Abstract:Yellow head virus (YHV) is a highly lethal pathogen that causes majority of shrimp diseases. In this study we conducted the epidemiology study and revealed the existence of YHV and the current infectious situation in China. We collected 299 samples in 2012-2014 and detected YHV using the nested RT-PCR method that was recommended in the World Organization for Animal Health (OIE) manual. Genes of some YHV-positive samples were sequenced and underwent the phylogenetic tree analysis. The epidemiology survey showed that the YHV infection rate was 11% and this virus was found in cultured Litopenaeus vannamei, Fenneropenaeus chinensis, Marsupenaeus japonicus and Macrobrachium rosenbergii in China. F. chinensis showed the highest infection rate. F. chinensis and M. rosenbergii were newly reported as natural hosts of YHV. We then amplified a 1002 bp fragment located in the ORF1b region. The alignment results suggested that YHVs from the six infected samples shared 81.0%-90.5% sequence identity with the six YHV genotypes archived in NCBI. The phylogenetic tree analysis showed that YHVs from the six samples clustered in one branch, but they did not share the same branch with the six already known YHV strains. YHVs in the 6 samples were relatively closely related to YHV Type 1 (YHV-1) and had higher similarity with YHV-1b than with YHV-1a. We also amplified a 509 bp sequence located in the ORF3 region. It encoded gp116 and was different from the 545 bp of YHV Type 1a (YHV-1a), the 383 bp of YHV-1b and the 476 bp of YHV-2 (GAV). We next compared the ORF2 sequence of two samples and found that they were 99.8% identical in gene sequence and had the same protein sequence. Their sequence similarities with YHV-1 and YHV-2 (GAV) were 85.9% and 80.9% respectively. Results of gene sequence alignment suggested that the YHV found in China was a new pathogenic strain.
