Epidemiological Studies On Abalone Shriveling Syndrome-associated Virus And Herpes-like Virus

The aquaculture industry of abalone in the southern coast of China has been severely affected by continuous disease outbreaks with high mortality from 1999 to 2002. The appearance of lethal diseases was first outbreak in Dongshan district, Fujiang province. Then it spreaded to Guangdong, Hainan and Taiwan in the next years, the abalone industry in southern China was close to collapse. Studies indicated that disease was caused by low-temperature viruses. The abalone shriveling syndrome-associated virus(Ab SV) and abalone Herpes-like Virus(Ab HV) were two of the suspected low-temperature viral pathogens. However, there are still many uncertainties about the exact identity of the pathogen and other features such as water temperature and host species preference. In this study, quantitative PCR was used to study the epidemiology of Ab SV and Ab HV. A total of 1082 abalone samples were collected monthly from April 2012 to October 2013 at seven sites along the coast of the South China Sea including Dongshan district in Fujiang province, Huilai, Huidong, Shantou and Shanwei in Guangdong Province, and Sanya district in Hainan Province. The data of April 2012 to January 2013 which was accomplished by Yangrui in our lab last year was used to make a more comprehensive analysis of the prevalence of Ab SV and Ab HV. Our results showed that Ab SV and Ab SV were still present in cultured abalone in China. Ab SV began in August or September, and it peaked in September or October. In other months, the infections were notably lower. Moreover, the Ab SV content was higher in the low latitudes such as Huidong, Shanwei in Guangdong province. Analysis of the relationship between the water temperature and viral abundance suggested that optimum water temperature of Ab SV is 25-29°C. The Ab HV began in September or October, and it peaked in November or December, which was one month later than the epidemic time of Ab SV. As the change of temperature, it suggested that there were two optimum water temperature ranges of Ab SV: 17-19℃and 26-29℃.However, we did not observe any effect of the culture method(artificial pond culture or open sea raft culture), shell length, or host species(H. diversicolor and H. discus) on the Ab SV load or Ab HV load.Ab SV is a new kind of virus found in aquaculture, whose morphology is still unknown. It is necessary to establish methods of the Ab SV isolation for the further morphological structure identification. It showed that Ab SV located in the position of the buoyant density of 1.35-1.42 g/ml by the cesium chloride gradient ultracentrifugation. 1% formaldehyde solution was added to prevent the virus to be depolymerized by the cesium chloride, which had a good protection and isolation of Ab SV. It was the first time to establish chromatography technology to isolate Ab SV. The result showed that the Ab SV was isolated when the electrical conductivity was 32-40mg/cm. The Ab SV sample isolated by chromatography was vacuum concentrated for further SDS-PAG analysis. It showed some different protein bands with the negative abalone sample(with no Ab SV).In the experiment of Ab SV and Ab HV infection, both Haliotis diversicolor and H. discus did not infected Ab SV by injection and soaking. While Haliotis diversicolor successfully infected Ab HV by the two kinds of artificial infection methods, and all died within 16 days.The death of Haliotis diversicolor with the Ab HV infection by injection began at the 4th day. At the 8th day, the Number of death reached the highest. Haliotis diversicolor infected Ab HV by soaking began to die at the 7th day and at the 11 th day, it reached death peak. The course of disease was longer with the Ab HV infection by soaking, indicated it had a certain latent period of the virus infection. Haliotis diversicolor infected Ab HV successfully when the water temperature was 19℃. the epidemiological survey of Ab HV showed that its optimum water temperature was 17-19℃.Both of them coincided with previous researches which indicated Ab HV as a low-temperature virus. In addition, H. discus was failure to be infected by Ab HV in this study, which suggested that H. discus had the ability of Ab HV resistance.