Thanks to an anonymous contributor at Wikipedia
"Another vintage Warner Bros. cartoon, 'Curtain Razor,' has Porky Pig as a theatrical agent auditioning acts, including a shaggy dog, who turns out to be the transport of a flea circus, which proceeds to set itself up, perform, and return to the dog, on command."
The Looney Toons film is from 1949 and the dog shuffles into the office and blows a whistle to command the fleas. The act does look to be more of a funfair than a circus but they do have a big top which presumably contains all of the acts. Apparently the film was retitled for TV changing from "Curtain Razor" (a reference to the Razorback pig) to "Show Stopper" and was remade with Daffy for "Show Biz Bugs" in 1957
The BFI have some good information about this film in their film at TV Database.
BFI film and TV entry for Curtain Razor
You can also watch this on youtube (at the time of writing)