Birth order doesn’t influence MS risk, study shows

Contrary to what the “hygiene hypothesis” suggests, the youngest children in a family are not less likely than older siblings to develop multiple sclerosis (MS), new research suggests.

According to the hygiene hypothesis, being too clean is not a good thing.

Infections at an early age actually trains the immune system to respond appropriately to the environment, and this protects kids against certain diseases like asthma and MS, so the argument goes.

Having older brothers and sisters should raise the odds of such infections, and therefore reduce the risk of MS.

Multiple sclerosis, or MS, is a lifelong autoimmune disorder that can cause severe disability. An autoimmune disorder is one in which the body produces antibodies that attack its own tissue. People with MS produce antibodies that attack the white matter in the brain and spinal cord.

To look into this idea, Dr. A. Dessa Sadovnick, from the University of British Columbia in Vancouver, Canada, and colleagues analyzed data from 10,995 MS patients and 26,336 healthy siblings drawn from a large population-based registry.

Late birth order did not cut the risk of MS, the researchers report in the online issue of The Lancet Neurology medical journal.

In fact, in large families with only one affected sibling, there was some evidence that late birth order actually raised the risk, as affected siblings were slightly younger than unaffected siblings.

What are the causes and risks of the disease?

The cause of multiple sclerosis is not known. There are four major scientific theories about the cause of MS:

Environmental. Worldwide, MS seems to be more common around the 40th parallel in the Northern and Southern hemispheres and is more prevalent in northern Europe. A person who is born in one of these areas but moves to another area before adolescence has a lower risk of developing MS.

Genetic. Having a parent or sibling with MS significantly increases a person’s risk of MS.

Immunologic. It is generally accepted that multiple sclerosis is an autoimmune disorder.

Viral. It is possible that a viral infection can trigger MS.

Some neurologists believe that MS develops because a person is born with a genetic predisposition to react to an environmental agent. When that person comes into contact with the agent, the contact triggers an autoimmune response that causes MS.

“This study does not support the prediction of the hygiene hypothesis,” the team concludes.

SOURCE: Lancet Neurology, online August 22, 2005.

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