Risk Factors

An estimated 400,000 Americans and 2.5 million people worldwide suffer from MS.

General Risk Factors

Age. Onset occurs between the ages of 20 to 40 years in 70% of patients with the average age being 30 and the peak incidence occurring in the mid-twenties. The disease can still occur in both younger and older individuals. It rarely develops before age 15 or after age 60, however.

Gender. Women are affected twice as often as men (although among older individuals there is less of a gender difference). The incidence in women also appears to be increasing, at least in the US. In a 2002 study, prevalence of MS in women in the early 1990s was 50% higher than in the early 1980s. Men may be more disabled by the disease than women.

Ethnicity. Multiple sclerosis occurs worldwide but is most common in Caucasian people of northern European origin, especially those of Scottish descent. It is extremely rare among Asians, Africans, and Native Americans. Specific groups (gypsies, Eskimos, Bantus) have never reported a case. While the risk of MS for African Americans is around half of that for Caucasian Americans, a recent study suggested that African Americans are more likely to develop a more aggressive form of the disease and to suffer impaired mobility.

Geography. The risk for MS is higher in different regions of the world. In general, MS is more prevalent in temperate regions than in the tropics. Specifically, prevalence is highest in northern and central Europe (except northern Scandinavia), Italy, southern Australia, and northern regions of North America. Middle-risk areas include southern Europe (except Italy), southern US, northern Australia, and northern Scandinavia. Low-risk areas include parts of Africa and Asia, the Caribbean, Mexico, and possibly northern South America. It is unclear whether this pattern is attributable to environmental factors, genetics, or both.

Family History. A family history of the disease also puts people at risk for MS, although the risk for someone inheriting all the genetic factors contributing to MS is only about 2 - 4%. Nevertheless, when siblings have the disease, they are more likely to have the same degree of severity. Among identical twins the risk is about 25 - 30%.

Factors Associated with a Higher Risk for Multiple Sclerosis

Cow's Milk During Early Infancy. Breast milk contains factors that may help regulate the immune response, and there is some evidence that infants fed only on cow's milk may have a higher risk for either diabetes type 1 (another type of autoimmune disease) or multiple sclerosis later in life. Studies on national differences in diabetes suggest that the risk may vary with different milk proteins, suggesting that not all cow's milk is the same and some proteins carry higher risks than others.

Factors Associated with Lower Risk for Multiple Sclerosis

The Hygiene Theory: Early Infections as Protection Against Allergies and Autoimmune Diseases. Over the past decades, there has been a dramatic increase in asthma, allergies, and autoimmune diseases, such as multiple sclerosis, Crohn's disease, and type 1 diabetes. One theory blames this rise on the reductions in childhood infections that have occurred with modern hygiene and antibiotic use. Studies supporting this have observed a higher incidence of allergies and autoimmune diseases, including MS, among populations with good hygiene and in animals that have been raised in a germ-free environment. The basic theory rests on the idea that early infections stimulate production of specific immune factors that protect against allergies and autoimmune diseases. The exact mechanisms of these effects are as yet unknown. Of note, there is no evidence indicating either a positive or a negative role of vaccinations in the development of autoimmune or allergic diseases.

Exposure to Sunlight. In a 2003 study, higher exposure to sunlight during childhood and early adolescence was associated with a lower risk for MS, perhaps because UV radiation produces higher levels of vitamin D, which has been associated with protection against MS. The effect of sunlight during winter seemed to be more protective than summer light. Unfortunately, higher exposure to sunlight also coincides with a higher risk for skin cancer, which is far more common than MS.

Estrogen and Oral Contraceptives. Higher estrogen levels may temporarily lower the risk of developing multiple sclerosis. Studies indicate that oral contraceptives (which contain estrogen) and pregnancy delay the onset of multiple sclerosis. The risk for a first clinical attack increases, however, in the 6 months after a woman gives birth.