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 (E(129)-->V(129) and A(142)-->S(142)) 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 (S(181)-->L(181), H(183)K(184)-->R(183)G(1841), N(193)-->D(193), E(205)-->K(205), R(217)-->K(217), N(235)-->D(235)). It seems likely that at least some of these differences result from mutations leading to isolate attenuation.