Emergency response to emerging disease: AHPND in shrimp

Outbreaks of acute hepatopancreatic necrosis disease (AHPND) have caused great economic losses to many shrimp producing countries in Asia since its first appearance in 2009. The causative agent was first reported in 2013 as specific isolates of Vibrio parahaemolyticus (VPAHPND) that were later found to harbor a plasmid (pVA) encoding the Pir-like binary toxin genes PirvpA and PirvpB. More recent information indicates that pVA plasmid and variants occur in many Vibrio parahaemolyticus serotypes and also in other Vibrio species such as V. campbellii, V. harveyi and V. owensii. Information on such genomic and proteomic studies of different VPAHPND isolates from different countries are reviewed. A cohort study carried out in Thailand in 2014 indicated that AHPND outbreaks account for only a portion of the disease outbreaks reported by shrimp farmers as outbreaks of early mortality syndrome (EMS). It is urgent that the etiology of the other EMS-associated mortalities be investigated and not be overlooked. It is recommended that a regional research network and surveillance program for newly-emerging or re-emerging pathogens be established to speed up the process of diagnosis and the implementation of coordinated control measures and to avoid a repeat of the EMS/AHPND scenario

Detection and differentiation of yellow head complex viruses using monoclonal antibodies

Three monoclonal antibodies (MAbs) raised against pathogenic yellow head virus (YHV) from Thailand were tested against tissues of shrimp from Thailand, Australia, Ecuador and India that were purported to be infected with yellow head complex viruses. MAbs V-3-2B and Y-18 were specific to gp116 and gp64 envelope proteins, respectively, while Y-19 was specific to a 20 kDa putative nucleoprotein p20. As a preliminary step, the site of reactivity of the 3 MAbs in YHV was determined by immuno-electron microscopy using ultra-thin sections of YHV-infected shrimp tissue and negatively stained, semi-purified YHV particles. As expected, MAb Y-19 reacted with viral nucleocapsids in ultra-thin sections but not with negatively stained, whole virions; MAb V-3-2B did react with negatively stained, whole virions, but not with virions or nucleocapsids in ultra-thin sections. Unexpectedly, MAb Y-18 did not react with whole or sectioned virions. By immunohistochemistry, MAbs Y-19 and Y-18 reacted with Penaeus monodon tissues infected with either YHV or with gill-associated virus (GAV) from Australia, while MAb V-3-2B reacted with YHV only. In addition, all the YHV and GAV tissue samples gave positive in situ hybridization reactions with a cDNA probe specific to the ORF1b gene of YHV. They also gave expected differential RT-PCR results for YHV and GAV. By contrast, 2 natural Thai shrimp specimens with no gross signs of disease gave similar immunohistochemical reactions and RT-PCR reactions to GAV. However, sequencing of their RT-PCR products showed that they shared 92.7% identity with GAV, but only 79.0% identity with YHV. Although specimens from Ecuador and India displayed histopathology suggestive of YHV infection, they gave negative immunohistochemical reactions with all 3 Mabs, and negative in situ hybridization results. Additional work is required to determine whether a virus from the yellow head complex was responsible for their observed histopathology. These data show that the 3 YHV MAbs could be used in diagnostic situations to differentiate some viruses in the yellow head virus complex