Whether you're the oldest, youngest or middle child in your family, your risk of contracting multiple sclerosis (MS) is about the same as every one of your siblings, younger or older. That's the opinion of Canadian doctors who published a study on the effect that birth order in families has on MS risk.1
Hygiene Hypothesis
Earlier studies, such as one published earlier this year,2 suggested that the so-called hygiene hypothesis played a role in a person's risk of developing MS, based on his or her contact with siblings.2 According to the hypothesis, the cleanliness of our society is not allowing children to become exposed to disease-causing organisms.
As a result, their immune systems don't become primed as they would if they were exposed to an infection. This, in turn, increases the risk of disease, given a weakened immunity.3 On the other hand, as earlier studies suggest, being exposed more often to siblings who pass on infection helps boost a child's immune system, and fights off diseases like MS. In this context, experts suggest that a virus, such as Epstein-Barr, may play a role in the origins of MS. Fighting off the virus, as such, would help keep the disease at bay.
Birth Order Doesn't Affect Your Risk: Study
The latest study from Canada, however, tosses that theory aside. A. Dessa Sadovnick, PhD, in the department of Medical Genetics at the University of British Columbia and her colleagues measured the risk of MS based on a particular child's birth order in a family, theorizing that contact with older siblings would decrease the risk.
The investigators collected data on nearly 11,000 MS patients from a large population-based registry. The information was compared to that of 26,336 people without the disease, collected from the same registry.
After thoroughly analyzing all the information, Sadovnick and her team "found no relation between MS risk and birth-order position." In families with at least seven siblings, slightly more siblings who were born later had MS. But the same was found for first-born siblings, as well, the study authors wrote.
Thus, the findings do not support the hygiene hypothesis, they concluded, "and could be due to a cohort effect resulting from increasing MS incidence. Birth order has no effect on MS risk in most families, and there is no support for the hypothesis that having older siblings protects against MS."
1. Sadovnick AD, Yee IM, Ebers GC. Multiple sclerosis and birth order: a longitudinal cohort study. Lancet Neurol Epub 24 Aug 2005.
2. Ponsonby AL, van der Mei I, Dwyer T et al. Exposure to infant siblings during early life and risk of multiple sclerosis. JAMA 2005 Jan 26;293(4):463-9.
3. Goldstein S. The Hygiene Hypothesis. Allergy&Asthma Advocate. American Academy of Allergy, Asthma & Immunology. Available at: http://www.aaaai.org/patients/advocate/2004/winter/hygiene.stm. Accessed August 26, 2005.
John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include overseeing health news coverage for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.
