Tsetse Fly and Key Enzyme (Cathepsin B) (IMAGE) Arizona State University Caption This image depicts a tsetse fly coupled with the structure of the key enzyme (cathepsin B) that enables the single-celled parasite that causes African trypanosomiasis (or sleeping sickness) in humans. Credit Greg Stewart/SLAC National Accelerator Laboratory Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.