The John Cunningham (JC) virus is a common and usually harmless virus. In rare cases, however, it can have a severe impact on people with multiple sclerosis (MS).

At least half of all adults have the JC virus in their system. In most people, it is dormant and does not directly cause health problems.

However, it can trigger a serious condition called progressive multifocal leukoencephalopathy (PML). This can lead to severe complications.

A few of the medications that doctors use to treat MS can reactivate the dormant virus and lead to PML.

This article will look at the JC virus, the dangers of PML, and the link between the JC virus and MS. It will also cover how doctors diagnose and treat PML.

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The JC virus is present in the blood samples of 70–90% of people worldwide, according to one review. However, most people have no symptoms.

Children with the JC virus often show no symptoms, and the presence of the virus in the body usually does not cause complications in adulthood.

The virus may be present in the kidneys, bone marrow, gastrointestinal tract, and other body tissues.

According to the same review, the virus stays dormant in kidney tissue, and around 40–75% of people around age 30 years have evidence of the virus in their urine.

The JC virus typically causes no harm, but some people with the virus face a high risk of developing PML, which is a potentially life threatening condition.

PML is rare, but people with a suppressed immune system have a higher risk of it. This includes people with HIV and those receiving certain treatments for MS and other autoimmune conditions.

If the dormant JC virus in a person’s body reactivates due to immune suppression, the resulting infection and severe inflammation can lead to PML. PML occurs when the protective myelin coating on nerve cells in the brain breaks down, which leads to tissue damage.

PML is incurable and can have severe physical effects. Complications include dementia, blindness, paralysis, and seizures.

For 30–50% of people who receive a PML diagnosis, the condition is fatal within the first few months of diagnosis.

PML has three stages:

  • Stage 1: The person has an initial infection with the JC virus but does not experience symptoms.
  • Stage 2: The inactive virus stays in the urinary tract, bone marrow, tonsils, lungs, spleen, and gut tissue.
  • Stage 3: The active virus and inflammatory response damage the central nervous system.

Those with a higher risk of developing PML include:

  • people with impaired immune systems due to HIV or treatment with certain medications
  • people with autoimmune conditions, such as MS, who receive treatment with drugs that suppress the immune system
  • people with certain types of cancer, such as lymphoma and leukemia
  • people taking specific medications after an organ transplant

Medications that might lead to the development of PML in people with the JC virus include:

Reactivation of the JC virus might cause cell damage and destruction. The inflammatory reaction associated with the virus may lead to a breakdown of the protective coating of nerve cells, damaging the white matter of the brain.

PML is an aggressive, potentially fatal disease that affects the brain, and no cure is available at the moment. Its symptoms may begin rapidly and develop over a few weeks, or they may progress slowly, taking months to develop.

Symptoms of PML can include:

  • apparent clumsiness
  • weakness
  • paralysis or a lack of coordination
  • changes in vision, such as vision loss and blindness
  • speech impairments
  • changes in thinking and behavior
  • seizures
  • headaches
  • changes in gait

These symptoms can worsen over time.

A doctor will consider a person’s symptoms and medical history. They will also carry out a physical examination. An MRI of the brain can reveal lesions.

The symptoms of an MS relapse can be similar to those of PML. If a person with MS notices an unexpected and persistent worsening of symptoms, they should seek medical advice.

There are no antiviral medications to cure infection with the JC virus. The main option is to discontinue the use of any drugs that may be contributing to PML and seek supportive treatment.

Some experts are currently investigating whether or not a drug called hexadecyloxypropyl-cidofovir can suppress the JC virus. Doctors use other drugs in specific cases.

As the immune system recovers, some people have a reaction known as immune reconstitution inflammatory syndrome (IRIS). Corticosteroids may help reduce the inflammatory effects of IRIS, but they are not suitable for people with HIV.

The JC virus is common in many people but usually remains dormant in the kidneys without causing any problems.

The link between the JC virus and MS is that some medications for treating MS suppress the immune system. This can cause the virus to become active, leading to a potentially fatal condition called PML.

PML occurs due to an attack on nerve cells that breaks down their myelin coating and causes them to die, damaging the white matter of the brain. PML is difficult to diagnose, and no treatment is currently available.

People with MS who are using certain drugs to suppress the immune system should speak with their healthcare team about the risks of developing PML.