ABSTRACT:To examine the effect of selenium (Se) supplementation on shrimp growth and disease resistance after Taura Syndrome virus (TSV) challenge, groups of Pacific white shrimp (Penaeus vannamei) from a strain known to be susceptible to TSV were fed 3 diets: (1) standard commercial shrimp feed (0.58 ppm Se content) without Se supplementation (standard diet group); (2) standard commercial feed plus 0.3 ppm inorganic Se (Inorg Se-group); and (3) standard commercial feed plus 0.3 ppm organic Se (Org Se-group) over a 5-week period. Samples (3 shrimp) were collected from all replicates before feeding (pre-feeding or week 0) and at the 1st, 2nd, and 3rd weeks post feeding to determine weight, length, and number of granular haemocytes. After the feeding trial, the remaining shrimp were used for a final TSV challenge test. Significant differences in weight between groups (P < 0.05) were found on the fifth week post feeding in which the average weight of shrimp fed the standard feed was 7.2 ± 1.2 g compared to 6.1 ± 1.2 g in the inorg Se-group and 9.0 ± 1.1 g in the org Se-group. Significant increase in the number of total haemocytes and granular haemocytes (GHs) were found only in the org Se-group on the third week post feeding. On day 6 post TSV challenge, the number of survivors in the org Se-group (66.7%) was also significantly higher than those of the inorg Se-(35.5%) and standard diet-(13.3%) group. Nested RT-PCR analysis showed that the number of shrimp with severe infection in the org Se-group (2/5) was less than the inorg Se-(4/5) and standard diet-group (4/4). The RT-PCR results were similar to those observed by histological examination. The number of total haemocytes and GHs isolated from survivors of the standard diet-group were significantly lower than those isolated from survivors of the inorg Se-or org Se-group. The present results indicated that organic Se supplementation in shrimp feed could improve shrimp growth and survival after an experimental challenge with TSV.