SAN DIEGO — Researchers identified patients who tested positive for the Epstein-Barr Virus (EBV) 9 years after they were diagnosed with multiple sclerosis (MS), challenging the idea that MS always follows EBV.

The findings come in an analysis of nearly 100,000 patients with positive EBV tests in the Canadian province of Ontario from 2007 to 2002.

Of 300 people diagnosed with MS, 74 tested positive for EBV after their MS diagnosis. Previous research suggests EBV infection almost always precedes the development of MS. Although the number of cases was small, researchers said the total was larger than they expected.

“We required our cases to have at least three MS diagnostic codes so we cannot explain away these cases as being an isolated demyelinating attack or clinically isolated syndrome. Our findings suggest that cases of MS onset preceding EBV need to be further explored in other cohorts,” lead investigators Dalia Rotstein, MD, MPH, associate professor of medicine at the University of Toronto and MS specialist at Barlo MS Clinic at St. Michael’s Hospital Toronto, both in Toronto, Ontario, Canada, told Medscape Medical News.

The findings were presented on February 6 at the Americas Committee for Treatment and Research in Multiple Sclerosis (ACTRIMS) Forum 2026.

The EBV-MS Connection

Previous research has offered strong evidence that EBV infection increases the risk for MS, with some studies showing more than to 99% of patients with MS have been infected with the virus.

One often cited landmark study from 2022 showed a 32-fold increased risk for MS following EBV seroconversion in US military personnel. Only one person out of 801 was EBV negative in samples taken a median of 1 year before MS onset.

Those findings and others seem to suggest that EBV infection is a prerequisite for MS.

However, that study had a number of key limitations that Rotstein said limited the odds of detecting cases of MS diagnosis predating EBV from the outset.

“The US military population is predominantly male and White, and they captured people only from the age of military recruitment onward, which is older than the typical age for EBV seroconversion,” Rotstein noted.

“We were curious about what we would find when we looked for cases of MS onset before primary EBV infection in the general population,” she said.

Unanswered Questions

To explore that question, researchers identified 95,980 patients who tested positive for EBV using the Ontario Laboratories Information System database, which captures more than 80% of laboratory tests performed in Ontario. Of those, 300 had MS.

Among those with MS, 74 patients (75.7% female) had a positive EBV test after MS diagnosis.

In this group, the mean age at MS onset was 33.3 years, but the mean age of a positive EBV test was 42.5 years. The mean time from MS onset to a positive EBV test was 9.1 years.

“We don’t know yet exactly what is going on with apparent cases of MS that occur before EBV,” Rotstein said.

Possible explanations include that some patients were misdiagnosed with MS, EBV tests were false positives, or that patients had a latent EBV infection that reactivated after MS diagnosis.

“Given the number of cases, we have to entertain the probability of a third hypothesis: There is some etiologic diversity underlying what we define clinically as MS,” Rostein said.

“For example, other research has suggested an association between the HHV-6A [human betaherpesvirus] virus and the development of MS. And like EBV, HHV-6A viral load seems to rise in advance of neurofilament light-chain changes,” Rotstein added.

The unusually late age of EBV infection in the MS cohort also gave researchers pause. The median age of a positive EBV test among those with MS was twice that of all 95,980 individuals in the cohort who also tested positive for the virus (42.5 years vs 21.5 years, respectively).

“This is older than the typical age for EBV, even when we consider the symptomatic cases of EBV infection which get tested, which typically occur in the late teenage years. However, we know that about 5% of the general adult population has never contracted EBV, and it is possible for EBV infection to transpire at older ages,” Rotstein said.

A ‘Fuse’ for MS?

When presenting the findings at ACTRIMS Forum, Rotstein noted several study limitations, including a lack of blood samples to test EBV status before MS onset, a lack of EBV testing after MS onset, and a lack of other information about MS cases prior to EBV testing.

Commenting for Medscape Medical News, Michael Levy, MD, PhD, associate professor at Harvard Medical School and Massachusetts General Hospital, both in Boston, who didn’t take part in the study, highlighted other limitations as well.

For example, EBV testing has a false positive rate of up to 10%, he said.

“Out of the 300 (0.31%) tested who met the algorithm for MS, if 10% could be false positive, that’s 30 people, which represents almost half of the cases that were detected after a visit for demyelinating disease,” Levy said.

He also questioned the “highly unusual” age at testing for EBV infection.

“That’s a very strange time to get infected with EBV and immediately prompted my skepticism. A more detailed examination of these cases is warranted to explain this finding,” he said. “Why were people tested for heterophiles antibodies or VCA IgM [viral capsid antigen immunoglobulin M] at this time in the patient’s journey? There are other causes of these antibodies to be positive including lymphomas and other infections.”

Joseph Sabatino, MD, PhD, assistant professor at the Weill Institute for Neurosciences, University of California San Francisco, also questioned the findings, especially the high age of patients in the cohort compared to when EBV is typically diagnosed.

Still, Sabatino said research into the link between EBV and MS is important.

“The big question here is: What does EBV do that allows MS to occur in only a very small fraction of people? It seems clear that MS can only develop in individuals with the ‘right’ combination of genetic and environmental risk factors,” said Sabatino, who was not part of the study.

“If EBV is a fuse for MS, then we really need to understand the components of the bomb that it’s setting off. By understanding that, we may come to find that there may be other triggers that could lead to MS independent of EBV,” Sabatino said.

The study was funded by the MS Canada and the National Multiple Sclerosis Society. Rotstein disclosed having relationships with the MS Canada, the National Multiple Sclerosis Society, the Canada’s Drug Agency, the Li Ka Shing Knowledge Institute of Unity Health Toronto, Peter and Susan Gordon, Guthy-Jackson Research Foundation, the University of Toronto Division of Neurology, Amgen, and Alexion. She reported receiving speaker or consultant fees from Alexion, Amgen, Biogen, EMD Serono, Novartis, Roche, and Sanofi Aventis. Levy reported having no disclosures. Sabatino reported having relationships with Novartis, Roche/Genentech, IgM, TG, and Sift.