Former US First Lady Rosalynn Carter has dementia

US President Jimmy Carter and his wife, Rosalynn, at the Democratic National Convention in 1976

Former US First Lady Rosalynn Carter has been diagnosed with dementia, according to a statement from her family.

The announcement comes as her husband, former President Jimmy Carter, receives hospice care at their home in Georgia.

"She continues to live happily at home with her husband," the statement said.

Former President Carter, 98, was placed on hospice care in February. He is the nation's longest-living president.

Mrs Carter, 95, the oldest living first lady, was said to be "enjoying spring... and visits with loved ones" despite her diagnosis.

President Carter and his wife, Rosalynn, served one term in office from 1977-81.

Since leaving the White House, Mr Carter and his wife have remained active, carrying out humanitarian work through his Carter Center.

Mrs Carter is said to be "enjoying the spring" with her husband

Mrs Carter is a long-time advocate for mental healthcare and has worked to reduce the stigma against mental illness. As first lady, she was the honorary chair of the President's Commission on Mental Health and helped pass a bill to increase spending on mental health services for disadvantaged communities.

Presidential historian Michael Beschloss hailed Mrs Carter on Twitter as "a magnificent champion for mental health in America for a half century".

Georgia Senator Raphael Warnock said he was praying for the former first lady and her husband "during these tough and tender times. So grateful for their continuing example of service and sacrifice".

Dementia is a general diagnosis for a loss of cognitive functions, including memory, language and problem-solving, that are severe enough to interfere with daily life, according to the Alzheimer's Association. Nearly one in three American seniors dies with Alzheimer's or another form of dementia, the association said.

After leaving office, Mrs Carter also founded the Rosalynn Carter Institute for Caregivers, which offers support, research and funding to Americans who care for aging family and loved ones.

"The universality of caregiving is clear in our family, and we are experiencing the joy and the challenges of this journey," the family said in its statement.

"We hope sharing our family's news will increase important conversations at kitchen tables and in doctor's offices around the country."