Individuals who have had infectious mononucleosis have an increased risk of developing multiple sclerosis (MS), even 30 years after infection, according to findings published in the Archives of Neurology.

While “mono” has been linked with MS before, “little is known about the characteristics of this association,” write Dr. Trine Rasmussen Nielsen, of Statens Serum Institut, Copenhagen, and colleagues.

The researchers monitored a group of 25,234 Danish patients who had had mononucleosis for the development of MS, starting on April 1, 1968 or in January the year after mononucleosis was diagnosed. The main outcome measure was the ratio of observed to expected MS cases (standardized incidence ratio).

There were 104 MS cases observed versus 45.91 MS cases expected - or 58 more cases than normal. This corresponds to a standardized incidence ratio of 2.27, or more than twice the risk of MS, Nielsen’s team reports.

The increased MS risk continued for more than 30 years after infectious mononucleosis. No differences in MS risk were seen according to sex or age, and the severity of mononucleosis did not appear to influence risk.

The investigators hypothesize that this long-term continuation of MS risk “may reflect a change in immunological status” after mononucleosis, which should be further explored.

SOURCE: Archives of Neurology, January 2007.