POSSIBLE VIRUS-LIKE ETIOLOGY CAUSING MASSIVE DEATH OF SCALLOP Chlamys
farreri IN NORTH CHINA
The scallop, Chlamys farreri, is one of the major species cultured in North China, and its culture in commercial scale has been performed more than 20 years. The highest production of about 1 million metric tons has been achieved in 1996, and valued 0.37 billion USD. The great expansion and intensification induced the occurrence of disease since 1990s, and became epizootics from 1997. In 1998, the farms in Shandong province lost 0.18 billion USD directly caused by the massive mortality of cultured scallop. It is believed that the disease so called Scallop Acute Viral Necrotic Disease(SAVND) has been becoming the major limiting factor in the development of the industry, and has stroke the economic process in the coast area of the region. The scallop with SAVND showed distinct gross signs and histopathological evidence proved the diseased condition. The lesions were found in gills, mantle, kidney, intestine and digestive gland, except adductor muscle and sexual gland. The typical infection was characterized as disordered and necrotic architecture, cellular disruption and disintegration often resulted in empty space amongst the multiple lesions. The epidermis of gill, intestine and mantle sloughed off the basal tissues was often observed. Shedding of the epidermal tissues resulted in the numeral reduction of tubules of the kidney and digestive gland. Cellular changes were nuclear hypertrophy, chromatin margination, pyknosis, karyorrhexis and lastly cellular disintegration. Other significant microorganisms were not observed in above-mentioned tissues. Electron microscopy examination of ultrathin sections revealed the presence of spherical virus-like particles in kidney, mantle, intestine and digestive gland of the moribund scallop collected during a severe mortality in Jiaozhou Bay in 2000. The virions were approximately 130 to 170 nm in diameter and had a bilaminal envelope, while the nucleocapsids were 90-140 nm. The virus occurred and distributed in the cytoplasmic vesicles of infected cell, and in scattered arrangement without occluding protein. The viral isolates were easily obtained, and consistent in size and morphology to those from ultrathin sections. The spikes of envelope were clearly observed with negative staining. Artificial infection was undertaken and the results showed that the injection and waterborne administration had high mortality of 75%, 60%, respectively. Meanwhile the control had 100% survival rate. It is clear that the virus might be the causative aetiology for the massive mortality, and corresponds to the outbreak
of the scallop in the region.
of the scallop in the region.