Both human and animal research suggests that viral infection is a likely environmental trigger of MS (Gilden, 2005; Sospedra and Martin, 2005). Postmortem analyses of the brains of MS patients have identified a number of viral agents, including mumps, measles, parainfluenza type I (Allen and Brankin, 1993), and human herpes virus simplex type 6 (Challoner et al., 1995). Although no single viral candidate has been implicated in all cases, it is possible that several viruses may serve as an environmental trigger of MS (Gilden, 2005; Sospedra and Martin, 2005). Indeed, several viruses have been found to initiate demyelination in humans (Johnson, 1994; Soldan and Jacobson, 2001) and animals (Dal Canto and Rabinowitz, 1982; Johnson, 1994; Soldan and Jacobson, 2001). Two common human herpes viruses that induce persistent infections are likely candidates in human MS: human herpes virus 6 (HHV-6) and Epstein-Barr virus (EBV). In the general population, both viruses are widespread, with seroprevalence rates of 80% and 90%, respectively (Martyn et al., 1993; Soldan et al., 2000; Wandinger et al., 2000; Moore et al., 2002). With both viruses, seroconversion tends to occur during adolescence, which corresponds with epidemiological studies suggesting that viral exposure during adolescence is a linked to the subsequent development of MS.
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