High Moon Studios, FreeStyle Games, and Nerve Software assisted in development of Ghosts
The credits of Call of Duty: Ghosts has listed about 12 members of High Moon Studios, who developed Transformers games, and 27 people from the DJ Hero creators, FreeStyle Games, as assisting in the development of Call of Duty: Ghosts. Nerve Software has been involved with Call of Duty for Black Ops 1, Black Ops 2, and Ghosts as well.
Both High Moon Studios and FreeStyle Games have not worked on a Call of Duty title before. It isn’t a surprise that Activision uses many studios for their games, but they have 3 publicly revealed, and never mention the smaller ones that do.