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.
Sleeping sickness, or Human African Trypanosomiasis (HAT), is caused by two distinct parasites. In East and Southern Africa, Trypanosoma brucei rhodesiense causes the Rhodesian form of the disease (about 2% of all reported cases [1]). In Central and West Africa, T. b. gambiense causes the Gambian form of the disease (G-HAT—about 98% of all reported cases [1]). The disease normally affects remote rural communities. The people most at risk are those working outdoors for long periods, as they are most exposed to the bite of the tsetse fly (Glossina spp.: Diptera), which transmits the parasites. The comparable diseases which occur in livestock, collectively termed African Animal Trypanosomiasis (AAT), are a significant brake on African development [2]. Among the 31 tsetse species, the most important vectors of G-HAT are Glossina fuscipes and Glossina palpalis, which are riverine tsetse species (Palpalis group).
Since the start of the 20th century, HAT has occurred in three huge epidemics. The most recent was in the 1990s when the annual cases officially reported to WHO peaked at 37,385 in 1998. It is widely acknowledged this severely underestimated actual numbers infected, which may have been as high as 450,000 in 1999 [3]. Untreated disease is normally fatal, so undoubtedly, many people infected in these epidemics died as a result. Although treatments for the disease have improved [4], they are still complex and difficult to administer particularly in the resource-poor settings where the disease thrives. There is no vaccine or chemoprophylaxis to prevent HAT and little prospect of either being developed in the near future. Vector control therefore remains the only means of protecting people from infection.
Rhodesian HAT (R-HAT) is a zoonosis. As a consequence, vector control plays a key part in its control, and medical interventions are only used for humanitarian purposes. In contrast, G-HAT is generally considered to be an anthroponosis, and control has relied heavily on active and/or passive case detection and treatment programmes [5]. However, modelling [6], historical investigations [7], and practical interventions [8,9] have clearly demonstrated the role that vector control can play in control of G-HAT, but it was considered too expensive and difficult to deploy in the resource poor settings of HAT foci. In consequence, a study was started in 2006 to try to find a simpler and cheaper alternative for vector control suitable for G-HAT foci.
The original hypothesis was that modifying insecticide-treated targets was the most likely means of producing a more cost-efficient vector control method for use in G-HAT foci. Two separate approaches were tried—to develop odours for use with targets or to change the visual characteristics of the target. The crucial finding was that a tiny target consisting of a small square of blue cloth flanked by a similar sized piece of black netting (Fig 1) was highly effective and would be about ten times more cost-effective than traps or large targets in control campaigns for the Palpalis (riverine) group tsetse flies responsible for the transmission of the vast majority of HAT [10–12]. This is in very strong contrast to Morsitans (savanna) group tsetse flies, which require much larger targets (1–2 m2). Importantly, it was found that all of the major G-HAT vectors responded well to tiny targets [13]. In addition, vegetation growth around tiny targets is a much smaller problem [14] than is the case for the large targets used against Morsitans group flies. In contrast to Morsitans group flies, odours seem to play only a minor role in the attraction of Palpalis group flies [15,16]. A modelling approach suggests that habitat geometry is the reason why Palpalis group flies are more dependent on sight than odour [17]. The general expectation is that relatively immobile insects in restricted habitats are more dependent on a thorough, vision-based search of their environment and that they are more wide-ranging in their diet.
Fig 1
A tiny target in a typical setting in Uganda.
Inevitably, the targets are gradually degraded by challenges in the environment, and the worst problems are floods, fallen targets, and the 6-month effective life of the insecticide in the tiny target [18]. As a consequence, current practice has been to deploy tiny targets once or twice per year, and the method has been successful in practice [9,18].
The aim in HAT foci is not to eradicate tsetse (although eradication should be embraced if feasible), but to stop transmission by reducing tsetse—human contact, and modelling suggests that this does not require complete removal of tsetse flies [6]. In addition, the reported time course of disease in humans is typically 3–4 years so that a fixed period of interrupted transmission may be sufficient to eliminate HAT in a focus. This approach is basically similar to the successful World Bank-funded OCP programme, which has led to the elimination of onchocerciasis as a public health problem in West Africa [19]. The approach had also been applied successfully in HAT foci of Ivory Coast in the 1980s and 1990s by Laveissiere and colleagues [8], although at that time the control techniques used were not considered to be sustainable and cost effective.
