Pathogenicity of a virus infection causing white spot syndrome in cultured penaeid shrimp in Taiwan

• Main Authors: Huang, Chang-Yi, 黃長義
• Other Authors: Chou Hsin-Yiu
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/75919501802173288027

碩士 === 國立海洋大學 === 水產養殖學系 === 84 === Outbreaks of a disease causing serious losses among population of culturedPenaeus japonicus in Taiwan are characterized by obvious white spots on the carapace, appendages and the iside surface of the body.The diseased shrimp alsodisplay signs of lethargy and reddish coloration of the hepatopancreas.For thisreason,Chou et al.(1995) and Wang et al.(1995)propose the name "white spot syndrome"(WSS).The present study describes histopathological changes in tissuesfrom diseased shrimp.Degenerated cells characterized by hypertrophied nucleiwere observed in the cuticular epidermis and connective tissue.Cellular necrosisand detachment of intestinal epithelium tissue and severe atrophy of the hepatopancreasin diseased shrimp were also observed.A non-occluded bacilliform viral particlewas found by electron microscopy in the epidermis of spontaneously infected shrimp.Virion were enveloped,330+20 nm in length and 87+7nm in diameter. Filtrates prepared from the epidermis of either diseased P. japonicus or diseasedP. monodon were inoculated 4 established fish cell lines.Cytopathic effects were not found in any of the 4 tested fish cell lines;the filtrates which were used aswaterborne inocula had no cytotoxicity.Healthy juvenile P. japonicus were immersedin filtrates prepared from rither diseased P. monodon or diseased P. japonicus,both exhibited marked white spot signs.Cumulative mortalities reached 100% within5 to 7 days and white spots were observed on the removed carapace of theseexperimentally infected shrimp.Moribund shrimp were collected and examined by transmission electon microscopy.The close resemblance in extrenal signs and viralmorphology between spontaneously diseased and experimentally infected shrimpindicated that the bacilliform virus may be the main causative agent of the diseasecharacterized by WSS in Taiwan.The virus was tentatively designated as WSSV(whitespot syndrome associated virus)(Chou et al.,1995; Wang et al., 1995). Shrimp infectivity test was also carried out by means of oral ingestion.Obvious white spots were observed on the exoskeleton of experimentally infectedyoung P. mpnodon 2 days after being fed with muscle of diseased P. monodon or P. japonicus.The results indicated that WSSV can be transmitted orally as well asvia water across shrimp species. Three sized groups of juvenile P. japonicus(meanweight 0.08g,0.16g and 0.26g respectively)were applied to infectivity test of WSSV.The results revealed that the larger shrimp groups were relatively low susceptibleto WSSV via waterborne inoculation and the pathogenicity of WSSV were significantlyaffected by catching and temperature stress. Healthy P. monodon, P. japonicus and P. penicillatus were immersed in filtratesfrom either diseased P. monodon or P. japonicus.No difference was found in the pathogenicity of WSSV from P. monodon and WSSV from P. japonicus.WSSV was foundhighly pathogenic to the 3 tested shrimp species and was readily transmitted acrossdifferent penaeid shrimp via waterborne contant. Using PCR with a specific primer set(Lo et al.,1996),WSSV was first detectedin the previously healthy P. monodon immersed in filtrate from diseased P.monodon6 hours postinoculation(HPI) and detected rate was 1/10(positive no./ tested no.).At 24 HPI and 48 HPI,the detected rates reached 9/10 and 9/9 respectively.Theappearence of WSSV in experimentally infected P. penicillatus postlarvae was detectedat 24 HPI and reached 100% at 72 HPI.Healthy young P. monodon fed with diseasedP. japonicus as well as those fed with diseased P. monodon became 70~90% WSSV-positive24 HPI and reached 100% at 48 HPI.With this specific primer and PCR,WSSV can be detected from healthy-looking shrimp.The technique will be useful for the screeningof carriers in shrimp larvae and parental spawners. Pilot studies have shown that Helice tridens collected shrimp farms are WSSV-positive by PCR with WSSV specific primer set.