The cause of MS is unknown; however, it is believed to occur as a result of some combination of genetic and environmental factors such as infectious agents. Geography MS is more common in people who live farther from the equator, although exceptions exist. These exceptions include ethnic groups that are at low risk and that live far from the equator such as the Samis, Amerindians, Canadian Hutterites, New Zealand Māori, and Canada's Inuit, as well as groups that have a relatively high risk and that live closer to the equator such as Sardinians, inland Sicilians, Palestinians, and Parsi. The cause of this geographical pattern is not clear. While the north–south gradient of incidence is decreasing, as of 2010 it is still present. MS is more common in regions with northern European populations and the geographic variation may simply reflect the global distribution of these high-risk populations. A relationship between season of birth and MS lends support to this idea, with fewer people born in the northern hemisphere in November compared to May being affected later in life. Environmental factors may play a role during childhood, with several studies finding that people who move to a different region of the world before the age of 15 acquire the new region's risk to MS. If migration takes place after age 15, however, the person retains the risk of their home country. There is some evidence that the effect of moving may still apply to people older than 15. Genetics HLA region of Chromosome 6. Changes in this area increase the probability of getting MS. MS is not considered a hereditary disease; however, a number of genetic variations have been shown to increase the risk. Some of these genes appear to have higher levels of expression in microglial cells than expected by chance. The probability of developing the disease is higher in relatives of an affected person, with a greater risk among those more closely related. An identical twin of an affected individual has a 30% chance of developing MS, 5% for a non-identical twin, 2.5% for a sibling, and an even lower chance for a half-sibling. If both parents are affected the risk in their children is 10 times that of the general population. MS is also more common in some ethnic groups than others. Specific genes that have been linked with MS include differences in the human leukocyte antigen (HLA) system—a group of genes on chromosome 6 that serves as the major histocompatibility complex (MHC). That differences in the HLA region are related to susceptibility has been known since the 1980s, and this same region has also been implicated in the development of other autoimmune diseases such as diabetes type I and systemic lupus erythematosus. The most consistent finding is the association between multiple sclerosis and alleles of the MHC defined as DR15 and DQ6. Other loci have shown a protective effect, such as HLA-C554 and HLA-DRB1*11. Overall, it has been estimated that HLA differences account for between 20% and 60% of the genetic predisposition. Modern genetic methods (genome-wide association studies) have revealed at least 200 variants outside the HLA locus that modestly increase the probability of MS. Infectious agents Many microbes have been proposed as triggers of MS, but none has been confirmed. One hypothesis is that infection by a widespread microbe contributes to disease development, and the geographic distribution of this organism significantly influences the epidemiology of MS. Two opposing versions of this hypothesis include the hygiene hypothesis and the prevalence hypothesis, the former being more favored. The hygiene hypothesis proposes that exposure to certain infectious agents early in life is protective; the disease is a response to a late encounter with such agents. The prevalence hypothesis proposes that an early, persistent, and silent infection increases risk of disease, and thus the disease is more common where the infectious agent is more common. Only in a few cases and after many years does it cause demyelination. Evidence for a virus as a cause include the presence of oligoclonal bands in the brain and cerebrospinal fluid of most people with MS, the association of several viruses with human demyelinating encephalomyelitis, and the occurrence of demyelination in animals caused by some viral infections. Human herpes viruses are a candidate group of viruses. Individuals having never been infected by the Epstein–Barr virus are at a reduced risk of getting MS, whereas those infected as young adults are at a greater risk than those having had it at a younger age. Although some consider that this goes against the hygiene hypothesis, since the non-infected have probably experienced a more hygienic upbringing, others believe that there is no contradiction, since it is a first encounter with the causative virus relatively late in life that is the trigger for the disease. Other diseases that may be related include measles, mumps and rubella. Other Smoking may be an independent risk factor for MS. Stress may be a risk factor, although the evidence to support this is weak. Association with occupational exposures and toxins—mainly organic solvents—has been evaluated, but no clear conclusions have been reached. Vaccinations were studied as causal factors; however, most studies show no association. Several other possible risk factors, such as diet and hormone intake, have been looked at; however, evidence on their relation with the disease is "sparse and unpersuasive". Gout occurs less than would be expected and lower levels of uric acid have been found in people with MS. This has led to the theory that uric acid is protective, although its exact importance remains unknown. Obesity during adolescence and young adulthood is a risk factor for MS.
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