Causative agent of undulant fever is . . . . . . . .

Causative agent of undulant fever is B. abortus.
Brucellosis is a highly contagious zoonosis caused by ingestion of unpasteurized milk or undercooked meat from infected animals, or close contact with their secretions. It is also known as undulant fever, Malta fever, and Mediterranean fever.
The first state federal cooperative efforts towards eradication of brucellosis caused by B. abortus in the U.S. began in 1934.