Werner MH, Huang D. Natalizumab-treated patients at high risk for PML persistently excrete JC polyomavirus. J Neurovirol. 2016 [Epub ahead of print]
Sixty-three natalizumab-treated patients with relapsing multiple sclerosis were screened for JC polyomavirus (JCV) viruria. Urinary-positive patients were longitudinally sampled for up to 24 weeks. Using methods that distinguish encapsidated virus from naked viral DNA, 17.5 % of patients were found to excrete virus, consistent with the prevalence of urinary excretion in the general population. Unexpectedly, urinary excretion was predominantly seen (>73 %) in patients with high JC antibody index (≥2.0). Active JCV infection, therefore, tends to occur in natalizumab patients that carry a high risk factor for the development of disease, directly linking JC infection to the risk factors for PML development.
If you have been exposed to JC virus you may make an antibody response to the virus and if this is detected it increases your risk of developing PML. This risk is higher in people with higher levels of antibodies, but in this study it shows that people with the higher levels of antibodies are more likely to have active infection of live virus which is shed in urine. So if you have more sheding virus you would probably be at a greater risk of PML infection