The evidence is provocative but not definitive; nonetheless, it should serve as a stimulus to further research
In this issue of the Journal, Daveson and colleagues1 describe a case of progressive multifocal leukoencephalopathy (PML) possibly caused by BK virus rather than JC virus. This finding is potentially very significant. The polyomaviruses BK and JC commonly infect humans and remain latent in immunocomptent individuals. Both are associated with clinical disease in the setting of immunosuppression. However, only JC virus has been causally associated with PML; BK virus has been causally associated with nephropathy, ureteric stenosis and cystitis. There have been previous case reports of BK virus causing a meningoencephalitis and PML as detailed by Daveson et al, but the evidence for causality has been tenuous, largely because of the lack of confirmation in tissue. The data provided by Daveson et al are more convincing, although not definitive.
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