Predicting the impact of chemoprophylaxis strategies against leprosy in South Tarawa, Kiribati

Abstract

Background Leprosy is widely thought to be a disease of the past, but globally there are still around 200,000 new cases annually. Current strategies of early diagnosis and treatment are proving ineffective in interrupting transmission. Providing targeted chemoprophylaxis to household contacts of confirmed cases or to the general population are proving to be promising new control strategies. Kiribati, a small island nation of around 110,000 people, is one of the few countries yet to reach the WHO leprosy elimination goal of a prevalence of less than one case per 10,000 population. In order to accelerate the interruption of transmission the Kiribati Ministry of Health plans to implement chemoprophylaxis in its capital, South Tarawa. This research aimed to determine the most effective chemoprophylaxis intervention strategy for South Tarawa. Methods The existing leprosy transmission model SIMCOLEP was quantified to simulate the population and the number of new leprosy cases in South Tarawa from 1989 to 2016. Quantification involved the assembly of data, parameterisation of the model, and the calibration of unknown demographic and disease parameters. Seven chemoprophylaxis interventions were simulated from 2017 to 2030 to predict the impact of adding chemoprophylaxis interventions to the current leprosy control strategy; household contact only, one round of mass, three rounds of mass, one round of mass with household contact, three rounds of mass (in consecutive years, every two years, and every five years) with household contact. These were all simulated at 100% and 80% adherence of individuals. Results The model predicted that the number of future leprosy cases would significantly decrease under the current control strategy alone, but that all interventions are expected to lead to faster and greater declines. Mass chemoprophylaxis leads to faster initial declines in cases than household contact only, with additional reductions with an increasing number of rounds. A combined strategy of three rounds of mass chemoprophylaxis implemented in consecutive years and household contact chemoprophylaxis is predicted to be the most effective approach because it results in the greatest reduction in cumulative cases over the prediction period. Lower adherence of individuals is predicted to reduce the speed and magnitude of the additional reductions for all interventions, however it does not impact on their relative effectiveness. Conclusion This model showed that all chemoprophylaxis interventions would be beneficial for reducing new leprosy cases in South Tarawa in the future. The most effective approach for interrupting transmission in Kiribati would be three rounds of mass chemoprophylaxis implemented in consecutive years, with ongoing household contact chemoprophylaxis. Decisions must also be aided by considerations of operational feasibility and cost-effectiveness.