Smoking may increase the risk of multiple sclerosis in people who have other risk factors for the neurological disorder, researchers said on Wednesday.

The findings suggest that smokers who have high levels of a protein that protects against the Epstein-Barr virus, a common herpes virus, were twice as likely as nonsmokers to get multiple sclerosis (MS), the researchers wrote in the online edition of the journal Neurology.

Previous studies have suggested that smoking and the virus-fighting protein were independent risk factors and this research looked at how they may be associated with each other, Claire Simon of Harvard University said in a telephone interview.

"we found that that association was stronger in people who reported smoking compared with people who did not report smoking," Simon said.

The study found no association between smoking and a gene related to the immune system gene called HLA-DR15, which is thought to be another risk factor for MS, she said.

Studying the potential risk factors simultaneously might provide clues about why some people get MS and others do not, Simon said.

MS is an incurable condition that affects more than 1 million people worldwide. the disease can cause mild symptoms in some people and permanent disability in others. Symptoms may include numbness or weakness in one or more limbs, partial or complete loss of vision, tingling or pain, electric-shock sensation with certain head movements, tremors and an unsteady gait.

Simon and colleagues analyzed information from 442 people with MS and 865 without the disease. All were participants in either the U.S.-based Nurses’ Health Study, the Tasmanian MS Study and the Swedish MS Study.

The team determined whether participants had either of the potential factors and looked at the participants’ smoking history. the researchers said they found a consistent association between MS, smoking and the body’s immune response to the Epstein-Barr virus across the three distinct, geographic regions.

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