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

Abstract

The purpose of this study was to confirm evidence that the shrimp parvovirus IHHNV no longer has a significant negative economic impact on farmed shrimp production, and that it should be delisted as a pathogen by the World Organization for Animal Health (WOAH). Current published evidence for delisting is presented in the introduction, and experimental evidence is presented from 11 commercial ponds cultivating the giant tiger shrimpPenaeus monodonin Thailand. All 11 ponds gave positive PCR test results for IHHNV using 2 methods recommended for IHHNV diagnosis by WOAH. However, when an additional in-house “IHHNV Long-Amp method” (IHHNV-LA) was also used to amplify 90% of the 4 kb IHHNV genome sequence, only 4 of the 11 ponds gave positive PCR test results. Further tests using normal histopathological analysis for the presence of pathognomonic Cowdry A type inclusions (CAI),in situhybridization (ISH) and immunohistochemistry (IHC) could confirm IHHNV infections in only 2 of those 4 ponds. In addition, a 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, as were all 11 ponds. Unfortunately, according to current practice, positive PCR test results with the WOAH methods can lead to rejection of traded shrimp products without assurance that they are not false positive results arising from EVE. Our results, results from others and our analysis of a recent decision of WOAH to maintain IHHNV as a listed disease suggest that WOAH must reconsider IHHNV for de-listing.