Life cycle of trypanosomes

It was initially suspected that transmission was purely mechanical (as in the case of T. evansi, which is transferred by biting flies or when carnivores bite their prey and get wounded in the mouth). In 1909 the German physician Dr. Kleine carried out transmission experiments from human patients to apes and monkeys. He, like Bruce, showed that infected tsetse flies were vectors. However, he also discovered that the parasite had to be present in the fly for a minimum period before it could cause infection. He subsequently showed that only the metacyclic forms in the salivary glands of the insect were infectious. When a tsetse fly bites an infected human patient, it can ingest parasites. After their arrival in the insect’s stomach, many (99%) of the parasites die because of interactions with specific lectins in the insect’s stomach (lectins are proteins that bind specifically to certain saccharides, such as are present on glycoproteins). The presence of bacterial endosymbionts in the insect possibly plays a role in whether or not an infection is produced in the fly. If the parasites survive, they become slender again ("procyclic trypanosome"). They must then manage to get into the salivary glands. It was initially thought that there were two ways for the parasites to do this. They may first pierce the intestinal wall and then penetrate directly into the salivary glands (this is now no longer accepted as a hypothesis), or they may undertake a complex migration in the insect. They would then migrate distally in the intestine around the posterior free extremity of the peritrophic membrane, then swim back between the intestinal wall and the peritrophic membrane. When they reach the level of the proventricular valve, they penetrate the peritrophic membrane that is being formed and swim up the oesophagus into the proboscis. Finally, they penetrate the hypopharynx and pass into the salivary glands. During this migration they change morphologically into “mesocyclic forms”. In the salivary glands they replicate as "epimastigotes". These then become infectious "metacyclic trypanosomes". However, the last word on this subject has not yet been said.

After about 3 weeks the fly can transmit parasites into its next victim when taking a blood meal. In man, the parasites multiply outside the cells, namely in blood, lymph and cerebrospinal fluid. Some can survive in certain areas (plexus choroideus in the brain), from whence they can cause further flare-ups.

Category: Medicine Notes