THE FATHER and stepmother of a severely malnourished Wisconsin teenager, have been charged with her mistreatment.

The couple have been accused of keeping the girl in a basement, torturing her and forcing her to eat her own feces over a period of more than five years.

During her ordeal, the girl was also allegedly sexually assaulted by her stepbrother, now aged 18.

The father and stepmother - who were not named to protect the teen's identity - were arrested on February 10 after the 15-year-old girl was found wandering near her home in Madison, Wisconsin's second-largest city, The Milwaukee Journal Sentinel reported.

The girl was taken to a hospital, and later told police that she had been living in a basement since 2006 while the rest of the family - including three stepbrothers, one of whom had sexually assaulted her - lived upstairs.

"She has to ask her stepmother ... if she can eat, drink water or use the upstairs bathroom. (The girl) stated she usually is denied food," court documents said.

The teen, who was severely underweight at 32kg, said that most of her meals came from whatever she picked off the floor or found in the rubbish. She said if she was caught eating without permission, her father and stepmother made her "throw out the food and/or vomit what she has eaten".

She was also forced to drink her own urine and eat her feces, she said, adding that she was not allowed to leave the basement which had no toolet, and had a door that was rigged to trigger off an alarm if she opened it.

The teenager fled the house on February 6 because she feared for her life after her stepmother allegedly threatened to slit her throat.

A doctor who examined the girl listed her condition as "serial child torture with prolonged exposure to definite starvation," according to court documents.

The girl's father and stepfather were charged with first-degree recklessly endangering safety, child abuse intentionally causing harm and neglecting a child.

Her stepbrother, who was also arrested on Februaury 10, was charged with first-degree sexual assault of a child, second-degree sexual assault of a child and child abuse intentionally causing harm.

All three were in custody with bail set at $20,000 each. Preliminary hearings are scheduled for February 23.