Multiple sclerosis and neuromyelitis optica in tropical and subtropical countries.

Among the indigenous people of tropical countries neuromyelitis optica is more common than multiple sclerosis which is exceptionally rare. It is postulated that these diseases are the result of virus-induced defects incorporated in the myelin at the time of myelination. These defects are located in those parts of the CNS which are undergoing active myelination at the time of the virus infection. A protective factor, which is capable of abolishing the defect is produced, directly or indirectly, by solar radiation. In tropical countries this factor abolishes most of the defects but it is unable to do so in the case of neuromyelitis optica because optic nerves and some parts of the spinal cord are myelinated before infants are exposed to such protective environmental factors. In temperate climates virus infections leave defects in many parts of the CNS and, lacking exposure to the protective environmental factor, these defects slowly mature to become plaques of multiple sclerosis.