The John Cunningham (JC) virus is linked to the autoimmune disease multiple sclerosis (MS). MS is a disorder of the central nervous system, which includes the brain and spinal cord. MS destroys the protective coating (called myelin) on nerve cells, leading to problems with balance, coordination, and muscle control.

The JC virus is common; most of the general population has been exposed to it. However, if someone who’s been exposed to the virus later develops a compromised immune system — by taking immunosuppressants to treat MS, for example — the JC virus may eventually resurface. If the virus crosses the blood-brain barrier, it can cause progressive multifocal leukoencephalopathy (PML), a dangerous brain infection.

Here we take a closer look at the relationship between the JC virus, PML, and MS.

What Is the JC Virus?

The JC virus can cause PML in some people. People are usually first infected with the JC virus in childhood. This first infection is typically harmless and causes no symptoms. However, sometimes the JC virus spreads throughout the body, targeting the kidneys, lymph nodes, white blood cells, and brain. If someone’s immune system is compromised, the JC virus can sometimes establish an infection known as PML.

What Is PML?

Progressive multifocal leukoencephalopathy is a potentially fatal brain infection. PML, like MS, is a demyelinating disorder (a disorder that destroys the myelin around nerve cells). PML symptoms can be similar to MS symptoms, including problems with muscle weakness, vision, speech, and cognition.

How MS Drugs, PML, and the Immune System Interact

Taking certain disease-modifying therapies used to treat MS can be a risk factor for developing PML. These therapies suppress important aspects of the immune system to slow MS progression. When the immune system is suppressed, the JC virus can move out of its latent form and develop into an active PML infection.

Of these medications, natalizumab — sold as Tysabri — is usually considered the drug with the highest risk. However, cases of PML have also been reported in people taking dimethyl fumarate (Tecfidera). Anti-JC virus antibodies have also been detected in people with MS taking fingolimod — sold as Gilenya — and rituximab — sold as Rituxan.

Monitoring for JC Virus Antibodies To Reduce Risk of PML

Your doctor may recommend that you undergo monitoring before beginning an immune-suppressing medication. This monitoring includes blood tests to check for JC virus antibodies. If JC virus antibodies are detected, you may still be able to take MS medications associated with PML risk, depending on the levels of antibodies in your blood. Higher levels increase the risk of developing PML.

If you are taking an immune-suppressing drug for MS, your doctor may recommend blood tests every six months to monitor your JC virus antibody levels.

Doctors will sometimes recommend remaining on natalizumab for a certain period of time before transitioning to another treatment. A member who tested positive for the JC virus shared: “My neurologist advised that I shouldn’t be at a high risk for PML until I’ve been on Tysabri for two years. As of right now, the plan is to keep monitoring for PML and remain on Tysabri for two years, then switch to something else.”

Always consult your doctor before discontinuing any medication.

What Are MyMSTeam Members Saying About the JC Virus?

Receiving a positive JC virus test result can be frustrating and scary. Some MyMSTeam members worry about switching treatments. “My JC virus levels have increased,” one member wrote. “My doctor is considering taking me off Tysabri. This is the ONLY MEDICINE THAT HAS WORKED FOR ME!”

Other members have decided to avoid any medication that can increase the risk of PML. “I don’t think I would take Tysabri if I were positive because I would have too much anxiety about PML,” another member commented.

MyMSTeam members also support each other when dealing with a positive JC virus result. “Doctors watch your numbers carefully,” one member wrote in response to a worried teammate. “They don’t want it to happen any more than you do.”

“It’s incredible; there are many more options for treatment than there were even a decade ago,” a member wrote to encourage another who was concerned about switching treatments.

Talk With Others Who Understand

MyMSTeam is the social network for people with multiple sclerosis and their loved ones. More than 184,000 members come together to ask questions, give advice, and share their stories with others who understand life with multiple sclerosis.

Do you have thoughts or concerns about the JC virus or PML? Share your experience in the comments below, or start a conversation by posting on your Activities page.