Xeno-monitoring the impact of Vector Control on trypanosome transmission in the Forecariah sleeping sickness focus (Guinea)

Abstract

BACKGROUND: Forecariah is one of the most active foci of gambiense human African trypanosomiasis (gHAT) in Guinea. The study aimed to evaluate the impact of a vector control intervention carried out in this focus from 2018 to 2021. To achieve this, thousands of deltamethrin-impregnated blue tiny targets were deployed annually. METHODOLOGY AND PRINCIPAL FINDINGS: Thirty-one sentinel traps were used to assess the following entomological and parasitological parameters: (i) the apparent density (AD) of tsetse flies, monitored twice per year; (ii) the tsetse fly infection rates, evaluated once per year; and (iii) the trypanosome species, determined by molecular approaches before and after three years of vector control. Prior to the VC campaign, significantly higher densities of Glossina palpalis gambiensis were observed in the mangroves (AD = 17.2 F/T/D) than in mainland areas (AD = 6.2 F/T/D). Just 18 months after the start of the VC campaign, the maximum reduction in AD was reached (mean AD = 1.1 F/T/D; p < 0.001; 92% reduction) and was maintained throughout the campaign. Prior to VC, 18 out of 31 (58%) sentinel traps presented at least one infected fly observed by microscopy, whereas only two out of 31 (6%; p = 0.002) traps were positive after three years of VC. Molecular analysis of the tsetse midgut revealed positivity for T. brucei sl DNA in 45.2% of the traps prior to the VC baseline. T. congolense and T. vivax were observed in 6.5% of traps. Although one T. b. gambiense infection, which is responsible for human disease, was detected three years after the introduction of vector control, a significant decrease in T. brucei sl PCR positivity was observed (45.2% versus 22.6%; p = 0.05), whereas a slight increase was observed for T. vivax (6.5% versus 16.1%), while T. congolense trap positivity (6.5%) remained stable. CONCLUSION: In Forecariah, the prevalence of the disease was reduced by 76% during the study period. Hence, these results further emphasize that implementing vector control alongside medical control is an effective way of reducing the transmission of parasites to humans.