A five-month-old baby boy was tragically mauled to death on his mother's birthday after the family's Great Dane mix broke into his bedroom.

The horror unfolded on Wednesday in the coastline community of Ormond Beach in Florida, after the boy's mother Kyleigh Wagner went out to celebrate her birthday.

The infant, named only as Carter, was left in the care of his great-grandmother and great-aunt, who put him in a bedroom when he went down for a nap.

After closing the door behind them, they let their three dogs out of a kennel, a tragic mistake that led their largest dog - a 130lb Great Dane-Husky mix - to storm into the home.

The boy's great-grandmother and great-aunt did not realize the horror unfolding for some time, the Volusia Sheriff's Office said, until they found him with severe injuries in the room.