April 18, 2011 -- Having a history of mononucleosis and living in an area that gets little sunlight both appear to increase the risk for developing multiple sclerosis, new research finds.
Multiple sclerosis (MS) is far more common in regions that get little sunlight most of the year, such as Scandinavia, Canada, and the Northern U.S.
Exposure to sunlight allows the skin to produce vitamin D, and vitamin D deficiency has been linked to MS in some studies. Having a history of mononucleosis, which is caused by the Epstein-Barr virus, has also long been suspected of being a possible trigger for MS.
In an effort to determine if the two possible risk factors could explain the geographic differences in multiple sclerosis incidence, longtime MS researcher George C. Ebers, MD, and colleagues at the University of Oxford in the U.K. conducted a study in England.
Their earlier studies in France, Scotland, and other regions of Europe suggest that differences in sunlight exposure affect MS risk even within small geographical areas.
For example, MS rates in Scotland were higher in Glasgow than in the sunnier city of Dundee.
“We wanted to find out if having had mononucleosis adds to the multiple sclerosis risk we have documented for low ultraviolet B (UVB) exposure,” Ebers tells WebMD.
To do this, the researchers examined all National Health Service (NHS) hospital admissions in England from spring of 1998 to spring of 2005, identifying almost 57,700 cases of multiple sclerosis and 14,600 cases of infectious mononucleosis.
They also used NASA satellite data on UV radiation exposure to calculate sunlight exposure within different areas of the country.
The researchers concluded that sunlight exposure and mononucleosis history together explained 72% of the variance in multiple sclerosis incidence within England during this period.
Exposure to sunlight accounted for about 61% of this variance, with having a history of mononucleosis accounting for the rest of the risk, the researchers found.
The findings do not prove that either low levels of sunlight exposure or having a history of mononucleosis cause multiple sclerosis. But Ebers says they point to the “pressing need” for larger studies to examine vitamin D, infectious mononucleosis, and MS.
“The evidence implicating UV radiation in MS is pretty strong,” he says. “It is less strong for infectious mononucleosis, but we need to learn more about this.”