The role of mammalian skin in harbouring and transmitting arthropod-borne protozoan parasites has been overlooked for decades as these pathogens have been regarded primarily as blood-dwelling organisms. Intriguingly, infections with low or undetected blood parasites are common, particularly in the case of Human African Trypanosomiasis caused by Trypanosoma brucei gambiense. We hypothesise, therefore, the skin represents an anatomic reservoir of infection. Here we definitively show that substantial quantities of trypanosomes exist within the skin following experimental infection, which can be transmitted to the tsetse vector, even in the absence of detectable parasitaemia. Importantly, we demonstrate the presence of extravascular parasites in human skin biopsies from undiagnosed individuals. The identification of this novel reservoir requires a re-evaluation of current diagnostic methods and control policies. More broadly, our results indicate that transmission is a key evolutionary force driving parasite extravasation that could further result in tissue invasion-dependent pathology.DOI: http://dx.doi.org/10.7554/eLife.17716.001

Abstract Background The diagnosis of gambiense Human African Trypanosomiasis (gHAT) typically involves two steps: a serological screen, followed by the detection of living trypanosome parasites in the blood or lymph node aspirate. Live parasites can, however, remain undetected in some seropositive individuals, who we hypothesize are infected with Trypanosoma brucei gambiense parasites in their extravascular dermis. Methods To test this hypothesis, we conducted a prospective observational cohort study in the gHAT focus of Forecariah, Republic of Guinea. Of the 5,417 subjects serologically screened for gHAT, 66 were enrolled into our study and underwent a dermatological examination. At enrolment, 11 seronegative, 8 unconfirmed seropositive and 18 confirmed seropositive individuals had blood samples and skin biopsies taken and examined for trypanosomes by molecular and immuno-histological methods. Results In seropositive individuals, dermatological symptoms were significantly more frequent, relative to seronegative controls. T.b. gambiense parasites were present in the blood of all confirmed cases (n=18) but not in unconfirmed seropositive individuals (n=8). However, T. brucei parasites were detected in the extravascular dermis of all unconfirmed seropositive individuals and all confirmed cases. Skin biopsies of all treated cases and most seropositive untreated individuals progressively became negative for trypanosomes 6 and 20 months later. Conclusions Our results highlight the skin as a potential reservoir for African trypanosomes, with implications for our understanding of this disease’s epidemiology in the context of its planned elimination and underlining the skin as a novel target for gHAT diagnostics.

Highlights • New triple reporter combining a red luciferase and a tdTomato fused by a Ty1-tag • Tested in two different stages of both T. brucei and L. major parasites • Tested in vitro and in vivo in tsetse flies and murine models • Useful for multimodal imaging at different scales, including intravital monitoring