Silent transmission of human African trypanosomiasis: isolation of a human-infective trypanosome from an asymptomatic pregnant woman in south-eastern Nigeria

BACKGROUND: Human African trypanosomiasis (HAT) is an endemic neglected tropical disease in Sub-Saharan Africa targeted for elimination by 2030. However, asymptomatic human infections may sustain cryptic transmission. Nigeria has not reported confirmed HAT cases in over a decade, but emerging evidence suggests ongoing silent transmission. Infection during pregnancy raises additional concerns, including adverse outcomes and vertical transmission. This study therefore investigated the presence of trypanosome infection in pregnant women attending the antenatal clinic in Enugu, Nigeria. METHODS: Peripheral blood from pregnant women attending antenatal care in Enugu State, Nigeria, were screened microscopically for the presence of trypanosomes using the micro-haematocrit centrifugation technique. A sample containing motile organisms suggestive of Trypanosoma spp. was further investigated through rodent inoculation for parasite amplification, followed by morphologic identification, Blood Incubation Infectivity Test (BIIT), and in vitro culture. Serologic assays (ELISA and CATT) were conducted on patient serum, and a multiplex PCR targeting both Trypanosomatidae and Trypanozoon-specific regions of the 18S rDNA was done using DNA extracted from the patient's blood and from inoculated rats. RESULTS: The pregnant woman was asymptomatic but mildly anaemic, with a history of recurrent miscarriage. Serum assay was positive by CATT but negative by ELISA. Parasitaemia was successfully established in experimentally infected rats, with Giemsa-stained smears showing long slender trypomastigotes consistent with the bloodstream form of the Trypanozoon complex. The isolate demonstrated resistance to human serum and established infection following BIIT, confirming human infectivity. In vitro culture yielded viable parasite populations. PCR amplified Trypanozoon-specific region of the 18S rDNA from both the inoculated rat and patient blood samples. CONCLUSIONS: This study provides direct parasitological evidence of asymptomatic human trypanosome infection in Enugu State, Nigeria, a region not previously considered an HAT focus. These findings highlight the potential role of asymptomatic carriers in sustaining transmission and underscore critical gaps in surveillance, particularly in antenatal populations and undocumented foci. Enhanced active surveillance integrating sensitive diagnostic approaches is essential to achieving HAT elimination goals.