JC virus testing: latest results
During treatment with Tysabri (natalizumab), one possible problem is the development of PML (progressive multifocal leukoencephalopathy), an often fatal brain disorder. The disease is caused by the JC virus. People normally acquire this virus at some point in their life and never know it. For the most part, the virus remains dormant and doesn’t causes any symptoms. Problems can arise, however, in people with a depressed immune response. Until recently, most cases of PML were in people with HIV/AIDS.
So exposure to the JC virus is one of the main risk factors for developing PML during treatment with Tysabri. (The other main risks are prior exposure to immunosuppressant drugs and being on Tysabri for over two years.)
Determining the PML risk is now easier with the recent approval in the U.S. and Canada of a new blood test. The Stratify test determines if you have anti-JC virus antibodies. If you do, that means you’ve had prior exposure to the virus.
The proportion of people in the population (or “prevalence”) with JC virus exposure was reported at the 2012 American Academy of Neurology annual meeting (Bozic and colleagues. Abstract S41.002). The information comes from the ongoing STRATIFY-2 study in the U.S.
The overall prevalence of anti-JC antibodies was about 55%, and was slightly higher in men (61%). As you might expect, exposure to the virus is increasingly likely as you get older. The prevalence was 45% in people younger than age 30, and about 58% in people older than age 60. By the time you’re in your 30s, you have a 50:50 chance of JC virus exposure.
The use of the Stratify blood test is likely to become part of the routine assessment in people who are being considered for treatment with Tysabri. A positive test (meaning JC virus exposure) won’t necessarily mean that you can’t start Tysabri. But it will mean your PML risk will be that much higher. So if you do start taking Tysabri, your doctor will probably suggest you try another therapy within a year or two because of the increasing risk of PML with more prolonged exposure to the drug.
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