Human herpesvirus type 6 and multiple sclerosis.

The aetiology of multiple sclerosis (MS) is unknown, but is probably the result of an interaction between predisposing genetic and environmental factors. The potential role of an infectious agent in MS aetiology stems from evidence of a geographical risk component, with disease being more prevalent in temperate regions. Also, several epidemics of MS have been recorded, suggesting a transmittable element to the disease. Although no one particular virus has been implicated as a causative factor, several herpesviruses have been incriminated due to their neurotropism, ubiquitous nature and ability to establish latency. The majority of evidence supporting a link between MS and human herpesvirus-6 (HHV-6) comes from case control studies comparing the presence of HHV-6 in brain tissue, cerebrospinal fluid (CSF) and peripheral blood mononuclear cells (PBMCs) in patients with and without MS. HHV-6 was detected in a high proportion of individuals without MS, indicating that HHV-6 polymerase chain reaction (PCR) positivity in itself is not sufficient for its causality in the development of MS. Studies of the association between HHV-6 and MS have been contradictory and the International Herpes Management Forum (IHMF) recommends that any future studies should use validated, sensitive methods and that masked samples should undergo independent analyses. If therapy trials are indicated, it is recommended that drugs with improved activity against HHV-6 are evaluated.