Occurrence of a lethal ranavirus in hybrid mandarin (Siniperca scherzeri × Siniperca chuatsi) in Guangdong, South China

A novel ranavirus with features similar to largemouth bass virus (LMBV) was isolated and then characterized from a natural mass mortality of adult hybrid mandarin (Siniperca scherzeri × Siniperca chuatsi). The isolated LMBV-like iridovirus was designated as mandarin ranavirus (MRV)-GD1301. The results of artificial infection showed that MRV-GD1301 was highly virulent to hybrid mandarin juveniles, and 100% mortality was observed within 5 days after infection via intraperitoneal injection. Moribund fishes typically have abnormally swollen abdomens with extremely severe ascites and exhibit exophthalmia. The characteristic clinical signs have been rarely recorded in other LMBV-associated fish diseases and other viral diseases in mandarin aquaculture. In contrast to the high lethality in hybrid mandarin, MRV-GD1301 showed avirulence to koi Cyprinus carpio, a susceptible fish species to LMBV-like koi ranavirus (KIRV) found recently in India. Our findings suggest that MRV is an emerging causative agent of mass mortality in mandarin species.