Younger siblings cut multiple sclerosis risk

Children who live with younger siblings during the first six years of childhood are much less likely to develop multiple sclerosis later in life, a new study suggests.

The finding backs the so-called “hygiene hypothesis” which proposes that exposure to infectious bugs early in life – lurking in household dirt or carried by younger siblings – reduces the risk of allergic and autoimmune diseases by stimulating the immune system.Multiple sclerosis (MS) is an autoimmune disease in which the body’s immune system is thought to attack the fatty coat which insulates nerve cells. Damage to this sheath stops the nerves conducting electrical signals properly.The study showed that living with a toddler sibling for over five years could reduce the risk of developing MS by almost 90%.”This possibly occurs by altering childhood infection patterns and related immune responses,” says Anne-Louise Ponsonby at the Australian National University in Canberra, who led the study.She says the results, if replicated, could help in understanding how to prevent MS, but adds: “It is important to emphasise that MS is a complex multifactorial disease with both genetic and environmental factors.”Viral antibodiesThe team examined 136 patients diagnosed with MS and 272 healthy matched controls in Tasmania, Australia. All the volunteers were interviewed to find out more about their childhood environment.Blood samples were also taken to test for specific antibodies to certain viruses, in particular for the Epstein-Barr virus (EBV) which can cause mononucleosis (glandular fever). High levels of antibodies against this virus or becoming ill with mononucleosis have previously been associated with a higher risk of developing MS.The study found that people who had been exposed to younger siblings for more than five years had an 88% reduced risk of suffering MS. Those with three to five years of contact with infant siblings had a 60% reduced risk. And those with one to three years living with a young sibling had a 43% reduced risk of developing MS.People without MS were also less likely to have high levels of EBV antibodies, or to have suffered from infectious mononucleosis.Repeated stimulation”The new study showed that, among healthy controls, higher infant contact reduced the risk of infectious mononucleosis or elevated EBV antibodies. That is, it reduced the risk of having these previously identified MS risk factors,” Ponsonby told New Scientist.A possible explanation, she says, “is that repeated stimulation of the immune system by common infant infections enhances the immune response against them”. That would apply in particular to viruses of the herpes group, of which EBV is a member. “The enhanced immune response may then reduce the risk of MS,” she says.MS affects about 2.5 million people worldwide. It has become increasingly common in recent decades. In Australia, its prevalence almost tripled from 20 to 59 people per 100,000 of the population between 1961 and 1996. The hygiene hypothesis links this leap with a decrease in childhood infections due to improvements in sanitation and healthcare in the developed world.(Source: Journal of the American Medical Association;vol293;p463: News Scientist: January 2005.)