Semliki Forest virus (SFV) normally causes an acute lethal encephalitis in mice following intranasal inoculation. However, animals co-administered with 10 LD SFV and defective interfering (DI) SFV survive the infection without clinical signs of disease. In this report we demonstrate the isolation of infectious virus from the brains of 12/169 protected mice up to 6.5 months post-infection. Although, with one exception, mice were clinically normal, five of 12 of the SFV isolates were identical to the original virus as judged by plaque morphology, maximum temperature for growth, virulence in mice and pathology. Others were less virulent (although not any were plaque or temperature-sensitive mutants) and on re-inoculation into fresh mice caused a demyelinating pathology which was not an attribute of the original inoculum. How the virulent virus can persist in brain, sometimes in amounts in excess of 100 LD, without causing disease remains to be determined.

Keyword(s): DI virus , persistent infection and SFV

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