By using data collected over decades in a database of more than 10 million active-duty military personnel, researchers have managed to nail down the connection between Epstein-Barr virus (EBV) infection and multiple sclerosis (MS).

The work, which was published in Science on January 13, 2022, "provides pretty definitive evidence that multiple sclerosis is a complication of infection with Epstein-Barr virus," corresponding author Alberto Ascherio told BioWorld.

Ascherio is a professor of epidemiology and nutrition at Harvard University's T.H. Chan School of Public Health.

EBV had been putatively linked to MS in previous studies. But almost 95% of the general population are infected with the virus, often from childhood, making it hard to tease out whether there is a causal link – and if so, which condition is cause and which effect.

Ascherio and his colleagues were able to answer this question by using serum samples from HIV screenings in active duty army and air force members. Recruits are screened for HIV when they join the army or air force, and every 2 years while they are on active duty. Even with a 95% rate of EBV infection, the sheer size of the database meant that the sample contained hundreds of thousands of individuals who were not infected with EBV at the time they joined up.

The researchers first compared serum samples from 801 service members with MS in their sample to roughly 1,600 matched controls. The first sample from 35 of the MS patients and roughly 100 controls showed no evidence of EBV infection. Of the MS patients, 34 had become infected with EBV when they were last sampled before their MS diagnosis. Overall, this meant that 800 of 801 MS cases had evidence of EBV infection. "I was really surprised by how clear-cut the results were," Ascherio said. "It is rare in science that you get a black and white answer... Usually, it's hints in one way or the other."

While there were also controls who became infected with EBV over the course of the study without developing MS, at 60%, their proportion was much lower.

To rule out that causality ran in the other direction – that is, MS deregulated the immune system, increasing the risk for viral infection – the team used an assay that measures the human immune response to the complete proteome of roughly 200 viral species – most known pathogenic viruses. Testing more than 100,000 peptides, the team did not see any differences in how the immune systems of both groups responded.

How EBV, which infects antibody-producing B cells, causes problems for the myelin sheath in the central nervous system, which is the main affected tissue in MS, is not yet clear. In a commentary that was published alongside with the paper, researchers from Stanford University listed cross-reactivity as one possibility. Antibodies from MS patients can react to certain parts of the viral protein EBNA-1. Interestingly, another part of EBNA-1 can be recognized by antibodies from patients with systemic lupus erythematosus, which has also been linked to EBV infection. Translationally, the work suggests that targeting EBV, whether through a vaccine or antivirals, could be a way to reduce or even eradicate MS.

The method, meanwhile, could be used to identify viral triggers for other disorders. Ascherio, whose team has also looked for infections that could trigger amyotrophic lateral sclerosis (ALS), said that "Many viruses cause rare complications in some people... [it's] more the rule than the exception."