Consequently, to test the utility of tiny targets, studies were started in G-HAT foci (typically 500–3,000 km2). To re-emphasise, the goal is to reduce tsetse numbers below a threshold for transmission for a defined period to either eliminate or reduce transmission in a HAT focus, thereby giving screen-and-treat programmes a far greater chance of success [20]. For example, a previously published model [6] has been used along with figures from West Nile, Uganda to calculate the impact of various levels of vector control on transmission in that region (Fig 2) [18]. In practice, the level of control actually achieved in that region was >90%, which exceeds the levels required to interrupt transmission (Fig 2) [18]. How long control must continue is a researchable question but, given the time course of the disease in humans, it is likely to be several years. Presumably, it is also dependent on the distribution of the parasite in the human population and/or the existence of reservoir hosts. Current discussions have been focusing on 4–5 years of control.
Fig 2
To obtain an estimate of the level of tsetse control required to stop transmission, a published model was rearranged [6].
The impact of landscape fragmentation due to human and climatic mediated factors on the structure of a population of Glossina palpalis gambiensis Vanderplank (Diptera: Glossinidae) was investigated in the Mouhoun river basin, Burkina Faso. Allele frequencies at five microsatellite loci, and metric properties based on 11 wing landmarks, were compared between four populations. The populations originated from the Mouhoun river and one of its tributaries. The average distance between samples was 72 km with the two most widely spaced populations being 216 km apart. The sampling points traversed an ecological cline in terms of rainfall and riverine forest ecotype, along a river enlarging from downstream to upstream and oriented south to north. Microsatellite DNA comparison demonstrated structuring between the populations, but not complete isolation, with an overall Fst = 0.012 (P < 0.001). Wing geometry revealed significant centroid size and shape differences between populations, especially between the two most distant populations. There was no significant correlation between gene flow and geographic distance at this scale, but there was a positive correlation in females between metric distances (wing shape differences) and geographic distances that might be attributed to the cline of environmental conditions. The impact of the fragmentation of riparian landscapes on tsetse population structure is discussed in the context of control campaigns currently promoted by Pan African Tsetse and Trypanosomosis Eradication Campaign.
Background Work to control the gambiense form of human African trypanosomiasis (gHAT), or sleeping sickness, is now directed towards ending transmission of the parasite by 2030. In order to supplement gHAT case-finding and treatment, since 2011 tsetse control has been implemented using Tiny Targets in a number of gHAT foci. As this intervention is extended to new foci, it is vital to understand the costs involved. Costs have already been analysed for the foci of Arua in Uganda and Mandoul in Chad. This paper examines the costs of controlling Glossina palpalis palpalis in the focus of Bonon in Côte d’Ivoire from 2016 to 2017. Methodology/Principal findings Some 2000 targets were placed throughout the main gHAT transmission area of 130 km 2 at a density of 14.9 per km 2 . The average annual cost was USD 0.5 per person protected, USD 31.6 per target deployed of which 12% was the cost of the target itself, or USD 471.2 per km 2 protected. Broken down by activity, 54% was for deployment and maintenance of targets, 34% for tsetse surveys/monitoring and 12% for sensitising populations. Conclusions/Significance The cost of tsetse control per km 2 of the gHAT focus protected in Bonon was more expensive than in Chad or Uganda, while the cost per km 2 treated, that is the area where the targets were actually deployed, was cheaper. Per person protected, the Bonon cost fell between the two, with Uganda cheaper and Chad more expensive. In Bonon, targets were deployed throughout the protected area, because G . p . palpalis was present everywhere, whereas in Chad and Uganda G . fuscipes fuscipes was found only the riverine fringing vegetation. Thus, differences between gHAT foci, in terms of tsetse ecology and human geography, impact on the cost-effectiveness of tsetse control. It also demonstrates the need to take into account both the area treated and protected alongside other impact indicators, such as the cost per person protected.
One health (OH) approaches have increasingly been used in the last decade in the fight against zoonotic neglected tropical diseases (NTDs). However, descriptions of such collaborations between the human, animal and environmental health sectors are still limited for French-speaking tropical countries. The objective of the current survey was to explore the diversity of OH experiences applied to research, surveillance and control of NTDs by scientists from French-speaking countries, and discuss their constraints and benefits. Six zoonotic NTDs were targeted: echinococcoses, trypanosomiases, leishmaniases, rabies, Taenia solium cysticercosis and leptospiroses. Invitations to fill in an online questionnaire were sent to members of francophone networks on NTDs and other tropical diseases. Results from the questionnaire were discussed during an international workshop in October 2019. The vast majority (98%) of the 171 respondents considered OH approaches relevant although only 64% had implemented them. Among respondents with OH experience, 58% had encountered difficulties mainly related to a lack of knowledge, interest and support for OH approaches by funding agencies, policy-makers, communities and researchers. Silos between disciplines and health sectors were still strong at both scientific and operational levels. Benefits were reported by 94% of respondents with OH experience, including increased intellectual stimulation, stronger collaborations, higher impact and cost-efficiency of interventions. Recommendations for OH uptake included advocacy, capacity-building, dedicated funding, and higher communities’ involvement. Improved research coordination by NTD networks, production of combined human-animal health NTD impact indicators, and transversal research projects on diagnostic and reservoirs were also considered essential.
