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The pathogenicity of the A7, M9 and L10 strains of Semliki Forest virus for weanling mice and primary mouse brain cell cultures.

The multiplication of the M9, A7 and L10 strains of Semliki Forest virus (SFV), both in weanling mice and primary mouse brain cell cultures, was compared. Following both intraperitoneal (i.p.) and intracerebral (i.c.) injection, the virulent L10 strain multiplied to higher titre in the mouse central nervous system (CNS) than did the less virulent M9 and A7 strains, whereas M9 multiplied to higher titre than A7. By the i.c. route, all three virus strains multiplied to higher titre than following i.p. injection. Multiplication of A7 and M9 in oligodendrocytes, but not neurons, was detected following i.c. injection. All three virus strains showed a tropism for cultured mouse glial cells rather than neurons. The L10 strain multiplied better in neurons than did A7 or M9. It is concluded that the mechanism of acute demyelination induced by the M9 and A7 strains is similar. Based on this and previous studies, it is proposed that infection of glial cells triggers immune-mediated demyelination. The virulence of the L10 strain is due to its ability to exceed a lethal threshold of damage to neurons before immune intervention can occur.