Author for correspondence: Alan Barrett. Fax +1 409 747 2415. e-mail abarrett@mspo5.med.utmb.edu.The nucleotide accession numbers of the sequences reported in this paper are AF052444-AF052446.
The 17D-204 vaccine manufactured in South Africa (17D-204-SA) and a large plaque variant (17D-LP) derived from it were highly virulent in adult mice. The LD50 of 17D-LP virus was 0 2 p.f.u. for mice following intracerebral inoculation. In comparison, a medium plaque variant derived from 17D-LP, termed 17D-MP virus, was found to be attenuated in adult mice following the same route of inoculation (> 104 p.f.u./LD50). Replication of 17D-MP virus was decreased in infected mouse brains compared to 17D-LP virus. Also, 17D-MP virus was slightly temperature sensitive at 39·5 °C. Compared to its parent viruses, 17D-204-SAand 17D-LP, 17D-MP virus had one unique mutation at nt 8045 in the genome which resulted in a single amino acid substitution (Pro → Ser) at residue 137 of the NS5 protein and appeared to be the mutation responsible for the attenuation of 17D-MP virus. This is the first time that altered virulence of a flavivirus caused by mutation in a non-structural protein gene, other than NS1, has been reported.
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