Hamsters experimentally infected with the neuroinvasive West Nile virus (WNV) strain NY385-99 frequently develop persistent renal infection and viruria. Viruses recovered from the urine of such animals no longer cause neurological disease when inoculated into naïve hamsters. To examine if this phenotypic change is stable, and if additional nucleotide changes occur during further passages, a urine isolate from a persistently infected hamster (WNV 9317B) was serially passaged in hamsters, and representative isolates from each passage were analysed for pathogenesis in hamsters and by nucleotide sequencing. The progeny viruses tested all resulted in asymptomatic infection when inoculated into hamsters and caused no mortality. Most of the original nucleotide changes were retained in these serial WNV isolates. Changes were distributed throughout the genome at 116 sites, ranging from 0.082 to 0.262 %, compared with the parent strain NY385-99, and they were mostly in coding regions. Our findings indicate that WNV underwent additional genetic changes during serial passage in hamsters, but there was no reversion to neurotropism and virulence.
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