A quantitative study on the relative virus load of white spot syndrome virus in infected tissues of tiger shrimp Penaeus monodon

A combination of molecular diagnostic techniques such as nested PCR, real time PCR, Digoxigenin (DIG) probe based in situ hybridization and dot blot were used to determine the virus load, tissue tropism and the viral spread in WSSV infected Penaeus monodon. Gills, gut, eye stalk, haemolymph, pleopod, muscle and hepatopancreas of WSSV infected shrimp collected at different time post infection period on analysis indicated gill tissues to be the primary target site for WSSV replication at early infection stage. Highest WSSV copies were observed in gill tissue of the infected shrimp at all time points post WSSV infection. DIG labelled 615 bp WSSV VP28 probe was able to detect WSSV in all the infected shrimp tissues. The highest intensity of the colour development by DNA dot blot was observed in infected gills, gut and muscle tissues of the shrimp. Histopathological observations by in situ hybridization of gill sections from WSSV infected P. monodon revealed progressive increase of positive hybridization signals with the increase in disease duration from 6 h to moribund stage of infection. These results indicate that the virus is able to replicate over time and WSSV pathogenesis proceed with increase in viral copy numbers with disease duration from 6 h to moribund stage in primary target tissue of WSSV infection such as gills.