Preliminary Study on Massive Mortality of Hatchery-Reared Half-Smooth Tongue Sole, Cynoglossus semilaevis, Associated with Viral Nervous Necrosis

Abstract:

In the years of 2012 and 2013, there was an outbreak of disease and massive death of half-smooth tongue sole (Cynoglossus semilaevis) 15–20 days post-hatching (dph 15–20) in a hatchery of northern China, and the cumulative mortality reached 90%–100% within one week. The outbreak occurred in July and August when the water temperature was 22–24℃. The symptoms included erratic swimming behaviors such as spiraling movement and fast swimming in circles, without hemorrhaging and ulceration on body surfaces. The histopathological examination revealed typical signs of viral nervous necrosis. The nerve cells of brain and retina underwent severe vacuolation and necrosis. RT-PCR with primers of nervous necrosis virus showed positive results in all samples from moribund half-smooth tongue soles. The RT-PCR products were then sequenced and the sequence alignment was carried out with BLAST. It was found that the sequence similarity was above 98% between the nervous necrosis virus in half-smooth tongue sole (CsCN NNV) and five strains of red-spotted grouper nervous necrosis virus (RGNNV). In contrast, the similarities between CsCN NNV and other three genotype strains of NNV: Striped jack nervous necrosis virus (SJNNV), Barfin flounder nervous necrosis virus (BFNNV), and Tiger puffer nervous necrosis virus (TPNNV), were only 71%–78%. Therefore, we inferred that CsCN NNV was a new strain with RGNNV genotype. It was the first case of naturally occurred RGNNV infection of half-smooth tongue sole. Our findings provided insights into the epidemic mechanism of RGNNV infection as well as the prevention of viral nervous necrosis in half-smooth tongue sole.