In 1921, the school opened its first overseas research laboratory in Freetown, Sierra Leone. This laboratory functioned continuously until the early stages of World War II and made many important discoveries in West Africa, including demonstrating that a species of black fly was responsible for transmission of filarial worm to humans, causing river blindness. It was work like this that was essential for the Liverpool school to understand tropical diseases and subsequently the school completed 32 expeditions to Africa and Central and South America over the following years.