Genetic diversity of the VP1 gene of duck hepatitis virus type I (DHV-I) isolates from southeast China is related to isolate attenuation

The complete sequence of an isolate (ZJ-V) of Duck hepatitis virus I (DHV-I), originally taken from the field in southeast China was determined. It was 7691 nucleotides long and had 5′- and 3′-terminal non-coding regions of 626 and 315 nucleotides, respectively. The poly(A) tail contained at least 22 residues and the single open reading frame encoded a polypeptide of 2249 amino acids. The VP1 gene was also sequenced from nine southeast China field isolates and three attenuated DHV-I vaccine strains. In phylogenetic analysis of the isolates and other published sequences, attenuated and tissue-adapted isolates (including ZJ-V) clustered as genotypes significantly different from the field isolates that had not been passaged in chicken/duck embryos. There were two consistent amino acid substitutions (E129 → V129 and A142 → S142) between all the field isolates and all the tissue-adapted ones. The carboxyl terminal region was generally the most variable and here the four attenuated Chinese isolates showed six consistent differences from the field isolates (S181 → L181, H183K184 → R183G1841, N193 → D193, E205 → K205, R217 → K217, N235 → D235). It seems likely that at least some of these differences result from mutations leading to isolate attenuation.