No single PCR test is sufficient to determine parvovirus IHHNV presence in or impact on farmed shrimp production

Abstract

The main purpose of this report is to provide hard evidence that the shrimp parvovirus IHHNV has not resulted "in significant consequences e.g. production losses, morbidity or mortality at a zone or country level" in Thailand since at least 2010. It also reveals that no single PCR test is sufficient to identify IHHNV-infected shrimp. It presents historical evidence and new evidence from 11 commercial ponds cultivating the giant tiger shrimp Penaeus monodon in Thailand. These ponds were selected because they were the ponds that gave positive PCR test results for IHHNV using 2 methods recommended for IHHNV diagnosis by WOAH. However, an additional in-house "IHHNV Long-Amp method" (IHHNV-LA) was also used to amplify 90% of the 4 kb IHHNV genome sequence, and it also gave false-positive test results with 2 of the 11 ponds. Further tests using normal histopathological analysis for the presence of pathognomonic Cowdry A type inclusions (CAI), in situ hybridization (ISH) and immunohistochemistry (IHC) could confirm IHHNV infections in only 2 of the 4 ponds PCR-positive using all 3 PCR methods. In addition, positive detection of CAI alone was equivalent to ISH or IHC in confirming IHHNV infection after a positive test with any of the PCR methods used. In summary, the recommended WOAH PCR methods gave false positive test results for IHHNV infection with 9/11 ponds (82%). All 11 ponds gave profitable harvests despite the confirmation of IHHNV in 2 ponds, where it was accompanied by various additional pathogens. Unfortunately, according to current practice, positive PCR test results with the WOAH methods alone sometimes leads to rejection of traded shrimp products without assurance that the test results are not false positive results that may arise from endogenous viral elements (EVE).