The Parasite
The causative agent of Sleeping Sickness is the single-celled protozoan parasite, Trypanosoma brucei brucei, which is a close relative of the pathogen that causes Chagas Disease (in fact, other common names for Sleeping Sickness and Chagas Disease are African Trypanosomiasis and American Trypanosomiasis, respectively). There are two sub-species of T. brucei that cause Sleeping Sickness in humans, Trypanosoma brucei gambiense (responsible for 98% of cases) and Trypanosoma brucei rhodesiense. These parasites are members of a group known as the hemoflagellates or ‘blood and tissue’ protozoans. This is because they can be found in circulating blood or, in the case of American trypanosomes, cardiac (heart) tissue of their hosts.
The Fly
The causative agent of Sleeping Sickness is a bizarre little relative of the house fly in the genus Glossina, most commonly known as the Tsetse fly. These flies have one of the strangest reproductive strategies in the insect world. One might almost say they think they’re people. They give birth to live young, they only give birth to one ‘baby’ at a time, and they even feed their young through a system of tubes that provide ‘milk’ proteins to growing larvae. While most insects produce high numbers of offspring and bank on a little cannon fodder, Tsetse flies take the more mammalian approach of caring for young until they’re less vulnerable. This process is surprisingly time consuming and energetically expensive for an insect.
While the ‘why’ of evolution is always tricky question to answer, susceptibility of larvae to pathogens and predators may provide one possible explanation for the unique reproductive strategy. The larvae are 3rd or 4th instars before they emerge from the female and they pupate almost immediately upon being born, limiting the chance for any pathogens or predators to take advantage of them in a vulnerable state. Still, both males and females are blood-feeders, a risky business in itself, with females requiring even more blood for the production of offspring.
[Note: much of the information in this section comes from my insect physiology course notes].
The Cycle
The life cycle of human African Trypanosomes begins with the taking of a blood meal by the Tsetse fly, which is when the protozoans are injected into the bloodstream of the host where they are carried to other parts of the body. They then multiply by a process called binary fission while in the blood, spinal fluid, or lymph. Another Tsetse fly may then take a blood meal and pick up new protozoans, which will then multiply (again by binary fission) in the midgut of the Tsetse fly. These will then travel from the midgut to the salivary glands, multiply once more, and wait for the fly to take another blood meal. Then the cycle begins, again. While humans are the primary host for T. brucei gambiense, it may also be found in animal reservoirs and T. brucei rhodesiense prefers large animals.
