Exposure to infant siblings decreases multiple sclerosis risk

Adults who report having had higher exposure to infant siblings during their first 6 years of life have a reduced risk of multiple sclerosis (MS), according to a study in the January 26 issue of JAMA.

The "hygiene hypothesis" proposes that early life infections may reduce the risk for allergic and autoimmune disorders, and can influence the developing immune system, according to background information in the article. Having siblings may increase the number of early-life infections, and lack of contact with siblings has been associated with several immune disorders. Younger siblings may be important because infants provide a source of common viral infections. Re-exposure to active viral infection is known to cause immune boosting. A protective role for early life infection in the development of MS is consistent with several features of MS, including the apparent recent increase in incidence that has accompanied a decline in childhood infection rates over time.

Anne-Louise Ponsonby, Ph.D., of the Menzies Research Institute, Hobart, Australia, and colleagues conducted a study to determine whether exposure to infant siblings in early life is associated with the risk of MS, and to explore the possible mechanism of any protective effect, such as elevated Epstein-Barr virus (EBV) antibodies. The study, conducted in Tasmania, Australia, from 1999 to 2001, included 136 cases (average age, 43.5 years) of magnetic resonance imaging–confirmed MS and 272 community controls (average age, 43.6 years), matched on sex and year of birth.

The researchers found that increasing duration of contact with a younger sibling aged less than 2 years in the first 6 years of life was associated with reduced MS risk: 1-3 years of infant contact, a 43 percent reduced risk; 3 to 5 years of infant contact, a 60 percent decreased risk; greater than 5 years of infant contact, an 88 percent reduced risk.

The researchers also found that a history of exposure to infant siblings was associated with a reduced IgG response (one of the major classes of immunoglobulins; the main antibody defense against infection) to EBV among controls. Controls with at least 1 infant-year contact had a reduced risk of infectious mononucleosis and a reduced risk of very high composite EBV IgG titers compared with other controls. Infectious mononucleosis and elevated EBV IgG titers have previously been shown to increase the risk of subsequent MS. The inverse association between higher infant contact and MS was independent of EBV IgG titer.

"Further work is required to confirm [these findings] and elucidate underlying mechanisms. The finding that higher infant contact in early life was associated with reduced EBV antibody titers and a reduced likelihood of infectious mononucleosis among healthy controls strengthens the inference that infant contact in early life may alter childhood infection patterns and related immune responses and reduce the risk of MS," the authors conclude.