A complaint was launched against the UK's National Health Service (NHS) after a family claimed that a 42-year-old patient was attacked by a rat as he lay sedated in his hospital bed.
The family of Jason Ketley says he was bitten repeatedly by the rodent last November shortly after going to bed in a specialist care unit at St. Ebba's Hospital in Epsom, about 15 miles south of London.
The Surrey Borders Partnership NHS Foundation Trust, which runs the hospital, described the incident as "genuinely shocking" and said an investigation was launched.
Jo Young, a director at the trust, said the complaint -- launched by Ketley's mother, Pat Boardman -- was being taken "extremely seriously."
She said records show that a mouse was seen in the hospital at the time.
Boardman called the suggestion that the rodent involved was a mouse rather than a rat "outrageous."
She told the Daily Mirror newspaper that her son, who is bipolar and was described as having a mental age of two, was left with "open bite marks" after the ordeal, which she said only ended when nurses knocked the rodent off the man and killed it.
"I'm appalled that this sort of thing could happen to my son in an NHS hospital in this day and age," she said. "He was completely helpless and terrified. It's a disgrace."
"Our records indicate that a mouse was seen in the house in November and that pest control was alerted immediately, but we are unable to provide a complete response to all the concerns raised until we know the outcome of our investigation," Young said in a statement to Sky News.
The "small rodent" was killed the same day, a spokesman said.
Ketley was taken to Epsom General Hospital for "precautionary treatment" and has fully recovered, the spokesman added.
The investigation is due to be completed by the end of March.