Diagnosis of sleeping sickness, a neglected tropical disease caused by African trypanosomes, relies on the detection of parasites in blood. Following their previous discovery that the skin is a reservoir for trypanosomes, scientists from the Institut Pasteur and colleagues have recently confirmed and quantified this phenomenon in humans, thereby improving our epidemiological understanding and opening new approaches to eliminate the disease.
African trypanosomes are parasites transmitted by tsetse flies and responsible for sleeping sickness. They proliferate in the blood and finally migrate to the central nervous system, ultimately resulting in death. Diagnosis generally involves two steps: a serological screening followed by the microscopic detection of live parasites in the blood. However, live parasites can remain undetected in some seropositive individuals who are therefore left untreated. In laboratory models, scientists from the Institut Pasteur's Trypanosome Transmission Group recently demonstrated that the skin was a major anatomical reservoir for trypanosomes, making them easily accessible to tsetse flies but difficult to detect in the blood. The scientists confirmed this phenomenon in humans in the sleeping sickness focus in Guinea. Of the 5,417 subjects serologically screened for sleeping sickness, 11 seronegative controls, 8 suspects and 18 confirmed cases had blood samples and skin biopsies examined for trypanosomes. Trypanosomes were found in the blood of all confirmed cases but not in suspected cases. But they were detected in the dermis of all the suspected and confirmed cases. These results demonstrate that the skin is an anatomical reservoir for African trypanosomes, including in individuals apparently not infected and who slip through the net of screening campaigns. These observations improve our understanding of the epidemiology of sleeping sickness in the effort to eliminate the disease, while also pointing to the skin as a novel target for diagnosis.