Yellow head virus (YHV) is a pathogen of the black tiger shrimp (Penaeus monodon) and, with gill-associated virus (GAV), is one of two known invertebrate nidoviruses. We describe sequences of the large replicase gene (ORF1a) and 5'- and 3'-terminal UTRs, completing the 26,662 nt sequence of the YHV genome. ORF1a (12,219 nt) encodes a approximately 462,662 Da polypeptide containing a putative 3C-like protease and a putative papain-like protease with the canonical C/H catalytic dyad and alpha+beta fold. The read-through pp1ab polyprotein contains putative uridylate-specific endoribonuclease and ribose-2'-O-methyl transferase domains, and an exonuclease domain incorporating unusual dual Zn2+-binding fingers. Upstream of ORF1a, the 71 nt 5'-UTR shares 82.4% identity with the 68 nt 5'-UTR of GAV. The 677 nt 3'-terminal region contains a single 60 nt ORF, commencing 298 nt downstream of ORF3, that is identical to N-terminal coding region of the 249 nt GAV ORF4. Northern blots using RNA from YHV-infected shrimp and probes directed at ORF1a, ORF1b, ORF2 and ORF3 identified a nested set of 3'-coterminal RNAs comprising the full-length genomic RNA and two sub-genomic (sg) mRNAs. Intergenic sequences upstream of ORF2 and ORF3 share high identity with GAV, particularly in the conserved domains predicted to mediate sgmRNA transcription.
Laem-Singh virus (LSNV) is a positive-sense single-stranded RNA (ssRNA) virus that was recently identified in Penaeus monodon shrimp in Thailand displaying signs of slow growth syndrome. A total of 326 shrimp collected between 1998 and 2007 from countries in the Indo-Pacific region were tested by RT-PCR for evidence of LSNV infection. The samples comprised batches of whole postlarvae, and lymphoid organ, gill, muscle or pleopod tissue of juvenile, subadult and adult shrimp. LSNV was not detected in 96 P. monodon, P. japonicus or P. merguiensis from Australia or 16 P. monodon from Fiji, Philippines, Sri Lanka and Mozambique. There was no evidence of LSNV infection in 73 healthy juvenile P. vannamei collected during 2006 from ponds at 9 locations in Thailand. However, LNSV was detected in each of 6 healthy P. monodon tested from Malaysia and Indonesia, 2 of 6 healthy P. monodon tested from Vietnam and 39 of 40 P. monodon collected from slow-growth ponds in Thailand. A survey of 81 P. monodon collected in 2007 from Andhra Pradesh, India, indicated 56.8% prevalence of LSNV infection but no clear association with disease or slow growth. Phylogenetic analysis of PCR amplicons obtained from samples from India, Vietnam, Malaysia and Thailand indicated that nucleotide sequence variation was very low (> 98% identity) and there was no clustering of viruses according to site of isolation or the health status of the shrimp. The data suggests that LSNV exists as a single genetic lineage and occurs commonly in healthy P. monodon in parts of Asia.
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