Studies On The Transmission Of Avnv (acute Virus Necrosis Virus) Via Marine Microalgae

Scallop (Chlamys farreri) is an economic shellfish in North China. However, the epizootics of a disease called Acute Viral Necrosis Disease (AVND) hampered the production seriously. According to former studies, Acute Viral Necrosis Virus (AVNV) is considered as its pathogen. Many technique of diagnosis of AVNV, including immunological and molecular, are founded, which are engaged in the study on the spread of AVNV via microalgae and the role of AVNV transmission.For researching whether microalgae could carry AVNV or not, this paper stained 17 species of normal microalgae culture with AVNV solution from ill scallops. Samples were picked out in a series of time, and tested by PCR, FQ-PCR. The results show that six in 7 species of microalgae could carry AVNV, they are Platymonas subcordiformis, Chlorella sp.,Tetraselmis tetrathele, Dunaliella viridis in Chlorophyta and Nitzschia sp. and Skeletonema costatum in Bacillariophyta. These 6 species of algae could carry AVNV at least 107 copies/L within 10 days. Meanwhile, a small amount of microalgae which was attached AVNV was used to stain AVNV-free algae. The result suggested that AVNV could not replicate in the cell of microalgae. The experiments about attachment of AVNV to microalgae indicated that the microalgae might be one possible horizontal transmission pathway of AVNV, but its role in the spread is vector rather than intermediate host.To understand the pathogenicity of microalgae-AVNV complex to scallops, we engaged 6 species of microalgae-AVNV mixture were in used to feed scallops compared with AVNV-free microalgae culture. Calculated the mortality and assayed the infection rate and the intensity quantitatively. The results of this experiment showed that, the death of scallops in Chlorella group and Tetraselmis group began at the second day after challenge, at the third day in Nitzschia group and Dunaliella group, and after the froth day in Platymonas group and Skeletonema group. After the forth day in Platymonas group, the sixth day in Skeletonema group and Dunaliella group, and the seventh day in other three groups, the experiment scallops got into a mass death period.During the experimental period of 9 days, the mortalities of Platymonas group, Tetraselmis group, Chlorella group, Dunaliella group, Nitzschia group and Skeletonema group were 76.47%,88.23%,86.27%,86.27%,78.43%å'Œ82.35%, respectively. All of these presented a highly significant difference (p<0.01) from the negative control group(17.65%).LAMP diagnosis result showes that the infection rate of Platymonas group is the highest,72.97%. The lowest one of infection rate is 55.56% tested in the Nitzschia group. All of the infection rate of scallops in the 6 group revealed the significant or highly significant differences from the control(15.38%). FQ-PCR protocol was explored in rechecking the AVNV intensities in the gill of experimenntal scallops quantitatively. The results showes that the amounts of copies per gram gill tissue were 7.54×106 and 1.82×106 respectively in the Dunaliella group and Nitzschia group, which have the significant differences (p<0.05) from the control (7.5×104 copies/g gill tissue).The differences of the AVNV intensities between the control group and the Platymonas group, Tetraselmis group, Chlorella group, Skeletonema group are all highly significant (p<0.01).The virus intensities of the four groups were 1.46×107, 4.04×106,4.42×106 and 2.72×106 copies/g gill tissue, respectively.Correlation analysis revealed that the relativity between the accumulated mortality and infection rate is significant (93.1%, p<0.01),but only 38.5% between the mortality and AVNV intensity and is 63.8% between the infection rate and virus intensity.The correlation coefficient of the AVNV loading on the microalgae-AVNV complex with the infection rate of scallops was 0.637 and was 0.721 between the AVNV loading on microalgae and the AVNV intensity of the scallops. But it is rare with the accumulate mortality (33.9%).This study suggests that, AVNV virus particles can attach to certain species of microalgae in a period of time. The scallops may be infected the virus by touch the micraoalgae-AVNV complex via feed or filter process. Then, they are to ill and finally to death. In sum, microalgae is an important factor in the pathway of horizontal transmission of AVNV.