Yellow Head Virus

Yellow head virus (YHV) infects the black tiger shrimp (Penaeus monodon) – one of the world's major aquaculture species. It is a highly virulent pathogen that can cause 100% mortality within a few days of the first signs of disease in a pond. YHV is a rod-shaped, enveloped (+) single-stranded RNA (ssRNA) virus with a helical nucleocaspsid. In genome organization and transcription strategy, it resembles coronaviruses, toroviruses, and arteriviruses with which it has been classified within the order Nidovirales (genus Okavirus, family Roniviridae). The 26 662 nt genome comprises four long open reading frames (ORFs). ORF1a and ORF1b encode nonstructural proteins that are expressed as polyproteins (pp1a and pp1ab) and processed to generate elements of a replicase complex. ORF1b is expressed only as an extension of ORF1a via a ribosomal frameshift. ORF2 encodes the nucleocapsid protein (p20). ORF3 encodes envelope glycoproteins (gp64 and gp116) and a small, nonstructural, triple-membrane-spanning protein (p22). YHV is one genotype in a complex of closely related viruses that are endemic in black tiger shrimp in the Indo-Pacific region. These include gill-associated virus which has been associated with less severe forms of disease in Australia and at least four other genotypes that cause low-level chronic infections in healthy shrimp.

Low viral loads and lymphoid organ spheroids are associated with yellow head virus (YHV) tolerance in whiteleg shrimp Penaeus vannamei

Yellow head virus (YHV) is an invertebrate nidovirus that has caused mass mortality in penaeid shrimp since 1990. Several YHV types are known, but only the original type (YHV-type 1 or YHV-1) is highly virulent. Most studies have focused on acute YHV-1 infections and there is limited work on YHV-1 survivors. We compared moribund and surviving (14%) whiteleg shrimp Penaeus (Litopenaeus) vannamei from an experimental challenge with YHV-1. Although grossly normal, all survivors were positive for YHV-1 by specific, reverse transcriptase polymerase chain reaction (RT-PCR) assays, histological analysis or transmission electron microscopy (TEM), indicating that they were not resistant but tolerant to YHV-1. On the other hand, real-time PCR analysis revealed that mean YHV-1 copies/ng total RNA for survivors (2.8x10(4) +/- 6.9x10(4)) were approximately 40 times lower (P<0.05) than those in moribund shrimp (1.2x10(6) +/- 6.7x10(5)copies/ng total RNA). This was confirmed by strong positive immunohistochemical and in situ hybridization (ISH) reactions for YHV-1 in lymphoid organ tubules (LOT) of moribund shrimp and weak positive reaction only in lymphoid organ spheroids (LOS) of survivors. TEM revealed morphologically complete YHV virions in both groups. Furthermore, immuno-TEM and Western blot analysis revealed that YHV-1 structural proteins gp116 and p20 were present at comparable reactive levels in each group. Thus, YHV-1 tolerance was not associated with absence of gp116 as previously reported for palaemonid shrimp. Instead, it was associated with the presence of YHV-positive LOS and a relatively low viral load.

A convenient immunochromatographic test strip for rapid diagnosis of yellow head virus infection in shrimp

A simple yellow head virus (YHV) "strip test" was developed using monoclonal antibody Y19 (against the p20 structural protein) conjugated with colloidal gold as the detector antibody. Rabbit anti-recombinant p20 (rp20) protein antibody was used as a capture antibody at the test line (T) and goat anti-mouse IgG antibody (GAM) was used as the capture antibody at the control line (C). The ready-to-use strip was housed in a plastic case for convenient application and stored in the desiccated plastic bag. A sample volume of 100 microl of either haemolymph or gill or appendage homogenates in application buffer was applied to the sample chamber at one end of the strip and allowed to flow by chromatography through the nitrocellulose membrane to the other end. In test samples containing YHV, the virus would bind to colloidal gold conjugated monoclonal antibody and the resulting complex would be captured by the rabbit anti-rp20 antibody at the test line to give a reddish-purple band. Any unbound monoclonal antibody conjugated with colloidal gold moved across the test line to be captured by the GAM to form a band at the control line (C). In the sample without YHV or below the limit of detection for the kit, only the control line was demonstrated. This method was about 500 times less sensitive than that of one-step RT-PCR, but slightly more sensitive than dot blotting. Therefore, it could be used for primary screening of individual shrimp or pooled shrimp samples to confirm high levels of YHV infection or YHV disease outbreaks. This kit can be used to detect gill associated virus (GAV) infection as well since the monoclonal antibody used in this kit cross-reacted well with GAV. The beneficial features of this kit are that simple, convenient, and rapid results that can be obtained without the requirement of sophisticated tools or special skills.