How do tsetse transmit trypanosomes?

When tsetse bite an infected animal, they ingest trypanosomes with the host's blood. These bloodstream forms develop and multiply within the fly and then migrate to the mouthparts (T. vivax and T. congolense) or the salivary glands (T. brucei). This process takes 5-13 days for Trypanosoma vivax, 15-23 days for T. congolense and 12-23 days for T. brucei. After this period, trypanosomes will be injected into a host as the fly feeds.

Infection rates in tsetse populations are generally low, ranging from about 5% for T. vivax and T. congolense to less than 1% for T. brucei. However, these rates vary according to factors such as:-

• the local climate,
• the species, age and physiological status of the flies and;
• the availability of infected hosts.