The shapeshifting surface
In the bloodstreams of mammals across sub-Saharan Africa, a microscopic parasite called Trypanosoma brucei executes a remarkable strategy of evasion. This single-celled organism is the cause of Human African Trypanosomiasis, also known as sleeping sickness. Its survival depends on a dense, protective layer of protein covering its entire surface. This layer, composed of about 10 million identical molecules of a single Variant Surface Glycoprotein (VSG), acts as a shield against the host's immune system.
The parasite's genome contains a collection of over 1,000 genes that code for different VSGs. At any given time, only one of these genes is expressed, ensuring the surface coat is uniform. However, the trypanosome population continuously switches which VSG gene is active. This process, known as antigenic variation, creates a constantly moving target for the immune system. As soon as the host mounts an effective antibody response against one VSG, a new subpopulation of parasites with a different VSG coat has already emerged, allowing the infection to persist for months or even years. This constant shapeshifting is a primary reason why developing a vaccine against sleeping sickness has proven so difficult.
The genetic mechanisms for switching coats are complex. They involve silencing of one expression site near the end of a chromosome and the activation of another. The parasite can also create entirely new "mosaic" VSGs by combining segments of different VSG genes, further expanding its defensive library.
From fly to human
The life of Trypanosoma brucei is split between a mammalian host and its insect vector, the tsetse fly (genus Glossina). Tanzania hosts the subspecies Trypanosoma brucei rhodesiense, which causes an acute form of sleeping sickness that progresses rapidly over weeks or months. This form of the disease is found in eastern and southern Africa.
When a tsetse fly bites an infected mammal, it ingests the parasites, which then undergo a complex developmental cycle. They travel from the fly's midgut to its salivary glands, transforming into the stage that is infective to mammals. The entire process within the fly takes about three weeks. When the infected fly next takes a blood meal, it injects the parasites into a new host's skin.
The disease unfolds in two stages. The first, or haemo-lymphatic stage, involves the parasite multiplying in the blood and lymph system, causing fever, headaches, and joint pain. The second, or meningo-encephalitic stage, begins when the parasite crosses the blood-brain barrier and invades the central nervous system. This leads to the characteristic symptoms that give the disease its name: confusion, poor coordination, and severe disruption of the sleep cycle. Without treatment, this stage is almost always fatal.