The African trypanosome Trypanosoma brucei gambiense is a parasite responsible for chronic Human African Trypanosomiasis, more commonly known as sleeping sickness. Diagnosing sleeping sickness generally involves two stages: serological screening (to search for antibodies against parasites in the bloodstream), followed by the microscopic detection of live parasites in the blood or a lymph node aspirate. But live parasites may remain undetected in some seropositive individuals. Following on from their previous discovery that the skin is a reservoir for trypanosomes, scientists from the Institut Pasteur and their colleagues in the TrypaDerm consortium have recently confirmed and quantified this phenomenon in humans, thereby improving our epidemiological understanding of sleeping sickness.
Trypanosomes are transmitted to humans via a bite from an infected tsetse fly. The parasites then proliferate in the blood and the lymph and finally migrate to the central nervous system, ultimately resulting in death. Large-scale campaigns over the past 30 years have been effective in reducing the number of new annual cases; there were less than a thousand in 2019. On the back of this success, WHO is aiming to eliminate the disease by 2030. But a number of questions about the biology of the parasites remain unanswered and could help explain the persistence or reemergence of transmission foci.
Live parasites can remain undetected in some seropositive individuals, preventing them from receiving treatment. In various 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 set out to verify and quantify this phenomenon in humans in the sleeping sickness focus in Forécariah, Guinea.
The skin, a new avenue for diagnosis
"To test the hypothesis, we carried out a prospective observational cohort study in the Forécariah focus in Guinea. Of the 5,417 subjects serologically screened for sleeping sickness, 66 agreed to take part in our study and underwent a dermatological examination", explains Brice Rotureau, Head of the Trypanosome Transmission Group at the Institut Pasteur. At the beginning of the study, 11 seronegative controls, 8 seropositive but unconfirmed suspects and 18 confirmed seropositive cases had blood samples and skin biopsies taken and examined for trypanosomes using seven molecular and immune-histological methods.
In the suspected and confirmed cases, dermatological symptoms were significantly more frequent than in seronegative controls. Trypanosomes were found in the blood of all confirmed cases but not in suspected cases. But parasites were detected in the dermis of all the suspected and confirmed cases. Six and 20 months after treatment, skin biopsies of the confirmed cases progressively became negative for trypanosomes. These results demonstrate that the skin is an anatomical reservoir for African trypanosomes, including in individuals that are apparently not infected and slip through the net of screening campaigns, becoming "healthy" carriers. "These observations improve our understanding of the epidemiology of sleeping sickness in the context of efforts to eliminate the disease, while also pointing to the skin as a novel target for diagnosis," concludes Brice Rotureau.
Discovery of trypanosomes in the skin of patients and suspects of sleeping sickness in Guinea. Credits : Institut Pasteur
A. Entrance to the Human African Trypanosomiasis (HAT) Management Center of the Prefecture of Forécariah in the Republic of Guinea where part of the study was carried out.
B. Dorsal view of the right shoulder of a patient from whom a small skin biopsy was taken.
C. Fine section of the patient's skin biopsy marked with antibodies recognizing the trypanosomes. We can clearly see the epidermis (in brown at the top) and the dermis matrix (in white in the middle and at the bottom).
D.High magnification views of extravascular trypanosomes (brown) in the patient's dermis (white). The scale bar measures 10 microns.
The ANR TrypaDerm consortium, coordinated by the Institut Pasteur and including teams from the Guinean National Control Program Against Human African Trypanosomiasis, the University of Glasgow and the French Research Institute for Development (IRD), is continuing its research, in particular with the aim of confirming these epidemiological observations at a larger scale and developing methods to detect trypanosomes in the skin.
Extravascular dermal trypanosomes in suspected and confirmed cases of gambiense Human African Trypanosomiasis, Clinical Infectious Diseases, 8 juillet 2020
Mariame Camara1, Alseny M’mah Soumah1,2, Hamidou Ilbouldo1,3,4, Christelle Travaillé5 4, Caroline Clucas6, Anneli Cooper6, Nono-Raymond Kuispond Swar6,7, Oumou Camara1,5, Ibrahim Sadissou4, Estefania Calvo Alvarez5, Aline Crouzols5, Jean-Mathieu Bart4, Vincent Jamonneau4, Mamadou Camara1, Annette MacLeod6,$, Bruno Bucheton1,4,$, and Brice Rotureau5,$,*
1 Programme National de Lutte contre la Trypanosomiase Humaine Africaine, Ministère de la Santé, Conakry, Guinea
2 Service de Dermatologie, Hôpital de Donka, Conakry, Guinea
3 Institut de Recherche en Sciences de la Santé (IRSS) - Unité de Recherche Clinique de Nanoro (URCN), Nanoro, Burkina-Faso
4 Institut de Recherche pour le Développement, Unité Mixte de Recherche IRD-CIRAD 177 InterTryp, Campus International de Baillarguet, Montpellier, France
5 Trypanosome Transmission Group, Trypanosome Cell Biology Unit, INSERM U1201 & Department of Parasites and Insect Vectors, Institut Pasteur, Paris, France
6 Wellcome Centre for Molecular Parasitology, College of Medical, Veterinary and Life Sciences, Henry Wellcome Building for Comparative Medical Sciences, Glasgow, Scotland, United Kingdom
7 Department of Parasitology, National Institute of Biomedical Research (INRB), Kinshasa, Democratic Republic of the Congo
$ Equivalent contributions
* Corresponding author: Brice Rotureau, PhD, Trypanosome Transmission Group, Trypanosome Cell Biology Unit, INSERM U1201 & Department of Parasites and Insect Vectors, Institut Pasteur