As the zoo celebrates its 200th birthday, photographer David Levene captures the people keeping their (sometimes very dangerous) patients healthy and happy. Introduction: Patrick Barkham Some images may be upsetting to young audiences How do you shift a sedated rhino? Can a dormouse be drugged? What happens to a lion with an unusually small ear canal? How does the world’s longest venomous snake respond to treatment? Not easily, was the answer to the final question, as Guardian photographer