Mechanical transmission of sleeping sickness by the tsetse fly

Up to the beginning of 1909 it was believed that the spread of trypanosome diseases, such as Sleeping Sickness and Nagana, was effected by the mechanical transmission of the parasite by the tsetse fly. The proboscis was supposed to be contaminated by being dipped in the infected blood, and some of the trypanosomes were pictured as remaining for some time within the tube and capable of being injected into a fresh animal at the next feed of the fly. Successful experiments were described which seemed to prove that the tsetse fly was capable of remaining infective for 48 hours, but not longer; and it was thought that any given Sleeping Sickness area would be free from danger a few days after the infected population had been removed from it. Dr. Kleine, however, at the end of 1908, showed that the tsetse fly remains infective for a much longer period, and that a period of non-infectivity of 20 days or more elapses before this power of passing on the parasite is gained. In other words, that the Trypanosoma gambiense undergose some process of development in the fly before it is able to infect a fresh animal. It was evident, then, that the mechanical theory had to be modified.