Murray Valley encephalitis virus field strains from Australia and Papua New Guinea: studies on the sequence of the major envelope protein gene and virulence for mice.

We have compared the nucleotide sequence of the gene encoding the major envelope (E) protein of a number of Murray Valley encephalitis virus (MVE) isolates from Australia and Papua New Guinea (PNG). The isolates, from widely separated geographic regions, were from four fatal human cases, a heron, and six mosquito pools and covered a period of 25 years. The sequences of the Australian strains were notable for their similarity, showing not more than 1.7% nucleotide sequence divergence in pairwise comparisons. There was 6.8% divergence in the E gene between the two available strains from PNG, and 9-10% divergence between each of the PNG strains and the Australian prototype. These data are consistent with previous conclusions based on HaeIII restriction digest analysis of cDNA to virion RNA (M. Lobigs, I. D. Marshall, R. C. Weir, and L. Dalgarno, 1986, Aust. J. Exp. Biol. Med. Sci. 64, 571-585). We conclude that a single MVE genetic type exists in Australia. Separate foci of MVE evolution appear to exist in PNG, generating greater strain variation. For all MVE isolates the deduced length of the E protein was 501 amino acids. The E protein differed at no more than three positions between any two Australian strains. The PNG strains differed from the Australian strains at 6-11 residues depending on the virus pair. Differences in amino acid sequence did not occur at a position corresponding to a previously demonstrated neutralization determinant in yellow fever virus (M. Lobigs, L. Dalgarno, J. J. Schlesinger, and R. C. Weir, 1987, Virology 161, 474-478). Thus selection for neutralization resistance may not be a major evolutionary pressure in the field situation. In comparisons between the E protein amino acid sequence of the prototype strain and those of a number of other MVE strains, 7 out of 14 differences were at residues seen at the corresponding position for Japanese encephalitis virus (JE), consistent with the close serological relationship of MVE and JE. Five Australian MVE strains and two from PNG were tested for virulence by comparing LD50 values after intraperitoneal and intracranial inoculation of 21-day-old mice; all strains were virulent by this test.