Around 7:30 p.m., someone at a home on Amesbury Court called to report that a child at the residence suffered a severe dog bite to the face, Contra Costa County Animal Services Lt. Jane Andreotti said.
The boy was taken to Children's Hospital in Oakland where he immediately underwent surgery and remained this morning, Andreotti said.
She said the dog, a brown Chesapeake Bay Retriever, does not have a record of violent behavior.
Andreotti said the dog belonged to a family friend or relative, but that the owner was not around at the time of the attack.
Animal Services personnel took the dog from the home and placed it in rabies quarantine, where it is now being held.
An investigation into the attack is ongoing.