Feed for circus elephants varies from town to town as the show travels. The Bronx Zoo buys 1,200 tons of food annually but cannot simulate the diet found in the wild. In two years, Dr. Dierenfeld, who is 32 years old, has tested more than 3,000 blood samples of more than 200 species, primarily mammals and birds. In a laboratory at the new health center at the zoo, high-technology blood testing machines, furnished by Hoffmann-LaRoche Inc., can produce results in …