Limited impact of vector control on the population genetic structure of Glossina fuscipes fuscipes from the sleeping sickness focus of Maro, Chad.

Tsetse flies (genus Glossina) transmit deadly trypanosomes to human populations and domestic animals in sub-Saharan Africa. Some foci of Human African Trypanosomiasis due to Trypanosoma brucei gambiense (g-HAT) persist in southern Chad, where a program of tsetse control was implemented against the local vector Glossina fuscipes fuscipes in 2018 in Maro. We analyzed the population genetics of G. f. fuscipes from the Maro focus before control (T0), one year (T1), and 18 months (T2) after the beginning of control efforts. Most flies captured displayed a local genetic profile (local survivors), but a few flies displayed outlier genotypes. Moreover, disturbance of isolation by distance signature (increase of genetic distance with geographic distance) and effective population size estimates, absence of any genetic signature of a bottleneck, and an increase of genetic diversity between T0 and T2 strongly suggest gene flows from various origins, and a limited impact of the vector control efforts on this tsetse population. Continuous control and surveillance of g-HAT transmission is thus recommended in Maro. Particular attention will need to be paid to the border with the Central African Republic, a country where the entomological and epidemiological status of g-HAT is unknown.

Title: Impact limité de la lutte antivectorielle sur la structure des populations de Glossina fuscipes fuscipes dans le foyer de la maladie du sommeil de Maro, Tchad. Abstract: Les mouches tsé-tsé (genre Glossina) transmettent des trypanosomes mortels aux populations humaines ainsi qu'aux animaux domestiques en Afrique sub-saharienne. Certains foyers de la trypanosomiase humaine Africaine due à Trypanosoma brucei gambiense (THA-g) persistent au sud du Tchad, où un programme de lutte antivectorielle a été mis en place contre le vecteur local de la maladie, Glossina fuscipes fuscipes, en particulier à Maro en 2018. Nous avons analysé la structure génétique des populations de G. f. fuscipes de ce foyer à T0 (avant lutte), une année après le début de la lutte (T1), et 18 mois après (T2). La plupart des mouches capturées après le début de la lutte ont montré un profil génétique local (survivants locaux), mais quelques-unes d'entre elles présentaient des génotypes d'individus atypiques. Par ailleurs, la présence de perturbations des signatures d'isolement par la distance (augmentation de la distance génétique avec la distance géographique), l'absence de signature génétique d'un goulot d'étranglement, et un accroissement de la diversité génétique entre T0 et T2 sont des arguments forts en faveur de la recolonisation de la zone par des mouches d'origines variées, tout en témoignant des effets limités de la campagne de lutte dans ce foyer. Ces résultats conduisent à recommander une lutte et une surveillance continues dans le foyer de Maro. Une attention particulière devra par ailleurs être prêtée à l'autre côté de la rive, située côté République Centre Africaine, dont le statut épidémiologique reste inconnu concernant les tsé-tsé et la THA-g.

Vector competence of sterile male Glossina fuscipes fuscipes for Trypanosoma brucei brucei: implications for the implementation of the sterile insect technique in a sleeping sickness focus in Chad.

BACKGROUND: Human African trypanosomiasis (HAT) is a neglected tropical disease caused by Trypanosoma brucei gambiense transmitted by tsetse flies in sub-Saharan West Africa. In southern Chad the most active and persistent focus is the Mandoul focus, with 98% of the reported human cases, and where African animal trypanosomosis (AAT) is also present. Recently, a control project to eliminate tsetse flies (Glossina fuscipes fuscipes) in this focus using the sterile insect technique (SIT) was initiated. However, the release of large numbers of sterile males of G. f. fuscipes might result in a potential temporary increase in transmission of trypanosomes since male tsetse flies are also able to transmit the parasite. The objective of this work was therefore to experimentally assess the vector competence of sterile males treated with isometamidium for Trypanosoma brucei brucei. METHODS: An experimental infection was set up in the laboratory, mimicking field conditions: the same tsetse species that is present in Mandoul was used. A T. b. brucei strain close to T. b. gambiense was used, and the ability of the sterile male tsetse flies fed on blood with and without a trypanocide to acquire and transmit trypanosomes was measured. RESULTS: Only 2% of the experimentally infected flies developed an immature infection (midgut) while none of the flies developed a metacyclic infection of T. b. brucei in the salivary glands. We did not observe any effect of the trypanocide used (isometamidium chloride at 100 mg/l) on the development of infection in the flies. CONCLUSIONS: Our results indicate that sterile males of the tested strain of G. f. fuscipes were unable to cyclically transmit T. b. brucei and might even be refractory to the infection. The data of the research indicate that the risk of cyclical transmission of T. brucei by sterile male G. f. fuscipes of the strain colonized at IAEA for almost 40 years appears to be small.

Delivering 'tiny targets' in a remote region of southern Chad: a cost analysis of tsetse control in the Mandoul sleeping sickness focus.

