Molecular epidemiology and phylogenetic analysis of diverse bovine astroviruses associated with diarrhea in cattle and water buffalo calves in China

ALFRED, Niyokwishimira; LIU, Huan; LI, Mu Lan; HONG, Shao Feng; TANG, Hai Bo; WEI, Zu Zhang; CHEN, Ying; LI, Fa Kai; ZHONG, Yi Zhi; HUANG, Wei Jian

Appreciation of the genetic diversity and evolution of astroviruses among wild and domestic animal populations is important to fully understand this challenging gastrointestinal virus [21] . However, the organization of the BAstV genome has not yet been fully described. In this study, a sequence motif upstream of ORF2 was predicted to be the signal of the putative promoter for subgenomic RNA (sgRNA) synthesis in bovine astrovirus as well as that for the ORF1b/ORF2 overlap sequence and the most 3′-end of ORF2. It has been suggested that in astroviruses [13, 23] , a conserved sequence acts as a putative promoter for sgRNA synthesis upstream of ORF2. In human astroviruses, the ORF1b/ORF2 overlap is 8 nt in length, while in duck astroviruses, the ORF1b/ORF2 junctions are not overlapped [6, 13, 23] . In contrast, the overlap in the ORF1b/ ORF2 junction of bovine astroviruses may be longer (56 nt) than that of the other known astroviruses. Moreover, compared Fig. 1 . Phylogenetic relationships between the Guangxi bovine astroviruses and other mammalian astroviruses retrieved from the NCBI database. Phylogenetic trees were constructed based on both predicted partial ORF2 (Fig.1a) and full-length ORF2 (Fig.1b) amino acid sequences of the Guangxi bovine astrovirus and documented full-length ORF2 sequences from other mammal astroviruses. The phylogenetic trees were generated using the Bootstrap Test Phylogeny algorithm with the neighbor-joining method included in the MEGA 4.1 software package. Each genotype of the genus Mamastrovirus was represented at least by one or more prototype strains, and genotypes are highlighted. Classification was based on the Taxonomic Proposals submitted to the ICTV by the Astroviridae Study Group in 2010. The Guangxi isolates derived from cattle are marked by (•), and isolates derived from water buffalo are marked by (■). a with a previous report [12] , the 3′-end of ORF2 of bovine astroviruses was highly conserved, although our results suggested further studies and analysis of more sequences from several novel bovine astroviruses to fully characterize ORF2 of bovine astrovirus. In summary, this study is of interest at both the epidemiological and genetic levels. These results will certainly contribute to the understanding of the evolution and pathol-b ogy of bovine astrovirus in cattle and water buffalo, as not only did we provide useful reference material for further studies, but also isolated bovine astrovirus in water buffalo for the first time. The present study is thus far among the largest epidemiological investigations of bovine astrovirus conducted at the farm level in the dairy industry in China.