Reduced growth performance of Black Tiger shrimp (Penaeus monodon) infected with infectious hypodermal and hematopoietic necrosis virus

In Australia, Infectious hypodermal and hematopoietic necrosis virus (IHHNV) occurs endemically in Black tiger shrimp (Penaeus monodon) and has been detected at high prevalence in farmed shrimp. Here we examined the role of high IHHNV infection prevalence and loads in reduced growth and survival in P. monodon reared under commercial conditions in 0.16 ha research ponds. TaqMan real-time quantitative (q)PCR testing of wild broodstock from the East Coast of Queensland identified lower and higher IHHNV infection loads among females used to generate 2 cohorts of progeny. Tracking of IHHNV loads in eggs and juveniles sampled progressively over grow-out identified these to be substantially higher and to increase more rapidly in 2 ponds stocked with a batch of postlarvae (PL) pooled from 3 females with high-level IHHNV infection at the time they spawned compared to 2 neighbouring ponds stocked with PL pooled from 4 females, of which only 1 possessed IHHNV at a relatively high load. From 120 days of culture (DOC) onwards, the growth performance in the IHHNV-high ponds began to progressively reduce compared to that in the IHHNV-low ponds. When harvested over 2 weeks starting at 155 DOC, the averaged harvest yield of the 2 IHHNV-high ponds was the equivalent of 3.72 t/ha lower than the 2 IHHNV-low ponds, and estimated survival was also markedly lower (79.5-84.5% compared to 95.9-99.8%). As real-time qPCR testing identified no involvement of either Gill-associated virus (GAV) or Yellow head virus genotype 7 (YHV7), other viruses known to cause such production losses in Australia, the data support a sustained high IHHNV infection burden as the primary contributor to the reduced growth and survival. Using a basic extrapolation of yield, stocking low-level IHHNV PL has resulted in an increase of farm gate value of $67,000 per hectare compared to stocking high-level IHHNV PL. Among other problems such as severe shell deformities that acute IHHNV infection has recently been identified to cause in locally-farmed P. monodon, the potential impacts on growth and survival identified here reinforce the value in screening-based selection of IHHNV-free/low broodstock for use in hatcheries and in breeding programs employing specific pathogen-free stocks, and strategies to select for IHHNV resistance/tolerance.