BACKGROUND: Since 2012, the World Health Organisation and the countries affected by the Gambian form of human African trypanosomiasis (HAT) have been committed to eliminating the disease, primarily through active case-finding and treatment. To interrupt transmission of Trypanosoma brucei gambiense and move more rapidly towards elimination, it was decided to add vector control using 'tiny targets'. Chad's Mandoul HAT focus extends over 840 km2, with a human population of 39,000 as well as 14,000 cattle and 3000 pigs. Some 2700 tiny targets were deployed annually from 2014 onwards. METHODS: A protocol was developed for the routine collection of tsetse control costs during all field missions. This was implemented throughout 2015 and 2016, and combined with the recorded costs of the preliminary survey and sensitisation activities. The objective was to calculate the full costs at local prices in Chad. Costs were adjusted to remove research components and to ensure that items outside the project budget lines were included, such as administrative overheads and a share of staff salaries. RESULTS: Targets were deployed at about 60 per linear km of riverine tsetse habitat. The average annual cost of the operation was USD 56,113, working out at USD 66.8 per km2 protected and USD 1.4 per person protected. Of this, 12.8% was an annual share of the initial tsetse survey, 40.6% for regular tsetse monitoring undertaken three times a year, 36.8% for target deployment and checking and 9.8% for sensitisation of local populations. Targets accounted for 8.3% of the cost, and the cost of delivering a target was USD 19.0 per target deployed. CONCLUSIONS: This study has confirmed that tiny targets provide a consistently low cost option for controlling tsetse in gambiense HAT foci. Although the study area is remote with a tsetse habitat characterised by wide river marshes, the costs were similar to those of tiny target work in Uganda, with some differences, in particular a higher cost per target delivered. As was the case in Uganda, the cost was between a quarter and a third that of historical target operations using full size targets or traps.

Developing and quality testing of microsatellite loci for four species of Glossina.

Microsatellite loci still represent valuable resources for the study of the population biology of non-model organisms. Discovering or adapting new suitable microsatellite markers in species of interest still represents a useful task, especially so for non-model organisms as tsetse flies (genus Glossina), which remain a serious threat to the health of humans and animals in sub-Saharan Africa. In this paper, we present the development of new microsatellite loci for four species of Glossina: two from the Morsitans group, G. morsitans morsitans (Gmm) from Zimbabwe, G. pallidipes (Gpalli) from Tanzania; and the other two from the Palpalis group, G. fuscipes fuscipes (Gff) from Chad, and G. palpalis gambiensis (Gpg) from Guinea. We found frequent short allele dominance and null alleles. Stuttering could also be found and amended when possible. Cryptic species seemed to occur frequently in all taxa but Gff. This explains why it may be difficult finding ecumenical primers, which thus need adaptation according to each taxonomic and geographic context. Amplification problems occurred more often in published old markers, and Gmm and Gpg were the most affected (stronger heterozygote deficits). Trinucleotide markers displayed selection signature in some instances (Gmm). Combining old and new loci, for Gmm, eight loci can be safely used (with correction for null alleles); and five seem particularly promising; for Gpalli, only five to three loci worked well, depending on the clade, which means that the use of loci from other species (four morsitans loci seemed to work well), or other new primers will need to be used; for Gff, 14 loci behaved well, but with null alleles, seven of which worked very well; and for G. palpalis sl, only four loci, needing null allele and stuttering corrections seem to work well, and other loci from the literature are thus needed, including X-linked markers, five of which seem to work rather well (in females only), but new markers will probably be needed. Finally, the high proportion of X-linked markers (around 30%) was explained by the non-Y DNA quantity and chromosome structure of tsetse flies studied so far.

Adding tsetse control to medical activities contributes to decreasing transmission of sleeping sickness in the Mandoul focus (Chad).

BACKGROUND: Gambian sleeping sickness or HAT (human African trypanosomiasis) is a neglected tropical disease caused by Trypanosoma brucei gambiense transmitted by riverine species of tsetse. A global programme aims to eliminate the disease as a public health problem by 2020 and stop transmission by 2030. In the South of Chad, the Mandoul area is a persistent focus of Gambian sleeping sickness where around 100 HAT cases were still diagnosed and treated annually until 2013. Pre-2014, control of HAT relied solely on case detection and treatment, which lead to a gradual decrease in the number of cases of HAT due to annual screening of the population. METHODS: Because of the persistence of transmission and detection of new cases, we assessed whether the addition of vector control to case detection and treatment could further reduce transmission and consequently, reduce annual incidence of HAT in Mandoul. In particular, we investigated the impact of deploying 'tiny targets' which attract and kill tsetse. Before tsetse control commenced, a census of the human population was conducted and their settlements mapped. A pre-intervention survey of tsetse distribution and abundance was implemented in November 2013 and 2600 targets were deployed in the riverine habitats of tsetse in early 2014, 2015 and 2016. Impact on tsetse and on the incidence of sleeping sickness was assessed through nine tsetse monitoring surveys and four medical surveys of the human population in 2014 and 2015. Mathematical modelling was used to assess the relative impact of tsetse control on incidence compared to active and passive screening. FINDINGS: The census indicated that a population of 38674 inhabitants lived in the vicinity of the Mandoul focus. Within this focus in November 2013, the vector is Glossina fuscipes fuscipes and the mean catch of tsetse from traps was 0.7 flies/trap/day (range, 0-26). The catch of tsetse from 44 sentinel biconical traps declined after target deployment with only five tsetse being caught in nine surveys giving a mean catch of 0.005 tsetse/trap/day. Modelling indicates that 70.4% (95% CI: 51-95%) of the reduction in reported cases between 2013 and 2015 can be attributed to vector control with the rest due to medical intervention. Similarly tiny targets are estimated to have reduced new infections dramatically with 62.8% (95% CI: 59-66%) of the reduction due to tsetse control, and 8.5% (95% 8-9%) to enhanced passive detection. Model predictions anticipate that elimination as a public health problem could be achieved by 2018 in this focus if vector control and screening continue at the present level and, furthermore, there may have been virtually no transmission since 2015. CONCLUSION: This work shows that tiny targets reduced the numbers of tsetse in this focus in Chad, which may have interrupted transmission and the combination of tsetse control to medical detection and treatment has played a major role in reducing in HAT incidence in 2014 and 2015.