Postlarvae and juveniles of a selected line of Penaeus stylirostris are resistant to infectious hypodermal and hematopoietic necrosis virus infection

A susceptibility study of postlarvae (PL) and juvenile Super Shrimp®, a selected line of Penaeus stylirostris, was conducted to compare their resistance to infectious hypodermal and hematopoietic necrosis virus (IHHNV) infection to that of a specific pathogen free (SPF) population of P. vannamei. Super Shrimp® PLs were fed with IHHNV-infected shrimp tissue for 2 days and then maintained on a pelletized ration for an additional 28 days. PLs were sampled at days 0, 1, 2, 3, 4, 6, 10, 15, 20, 25 and 30. There was no apparent mortality during the experimental period. Tissue DNA extracted from the PLs was analyzed for the presence of IHHNV by PCR. Low levels of IHHNV were detected only in DNA extracts from samples at days 1, 2, and 3. No IHHNV DNA was detected from days 4 to 30. The days that the PLs were weakly IHHNV-PCR positive were during the period that they were being fed with IHHNV-tissue, and thus, the IHHNV DNA signal was suspected to be from the infected tissue used as a feed. Through both histology and in situ hybridization, we confirmed that tissues of Super Shrimp® PLs were not infected with IHHNV. PCR results of another IHHNV challenge study with juveniles of Super Shrimp® were similar to those with PLs. These results indicate that IHHNV did not replicate in the PL and juvenile Super Shrimp®. In contrast, P. vannamei juveniles, which were used as a positive control, showed a more intense IHHNV infection, as determined by PCR detection, beginning at 6 days postchallenge and increasing throughout the remainder of the study. In addition, the IHHNV-infected P. vannamei at 30 days postchallenge showed histological changes characteristic of IHHNV infection and had a positive reaction for IHHNV with in situ hybridization. Our studies show that Super Shrimp® are resistant to IHHNV infection. This is the first unequivocal demonstration of resistance to viral infection in shrimp.