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.