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Younger siblings protect against multiple sclerosis

CANBERRA, AUSTRALIA. Multiple sclerosis (MS) is a chronic neurological disorder that affects the central nervous system. Primarily affecting adults, MS results in impaired nerve signaling and problems with muscle control, strength, vision, and balance. The cause is unknown, but thought to involve environmental, viral, and genetic factors. The "hygiene hypothesis" is put forward as a possible explanation for many autoimmune disorders, including MS. It proposes that exposure to infections during early life has a protective effect on the developing immune system. A research team from the Australian National University has investigated the role of siblings in infection exposure and risk of MS.

They set up a case-control study in Tasmania, Australia, involving 136 MS patients and 272 healthy matched participants for comparison. Participants were interviewed and their blood tested for antibodies to certain viruses, in particular for the Epstein-Barr virus (EBV) which can cause mononucleosis (glandular fever) and has been linked to MS risk in previous studies. Results showed that in the first 6 years of life contact with a sibling aged less than 2 years was linked to a reduced MS risk, the effect increasing with years of exposure. Living with a younger sibling for over five years reduced the risk of developing MS by almost 90 per cent. Three to five years of contact led to a 60 per cent reduced risk and one to three years led to a 43 per cent reduced risk. Furthermore, when adults with high exposure to younger siblings did develop MS, they tended to develop it at an older age. Healthy participants with younger siblings showed a reduced IgG response to EBV, and were less likely to have developed mononucleosis.

The researchers conclude that repeated stimulation of the immune system by common infant infections may ‘down-regulate’ allergic and autoimmune disorders, reducing MS risk. The hygiene hypothesis suggests that the recent rise in MS is linked to the decrease in childhood infections through improved sanitation and healthcare. However the researchers add that MS is a complex multifactorial disease including both genetic and environmental factors.
Ponsonby, A et al. Exposure to Infant Siblings During Early Life and Risk of Multiple Sclerosis. Journal of the American Medical Association, Vol. 293, January 2005, pp463-469

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