Challenges and lessons from a vector control campaign targeting Glossina palpalis palpalis in an isolated protected forest area in Abidjan, Côte d'Ivoire

Abstract

Vector control (VC) is one of the strategies employed to manage African trypanosomoses. This study aimed at assessing the effectiveness of a VC campaign against Glossina palpalis palpalis using tiny targets (TTs) impregnated with insecticide in an isolated, protected forest in Abidjan, Côte d'Ivoire, while considering ecological, genetic, and operational factors. Between January 2020 and September 2022, 2,712 TTs were deployed at 684 sites, covering a total area of 1.7 km². VC monitoring was conducted using Vavoua traps during 12 evaluation surveys, between June 2020 and March 2023. Five months after the initial TT deployment, tsetse fly density had decreased by 98.53%. Although tsetse density remained low due to TT redeployment and reinforcement, there was a significant increase a few months after the last redeployment. VC appeared to have minimal impact on the genetic structuring of G. p. palpalis. This suggested recruitment of local surviving tsetse flies all along the VC campaign due to a low probability of tsetse coming into contact with TTs, or to the evolution of behavioral or physiological resistance to control efforts. The genetic study revealed that one of the microsatellite markers used, the GPCAG locus, exhibited a selection signature possibly in response to VC. This could partly explain the challenges encountered in eliminating a seemingly isolated tsetse population thriving in a particularly favorable habitat.

Keywords: African trypanosomoses; Population genetics; Resistance; Tiny target; Tsetse flies; Vector control.

Figure 1

Study area (Source: Institut Pierre Richet, 2024). Figure adapted from Figure 1 of Konan [46]. It represents (red boundary) the three zones of the French military base in which the VC campaign was conducted and the three zones in which traps (in red) were set as part of the entomological surveys conducted outside the base.

Figure 2

Deployments and reinforcements of tiny targets during the vector control campaign. DX: Deployment; D1: First deployment; D2 to D5: Replacement of all previously set tiny targets; R: Reinforcement with additional tiny targets.

Figure 3

Distribution of control devices in the study area in September 2022 (last reinforcements) (Source: Institut Pierre Richet, 2024).

Figure 4

Distribution of vector control monitoring sites (Source: Institut Pierre Richet, 2024).

Figure 5

Boxplot of the number of captured tsetse flies during the entomological evaluation in the 25 T₀ sentinel traps according to zones. The boxplot (vertical bars in grey) shows the median and interquartile range, the whiskers show the 10th and 90th centiles and the black dots show all values >90th centile. The average catches per trap and per day (ADT) are represented by the blue dots.

Figure 6

Boxplot of the number of captured tsetse flies with the T₀ and supplementary traps. The boxplot (vertical bars in grey) shows the median and interquartile range, the whiskers show the 10th and 90th centiles and the black dots show all values >90th centile. The average catches per trap and per day (ADT) are represented by the blue dots.

Figure 7

Frequencies of the main alleles (219, 213, 210, 216, 222, and 225) at the GPCAG locus before and after the VC campaign. C0: Baseline entomological survey in May 2019; C23: Entomological evaluation in March 2023 (more than 3 years after the implementation of the VC).

Figure 8

Factorial Correspondence Analysis obtained with the seven neutral loci. CohortX: Individuals are represented by empty circles of different colors (one color for one cohort).