Final answer:
Avian tuberculin is used as a control in the Single Intradermal Comparative Cervical Tuberculin (SICCT) test to differentiate between bovine TB responses and other mycobacteria.
Step-by-step explanation:
The Student's question pertains to the Single Intradermal Comparative Cervical Tuberculin (SICCT) test, which is used to diagnose tuberculosis (TB) in animals, specifically in cattle. The SICCT test includes two injections: one with avian tuberculin and the other with bovine tuberculin. The avian tuberculin serves as the control to differentiate between a bovine TB response and a response to other mycobacteria that might cause a false positive reaction. Therefore, the answer to the question 'What is used as a control in SICCT?' is avian tuberculin.