Abstract Background In recent years, a programme of vector control, screening and treatment of gambiense human African trypanosomiasis (gHAT) infections led to a rapid decline in cases in the Mandoul focus of Chad. To represent the biology of transmission between humans and tsetse, we previously developed a mechanistic transmission model, fitted to data between 2000 and 2013 which suggested that transmission was interrupted by 2015. The present study outlines refinements to the model to: (1) Assess whether elimination of transmission has already been achieved despite low-level case reporting; (2) quantify the role of intensified interventions in transmission reduction; and (3) predict the trajectory of gHAT in Mandoul for the next decade under different strategies. Method Our previous gHAT transmission model for Mandoul was updated using human case data (2000–2019) and a series of model refinements. These include how diagnostic specificity is incorporated into the model and improvements to the fitting method (increased variance in observed case reporting and how underreporting and improvements to passive screening are captured). A side-by-side comparison of fitting to case data was performed between the models. Results We estimated that passive detection rates have increased due to improvements in diagnostic availability in fixed health facilities since 2015, by 2.1-fold for stage 1 detection, and 1.5-fold for stage 2. We find that whilst the diagnostic algorithm for active screening is estimated to be highly specific (95% credible interval ( CI ) 99.9–100%, Specificity = 99.9%), the high screening and low infection levels mean that some recently reported cases with no parasitological confirmation might be false positives. We also find that the focus-wide tsetse reduction estimated through model fitting (95% CI 96.1–99.6%, Reduction = 99.1%) is comparable to the reduction previously measured by the decline in tsetse catches from monitoring traps. In line with previous results, the model suggests that transmission was interrupted in 2015 due to intensified interventions. Conclusions We recommend that additional confirmatory testing is performed in Mandoul to ensure the endgame can be carefully monitored. More specific measurement of cases, would better inform when it is safe to stop active screening and vector control, provided there is a strong passive surveillance system in place. Graphical Abstract
Four out of six Azawak zebu bulls raised in northern Burkina Faso were found to be infected with trypanosomes, including Trypanosoma congolense, six months after they had been transferred, uninfected, to the CIRDES experimental farm at Banankélédaga (Southwest Burkina Faso). Entomological surveys are carried out regularly in the area around this farm but, in one year, only 10 tsetse flies were captured, none of which showed infection in the midgut. However, a large number of tabanids were captured in the Glossina traps and dissection of some of them showed the presence of trypanosomes in their midgut. DNA amplification with the polymerase chain reaction (PCR) technique showed that the trypanosomes found in two tabanids belonged to the savannah type of T. congolense.
En Afrique subsaharienne, la Trypanosomiase Humaine Africaine (THA ou maladie du sommeil) est une maladie parasitaire negligee provoquee par un trypanosome (protozoaire flagelle), transmis a l'homme par la mouche tse-tse (Glossina spp.). Cette mouche hematophage est egalement en grande partie responsable de la transmission de trypanosomes aux animaux, provoquant la Trypanosomose Animale Africaine (TAA ou nagana). Ce complexe de maladies a un impact important sur la sante humaine et animale et sur la productivite en Afrique. L'elevage du betail dans les zones endemiques est difficile, et entraine des pertes economiques de plusieurs milliards de dollars par an. La mouche tse-tse reste cependant un insecte vulnerable, sensible aux modifications environnementales et aux insecticides. En 2015 en Cote d'Ivoire, il a ete mis en evidence une grande zone forestiere (200 km2) ou les glossines (Glossina palpalis palpalis) ont totalement disparues, en l'absence de lutte anti-vectorielle bien que l'environnement semble rester propice a leur developpement. L'hypothese sous-jacente a la disparition des glossines serait liee aux fortes densites de population humaines et aux pratiques agricoles associees qui se caracterisent par une utilisation importante de de produits phytosanitaires et qui affecteraient la survie des mouches. Apres avoir caracterise les produits les plus utilises par les agriculteurs, nous les avons testes en bioessays par contact tarsal sur l'espece Glossina palpalis gambiensis. Cette etude actuellement en cours nous permettra de mettre en evidence l'impact collateral des traitements phytosanitaires en foret de Cote d'Ivoire sur les populations de glossines.
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