Modelling framework to demonstrate elimination of a vector population: tsetse elimination in Chad

Abstract

Every year, over 700 000 people, particularly children under five, die from vector-borne diseases worldwide. Effectively controlling current endemics and preventing new outbreaks requires an integrated approach that can lead to the elimination of both vectors and diseases. In the last two decades, integrating medical interventions and vector control has significantly reduced the incidence of Gambian Human African Trypanosomiasis (gHAT), with the World Health Organization validating eight countries as having eliminated the disease as a public health problem. However, elimination of the tsetse vector has not been confirmed, leaving the possibility of re-emergence. We developed a five-step modelling framework to assess vector elimination by calculating: (i) the probability of vector capture; (ii) the probability of observing a series of zero catches, even without actual elimination; (iii) the probability of natural elimination; (iv) the probability of failing to detect a rebound; and (v) the reinvasion risk. Our case study is g-HAT in Mandoul, Chad and the elimination of G. fuscipes fuscipes. We used vector control from 2014 to 2025 with no tsetse detected since 2018. We cannot yet conclude, with more than 90% confidence, that tsetse has been eliminated from Mandoul, nor that any remnant population will be naturally eliminated. However, since vector control was stopped in April 2025, we estimate that with continued sampling over the next two years, and no tsetse detected, elimination could be demonstrated with 99% confidence. Our multi-step modelling framework can be applied to other vectors, providing policymakers with clear guidelines for ongoing and future efforts. Significance StatementThe World Health Organisation has set the elimination of transmission of several neglected tropical vector-borne diseases, including human African trypanosomiasis (sleeping sickness), as a target for 2030. We show that deliberate elimination of tsetse, the vector, is feasible and can be demonstrated. We draw on our large-scale intervention in Mandoul, Chad where 3000 insecticide-treated Tiny Targets were deployed between 2014 and 2025, with no tsetse detected since 2018. While small undetected remnant populations cannot be entirely excluded, they would rapidly rebound in the absence of control, rendering them detectable. If no tsetse are caught over the next two years, it will confirm elimination. This illustrates a pathway for assessing and achieving vector elimination as a cornerstone of disease eradication.