en fish disease fish disease پژوهشي Orginal research papers <p>  White Spot Virus (WSV) is currently the most serious viral pathogen of shrimp worldwide it causes mortality up to 100% within 7-10 days in commercial shrimp farms. Infected Indian white shrimp <i>Fenneropenaeus indicus</i> samples were collected from Guatr shrimp site in Sistan and Baluchestan province in south of Iran and WSV infection was confirmed by Nested PCR<strong>. </strong>WSV was isolated from infected shrimp samples by centrifugation and filtration and multiplied in crayfish by intramuscular inoculation, the isolated virus was called WSV/IRN/1/2010.<strong> </strong>In order to determine the dilution resulting in 90-100% mortality in <i>Penaeus semiculcatus,</i> diluted virus stock in steps from 10⁰ till 10⁵times in sterile PBS was injected intramuscularly to 14 shrimps in each group. Also the virus stock was diluted in steps from 1/2 till 1/32 times in sterile PBS and injected intramuscularly in <i>Astacus leptodactylus</i> crayfish<i>. </i>Therefore the LD₅₀ of live virus stock in <i>Astacus leptodactylus</i> and <i>Penaeus semiculcatus </i>crayfish were calculated by the Karber method 10 ^3.29 /ml and 10 ^5.35 /ml, respectively. <strong></strong></p> White Spot Virus, Titration, Karber Formula, Astacus leptodactylus, Penaeus semisulcatus 145 155 http://jifro.ir/browse.php?a_code=A-10-1-144&slc_lang=en&sid=1 F. Motamedi Sedeh fmotamedi@nrcam.org 10031947532846002749 10031947532846002749 Yes M. Afsharnasab 10031947532846002750 10031947532846002750 No M. Heidarieh 10031947532846002751 10031947532846002751 No S. K. Shafaee 10031947532846002752 10031947532846002752 No S. Rajabifar 10031947532846002753 10031947532846002753 No A. Dashtiannasab 10031947532846002754 10031947532846002754 No M. H. Razavi 10031947532846002755 10031947532846002755 No