What Is the JC Virus?
The John Cunningham virus, known more commonly as the JC virus, is a very common virus in the United States. It’s estimated that more than three-quarters of Americans have it. The average person carrying the JC virus will never know and is unlikely to suffer any side effects as a result of its being in his or her body.
However, for a small percentage of individuals with multiple sclerosis (MS), that is not the case. The JC virus can be activated when a person’s immune system is compromised because of disease or immunosuppressive medication. The virus can then be carried into the brain, where it infects the white matter of the brain and attacks the cells responsible for making myelin, the protective coating that covers and protects nerve cells. This infection is called progressive multifocal leukoencephalopathy (PML). (1) PML can be disabling, even fatal.
The Role of Immune-Suppressing Drugs
The JC virus often attacks when a person’s immune system is at its weakest—a weakened immune system can no longer fight off invading viruses. It’s the perfect opportunity for the JC virus to awaken, cross the blood-brain barrier, and begin attacking the brain. People with MS are at an increased risk for PML because their immune system is often compromised as a result of the condition. Further compounding the problem, several medications used to treat the symptoms of MS can compromise the immune system. These medicines include natalizumab (Tysabri) and corticosteroids. Prior treatment with immunosuppressant medications, including azathioprine, cyclophosphamide, methotrexate, mitoxantrone, or mycophenolate mofetil, can also increase the likelihood that a person with MS will develop PML after exposure to the JC virus.
In 2012, the FDA approved the Stratify JCV Antibody ELISA test. This JC virus–detecting test can determine if a person has been exposed to the virus and if it is present in his or her body. A positive test does not mean a person with MS will develop PML, but only JC virus–positive individuals can develop PML. (4) Even with a negative result, you’re not 100 percent safe—you can be infected with the JC virus at any point during your treatment. If you begin taking medications as part of your treatment for MS, it’s important you continue regular testing to see if you have been infected. Talk with your doctor about how often you should be tested for JC virus antibodies. If you become infected, regular testing will help you detect the infection faster, and the sooner it’s detected, the sooner you can begin treatment. (4)
What to Say to Your Doctor
Talk with your doctor about your risk for developing PML and how the medications you’re taking affect that risk. He or she may want to conduct an ELISA test out of an abundance of caution—especially if your doctor plans to prescribe the medicine Tysabri to you.
Tysabri is often prescribed to patients who have not responded well to other forms of MS treatment. Studies show that patients taking Tysabri are at an increased risk of developing PML compared to MS patients taking other disease-modifying medications. If your doctor recommends you begin treatment with Tysabri, talk to him or her about having the blood test first. If your result comes back negative, you are less likely to develop PML while on Tysabri. If your results come back positive, talk to your doctor about the danger of taking the medicine and the likelihood you will develop PML. A positive test may require that you and your doctor rethink your treatment plan.(4)