No one is sure what causes multiple sclerosis, but a preliminary study out Monday hints that a combination of low sun exposure and the "kissing disease" mononucleosis could raise a person's risk for the condition.
Looking at 7 years' worth of data from England, researchers found that areas with a relatively lower amount of sunlight and higher rates of infectious mononucleosis also had relatively higher rates of multiple sclerosis.
Together, sunlight intensity and mononucleosis cases explained 72 percent of the geographical variations in multiple sclerosis rates across England, the researchers report in the journal Neurology.
Exactly what the pattern means is not clear.
The researchers speculate that low sun exposure, which leads to lower vitamin D levels in the body, may alter the immune response to the virus that causes mono.
That, in turn, might make some people more susceptible to developing multiple sclerosis, or MS.
MS is thought to arise when the immune system mistakenly attacks the protective sheath around nerve fibers in the brain and spine -- leading to symptoms like muscle weakness, numbness, vision problems and difficulty with balance and coordination.
What triggers the abnormal immune response is still unknown, but experts have long suspected that certain viruses might be to blame. One of the leading candidates has been the Epstein-Barr virus that causes infectious mononucleosis in some people.
Several studies have also suggested that the vitamin D provided by sun exposure may offer some protection.
MS is more common farther away from the equator, and some research has found that people with MS report less lifetime sun exposure than those without the disease.
The "sunshine" vitamin is needed for healthy immune functioning.
"It's possible that vitamin D deficiency may lead to an abnormal response to the Epstein-Barr virus," Dr. George C. Ebers of Oxford University in the UK, one of the researchers on the new study, said in a written statement.
For their study, Ebers and his colleagues looked at data on hospital admissions for MS, infectious mononucleosis and certain other infections across England during the years 1998 to 2005.
They also used NASA data to estimate the intensity of ultraviolet light in different regions of the country in all seasons.
Overall, the researchers found that differences in UV intensity seemed to explain 61 percent of the geographical differences in how frequently MS occurs in the population. Taken together, UV intensity and mononucleosis rates seemed to explain 72 percent of the variations across the country.
None of that proves that low sunlight, vitamin D deficiency or Epstein-Barr can cause MS. The researchers looked only at geographical patterns, and, they write, "causative inferences must be tentative."
Indeed, some scientists doubt that Epstein-Barr plays any role in MS.
In a study published last year in the same journal, researchers found no evidence of Epstein-Barr in spinal fluid samples from people with MS, and little evidence of it in the brain lesions that mark MS.
At the time, senior researcher Dr. Donald Gilden, of the University of Colorado School of Medicine, told Reuters Health that his team felt "quite confident that (Epstein-Barr) has no role in causing MS."
Other researchers disagree, and are continuing to study how Epstein-Barr, in concert with other factors, might contribute to MS.
While Epstein-Barr causes mono in only a minority of people infected, the virus itself is extremely common. Up to 95 percent of people worldwide become infected at some point in their lives; afterward, the virus remains dormant in the body -- residing in certain immune-system cells.
Most people who harbor Epstein-Barr do not develop MS -- which in the U.S., affects about 350,000 people. Nor do most people who shun the sun.
But Ebers and his colleagues say there is a "pressing need" for further studies to see whether vitamin D levels and a history of mono might affect people's risk of developing MS.
If they do, the researchers write, that would open the door to using vaccines against Epstein-Barr -- which do not currently exist -- or vitamin D supplements to help prevent some cases of MS.