• Researchers recently studied the link between adolescent infections and multiple sclerosis (MS).
• They found that any hospital-treated infections in adolescence, rather than in earlier childhood, increased the risk of a multiple sclerosis diagnosis.
• Infections affecting the central nervous system (CNS) were most associated with MS risk.

Individuals who experience serious infections during adolescence face a greater risk of developing multiple sclerosis later in life, according to a study out of Sweden published in the scientific journal Brain.

The scientists estimated that the chance of an MS diagnosis goes up by 33 percent among those who had any hospital-diagnosed infections before age 20. However, they stressed that the odds overall of developing the disease after severe infection were still small.

“The vast majority of people who have such infection will not go on to develop MS, as other factors are required for MS pathogenesis [development of the disease],” said study author Scott Montgomery, Ph.D., a clinical epidemiologist and director of the clinical epidemiology group at Örebro University and Örebro University Hospital.

In an interview with MyMSTeam, Montgomery explained this study provides further evidence that adolescence is a period of heightened susceptibility to exposures linked with MS risk, and that there can be many years between exposure and MS diagnosis.

Certain infections are already established risk factors for MS. In particular, the Mayo Clinic cites Epstein-Barr virus (the virus that causes infectious mononucleosis) as having a link to the illness.

For this most recent analysis, Montgomery and his colleagues were interested in evaluating whether the risk extended to infections in a broader sense.

Drawing on health data from more than 2.4 million people born in Sweden between 1970 and 1994, the investigators identified 4,022 who were diagnosed with MS after age 20 (the average age at diagnosis was 30). From the same population of 2.4 million people, the researchers found 462,157 (19.07 percent) had hospital-diagnosed infections between birth and age 10. Another 338,352 individuals (13.96 percent) had hospital-diagnosed infections between age 11 and 19.

A statistical analysis revealed that none of the infections by age 10 were connected with risk of an MS diagnosis. Any infection in adolescence, however, heightened the chances of a multiple sclerosis diagnosis by one-third (33 percent), with infections affecting the central nervous system posing the greatest risk increase (85 percent).

“The main infection linked to MS previously is by the Epstein-Barr virus,” said Montgomery. “This paper indicates a variety of infections — both viral and bacterial — during adolescence are associated with subsequent MS.”

An infection in the CNS itself is likely to cause significant inflammation, Montgomery explained, and that could trigger autoimmune demyelination — damage to the myelin sheath, the protective covering that surrounds nerve fibers in the brain, optic nerves, and spinal cord.

Montgomery told MyMSTeam, “I suspect that only infections that can cross the blood-brain barrier or otherwise increase inflammation risk in the central nervous system could trigger the autoimmune processes leading to MS in genetically susceptible individuals.”

Respiratory infections in adolescence were also associated with MS, as were infections of the genitals or urinary tract — but to a lesser degree. The study authors suggested that these causes of inflammation in other parts of the body could lead to inflammation in the central nervous system. The research noted that not all sites of infection — such as the skin and gastrointestinal system — were associated with MS.

Dr. Christopher Lock, a multiple sclerosis specialist and general neurologist with Stanford Health Care, emphasized to MyMSTeam that these study results should not overly concern those who have had serious infections as young adults.

“It’s a very low risk of developing MS, so I probably wouldn’t worry parents,” said Dr. Lock, who was not affiliated with the study.

The Multiple Sclerosis Association of America reports the average likelihood of developing MS in the United States is less than half of 1 percent. “MS, like other autoimmune conditions, is a complicated interplay of genes and environment,” Dr. Lock said. “So other factors are involved for the disease to develop.”