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People who care for spouses with dementia are, on average, six times more likely to develop the condition than those who don't have an affected partner, say the researchers of a new study that tracked more than 1,000 married couples over 12 years.

Male caregivers are particularly vulnerable, according to the study published in the Journal of the American Geriatrics Society. Their risk of a man getting the cognitive disorder - characterized by symptoms such as memory and language loss, as well as mood swings - if his wife has it is 12-fold, compared with 3.7 fold for women.

The risk may be caused by the stress of caregiving and observing "the deterioration of their life partner," write the researchers from Johns Hopkins, Utah State University and Duke University.

"The distress of watching one's spouse suffer from dementia, and the physical and mental burden of providing dementia care, are potential causal factors."

The study authors screened 1,221 married couples aged 65 and older in Cache County, UT, visiting them every three years from 1995 to 2007. (The community's elderly residents had a longer life expectancy and lower incidence of chronic disease - which can complicate the dementia diagnosis - than other similar populations.) Of those 2,442 individuals, 255 were diagnosed with dementia: 125 men, 70 women and 30 couples.

Earlier research has shown that dementia caregiving is associated with depression, physical health problems and mortality. Dementia caregivers have also been shown to provide more assistance and report more personal sacrifices and stress than those caring for older adults who are physically impaired.

Still, most previous studies have focused on the emotional distress - not cognitive abilities - of spousal caregivers. The authors of the current study suggest the chronic stress may harm the hippocampus, a brain region responsible for memory. They report that the increased spousal risk is on par with a gene variant known to increase susceptibility to Alzheimer's disease.

The findings held up even when the researchers accounted for other factors that might influence the risk of developing dementia, such as socioeconomic status.

A population shift of aging boomers and increased life expectancy is contributing to a rise in the number of dementia sufferers: 40 per cent of those over the age of 85 have the condition, according to the U.S.-based Alzheimer's Association.

The authors are calling for more research "to develop interventions for more vulnerable individuals." They also want to develop ways to decrease stress for spousal caregivers.

"Caregiving has positive aspects, as well as negative ones. If we can boost the positive aspects and reduce the negative ones, we may be able to reduce a caregiver's risk of developing dementia," one of the authors, Johns Hopkins psychiatry professor Peter Rabins, said in a release.