Multiple sclerosis (MS) is a chronic (long-term) help condition that affects the brain and spinal cord. It disrupts the way the brain communicates with the rest of the body. MS is believed to result from a combination of factors — genetics, immune responses, and environmental conditions — and the way these elements interact.

Environmental factors linked to a higher risk of MS include smoking, vitamin D levels, and exposure to certain viruses. One of the earliest environmental clues researchers discovered is geography — specifically, where a person lives in the world.

Studies on the epidemiology of MS — or how MS affects different populations — show that the condition is generally more common in places that are farther from the equator.

It might seem surprising that MS, an autoimmune disease of the central nervous system, is connected to a person’s location. Researchers are still working to understand this pattern, but they have several theories.

Where Is MS Most Common?

According to the National Multiple Sclerosis Society, areas farthest from the equator have the highest prevalence of MS. Conversely, MS is rare in populations that live near the equator.

A 2016 study of the geographical distribution of MS found areas with the highest prevalence of MS to be North America, western Europe, and Australasia (New Zealand and Australia). Researchers found the lowest rates of MS in sub-Saharan Africa and Oceania — particularly among island nations such as Fiji, Guam, Samoa, and Tonga. Researchers noted that these findings reflect a distinct global pattern, where MS is more common in higher-income countries farther from the equator.

However, the study’s authors noted that in many countries with lower reported rates of MS — especially in parts of Africa, Asia, and Oceania — there was limited access to high-quality health records. This means the true number of people living with MS in these areas may be undercounted.

According to MS International Federation’s Atlas of MS, the countries and regions with the highest prevalence of MS (more than 150 cases per 100,000 people) include:

• San Marino (highest reported prevalence worldwide)
• Canada
• United States
• Germany
• Denmark
• United Kingdom
• Southern Australia
• Italy
• Iceland

Countries and regions with moderate MS prevalence (50 to 149 cases per 100,000) include:

• Russia
• Northern Australia
• Turkey
• Egypt
• Hungary

Lower MS prevalence (fewer than 50 cases per 100,000) is reported in:

• Most of Africa
• Most of South America
• Southeast Asia
• China

MS Patterns Within Countries

Patterns in MS prevalence don’t just appear between countries — they also show up within them. In Australia, for instance, people living in southern areas like Tasmania are nearly twice as likely to develop MS as those living in northern regions like Queensland. This north-south difference suggests that environmental factors may influence MS risk even within the same country.

How Moving Affects a Person’s Risk of MS

Researchers have found that moving to a new location before puberty can change a person’s risk of developing MS. If someone moves from a low-risk area (closer to the equator) to a high-risk area (farther from the equator) before they turn 15, their risk of developing MS becomes similar to that of people who grew up in the new location. The opposite is also true: Moving from a high-risk to a low-risk area before puberty appears to lower a person’s MS risk.

How Could Geography Influence the Risk of MS?

Neurology researchers believe that several factors associated with geography may help explain why MS is more common in some parts of the world than others.

Temperature

In general, temperatures get colder the farther a place is from the equator. MS is more common in temperate regions, where seasonal temperature changes are greater, than in tropical areas with steady, warmer climates.

Temperature itself likely doesn’t cause MS, but it may influence other risk factors. For example, viruses such as Epstein-Barr virus (EBV) — which has been linked to MS — may spread more easily in colder climates, where people spend more time indoors in close contact with others. In warmer climates, people may spend more time outside and be more spread out, which could reduce the spread of such viruses.

More research is needed to fully understand how temperature and viral infections might interact to affect MS risk.

Vitamin D

Vitamin D is made in the body when skin is exposed to sunlight. It plays a key role in calcium absorption, immune function, and brain health.

People who live closer to the equator get more year-round sun exposure and often have higher natural levels of vitamin D. According to the National Multiple Sclerosis Society, adequate vitamin D levels may help reduce the risk of developing MS. There’s also evidence that vitamin D supplements may improve symptoms in people already diagnosed with MS.

While scientists don’t yet know exactly how vitamin D protects against MS, vitamin D deficiency is one of the most consistently linked environmental risk factors for MS.

Diet

What people eat — and what foods are available where they live — may also influence MS risk.

Some studies suggest that people who live closer to the ocean tend to eat more fish, which is rich in omega-3 fatty acids and vitamin D. These nutrients are known to reduce inflammation and may help lower the risk of MS.

For example, a research study in Norway found that people in coastal regions who ate more fish had a lower rate of MS than people living inland, who ate less fish. Inland populations were more likely to eat diets high in saturated fats from meat and dairy products, which generally increase inflammation in the body. These populations also tend to have a higher prevalence of multiple sclerosis.

Although researchers can’t say for sure that a fish-rich diet directly prevents MS, it’s possible that regional dietary patterns play a role in the connection between geography and MS risk.

Genes and the Environment Both Affect the Risk of MS

While MS is more common in certain parts of the world, geography alone doesn’t determine who will develop the condition. Research shows that a mix of genetic and environmental factors likely shapes a person’s risk of MS.

For example, consider Rochester, Minnesota; London, Ontario; and Vancouver, British Columbia. These cities are at similar latitudes, meaning they’re roughly the same distance from the equator. Yet MS rates in Rochester are nearly double those in the two Canadian cities. This difference suggests that something beyond geography is at play.

Scientists believe that interactions among genetics, the immune system, and environmental exposures ultimately influence MS risk. Ongoing research continues to explore how these factors work together — and may one day lead to better ways to prevent MS.

If you’re living with MS, talk with your neurologist or another healthcare provider about treatment options and what’s right for your individual situation.

Talk With Others Who Understand

MyMSTeam is the social network for people with multiple sclerosis and their loved ones. On MyMSTeam, members come together to ask questions, give advice, and share their stories with others who understand life with MS.

Do you live in an area with a high prevalence of MS? How might your environment affect your MS? Share your experience in the comments below, or start a conversation by posting on your Activities page.