| The red swamp crayfish Procambarus clarkii,originated in the southern United States and Northern Mexico,was introduced into China from Japan in the 1980s.In the past,the species deemed as enemy because of its destructive effect on dams and crops.In recent years,it has become widely distributed,highly consumed in China due to its high nutritional value and unique flavor.High economic value of the species makes its industry developed very fast in China.In 2016,the production of this crayfish ranked first among the economic cultured crustaceans in China.With the increase of farming area,the disease of crayfish occurred frequently,which seriously restricted the development of crayfish farming industry.Since 2006,there has been a peak of morbidity and mortality in cultured crayfish around May every year,and the morbidity time is constantly ahead of schedule.Farmers call it "May Plague",also known as"Black May" and "May Magic Curse".It is generally believed that the pathogen of"May Plague" is White Spot Syndrome Virus(WSSV).At the end of April and the beginning of May,2018,a large number of Procambarus clarkii died in the pond culture area of Xinglong Township,Xuyi County.A survey of the local crayfish culture area showed that the dead crayfish mainly distributed in shallow water areas such as the edge of the pond.The low vitality and difficult turning crayfish generally appeared hepatopancreas whitening,hydrocephalus,duodenum and other phenomena.The results of samples collected in first day from infected crayfish farm showed that,three of the twelve liver samples of crayfish were positive for white spot syndrome virus,and another virus was 100%positive in the liver samples.Genomic sequence analysis showed that the new virus belonged to the family Dicistroviridae and named it as PcDV(Procambarus clarkia dicistro-like virus).On May 8,2018,we arrived at Xinglong Township,Xuyi City for the second time.In order to ensure tissue viability,we took samples from pool and brought them back to the laboratory for testing.The results of common PCR showed that 100%of the 12 hepatopancreas samples collected at pool were positive for PcDV and WSSV.The electron microscopic results showed that there were a large number of spherical particles with a diameter of 30 nm in the gill tissue of the infected crayfish,which was consistent with the appearance of PcDV,also large number of enveloped virus with 300nm which is consistence with WSSV.From October 2018 to February 2019,an epidemiological survey was conducted on the pond-cultured crayfish in three major crayfish culture provinces of Jiangsu,Anhui and Hubei.The results showed that 15.60%of crayfish carried PcDV,20.83%,4.17%and 0.00%in October 2018,25%,16.67%and 33.33%in November 2018,4.17%,8.33%in December 2018,4.17%and 4.17%in Jiangsu,Anhui and Hubei,respectively.The carrying rates of PcDV were 29.17%,0.00%and 20.83%in January,Jiangsu,Anhui and Hubei respectively.In February,2019,the carrying rates of PcDV were 20.83%,10.00%and 16.67%in Jiangsu,Anhui and Hubei,respectively.It was found that the relative expression of PcDV was low and there was no obvious change rule.The disease of Crayfish usually occurred in May,and the water temperature was about 25 C.The occurrence of this phenomenon was compared with that in 2001.The lower temperature from October 8 to February 2019 is related to some extent.We believe that the discovery of this new novel Procambarus clarkii dicistro-like virus provide a new perspective and research direction for the study of viral pathogens of "Black May" in crayfish,and provide a reference for the prevention and control of crayfish diseases and the healthy development of crayfish farming industry. |
