pathogen involved in this outbreak was CyHV‐2. To the best of our knowledge, this is the first report of a gibel carp disease caused by CyHV‐2 at low temperature. These findings are extremely important in reducing the risk of HVHN outbreaks in gibel carp aquaculture.