Population sizes and dispersal pattern of tsetse flies: rolling on the river?

Population Structure and Migration Patterns of the Tsetse Fly Glossina fuscipes in Congo-Brazzaville

Tsetse flies of the palpalis group, particularly Glossina fuscipes, are the main vectors of human African trypanosomiasis or sleeping sickness in Congo-Brazzaville. They transmit the deadly human parasite, Trypanosoma brucei gambiense and other trypanosomes that cause animal trypanosomiasis. Knowledge on diversity, population structure, population size, and gene flow is a prerequisite for designing effective tsetse control strategies. There is limited published information on these parameters including migration patterns of G. fuscipes in Congo-Brazzaville. We genotyped 288 samples of G. fuscipes from Bomassa (BMSA), Bouemba (BEMB), and Talangai (TLG) locations at 10 microsatellite loci and determined levels of genetic diversity, differentiation, structuring, and gene flow among populations. We observed high genetic diversity in all three localities. Mean expected heterozygosity was 0.77 ± 0.04, and mean allelic richness was 11.2 ± 1.35. Deficiency of heterozygosity was observed in all populations with positive and significant F IS values (0.077-0.149). Structure analysis revealed three clusters with genetic admixtures, evidence of closely related but potentially different taxa within G. fuscipes. Genetic differentiation indices were low but significant (F ST = 0.049, P < 0.05), indicating ongoing gene flow countered with a stronger force of drift. We recorded significant migration from all the three populations, suggesting exchange of genetic information between and among locations. Ne estimates revealed high and infinite population sizes in BEMB and TLG. These critical factors should be considered when planning area-wide tsetse control interventions in the country to prevent resurgence of tsetse from relict populations and/or reinvasion of cleared habitats.

Revisiting FIS, FST, Wahlund Effects, and Null Alleles

Null alleles and Wahlund effects are well known causes of heterozygote deficits in empirical population genetics studies as compared to Hardy-Weinberg genotypic expectations. Some authors have theoretically studied the relationship of Wright's FIS computed from subsamples displaying a Wahlund effect and FST before the Wahlund effect, as can occasionally be obtained from populations of long-lived organisms. In the 2 subsample case, a positive relationship between these 2 parameters across loci would represent a signature of Wahlund effects. Nevertheless, for most organisms, getting 2 independent subsamples of the same cohort and population, one with a Wahlund effect and the other without, is almost never achieved and most of the time, empirical population geneticists only collect a single sample, with or without a Wahlund effect, or with or without null alleles. Another issue is that null allele increase FIS and FST altogether and thus may also create such correlation. In this article, I show that, for organisms collected in a single sample, which corresponds to the most common situation, Wahlund effects and null alleles affect the values of both FIS and FST though in the opposite direction. I also show that Wahlund effect produces no or weak positive correlation between the 2 F-statistics, while null alleles generate a strong positive correlation between them. Variation of these F-statistics is small and even minimized for FST under Wahlund effects as compared to null alleles. I finally propose a determination key to interpret data with heterozygote deficits.

Using genetic and phenetic markers to assess population isolation within the southernmost tsetse fly belt in Africa

The effective control of tsetse flies (Diptera; Glossinidae), the biological vectors of trypanosome parasites that cause human African trypanosomosis and African animal trypanosomosis throughout sub-Saharan Africa, is crucial for the development of productive livestock systems. The degree of genetic isolation of the targeted populations, which indicate reinvasion potential from uncontrolled areas, will be critical to establish a control strategy. Molecular and morphometrics markers were used to assess the degree of genetic isolation between seemingly fragmented populations of Glossina brevipalpis Newstead and Glossina austeni Newstead present in South Africa. These populations were also compared with flies from adjacent areas in Mozambique and Eswatini. For the molecular markers, deoxyribonucleic acid was extracted, a r16S2 Polymerase chain reaction (PCR) was performed and the PCR product sequenced. Nine landmarks were used for the morphometrics study as defined by vein intersections in the right wings of female flies. Generalised Procrustes analyses and regression on centroid size were used to determine the Cartesian coordinates for comparison between populations. Both methods indicated an absence of significant barriers to gene flow between the G. brevipalpis and G. austeni populations of South Africa and southern Mozambique. Sustainable control can only be achieved if implemented following an area-wide management approach against the entire G. brevipalpis and G. austeni populations of South Africa and southern Mozambique. Limited gene flow detected between the G. austeni population from Eswatini and that of South Africa or Mozambique may imply that these two populations are in the proses of becoming isolated.

Negative Density-dependent Dispersal in Tsetse Flies: A Risk for Control Campaigns?

Tsetse flies are vectors of parasites that cause diseases responsible for significant economic losses and health issues in sub-Saharan Africa, including sleeping sickness in humans and nagana in domestic animals. Efficient vector-control campaigns require good knowledge of the demographic parameters of the targeted populations. In the last decade, population genetics emerged as a convenient way to measure population densities and dispersal in tsetse flies. Here, by revealing a strong negative density-dependent dispersal in two dimensions, we suggest that control campaigns might unleash dispersal from untreated areas. If confirmed by direct measurement of dispersal before and after control campaigns, area-wide and/or sequential treatments of neighboring sites will be necessary to prevent this issue.

The population genomics of multiple tsetse fly (Glossina fuscipes fuscipes) admixture zones in Uganda

Understanding the mechanisms that enforce, maintain or reverse the process of speciation is an important challenge in evolutionary biology. This study investigates the patterns of divergence and discusses the processes that form and maintain divergent lineages of the tsetse fly Glossina fuscipes fuscipes in Uganda. We sampled 251 flies from 18 sites spanning known genetic lineages and the four admixture zones between them. We apply population genomics, hybrid zone and approximate Bayesian computation to the analysis of three types of genetic markers: 55,267 double-digest restriction site-associated DNA (ddRAD) SNPs to assess genome-wide admixture, 16 microsatellites to provide continuity with published data and accurate biogeographic modelling, and a 491-bp fragment of mitochondrial cytochrome oxidase I and II to infer maternal inheritance patterns. Admixture zones correspond with regions impacted by the reorganization of Uganda's river networks that occurred during the formation of the West African Rift system over the last several hundred thousand years. Because tsetse fly population distributions are defined by rivers, admixture zones likely represent both old and new regions of secondary contact. Our results indicate that older hybrid zones contain mostly parental types, while younger zones contain variable hybrid types resulting from multiple generations of interbreeding. These findings suggest that reproductive barriers are nearly complete in the older admixture zones, while nearly absent in the younger admixture zones. Findings are consistent with predictions of hybrid zone theory: Populations in zones of secondary contact transition rapidly from early to late stages of speciation or collapse all together.

Tsetse fly evolution, genetics and the trypanosomiases - A review

This reviews work published since 2007. Relative efforts devoted to the agents of African trypanosomiasis and their tsetse fly vectors are given by the numbers of PubMed accessions. In the last 10 years PubMed citations number 3457 for Trypanosoma brucei and 769 for Glossina. The development of simple sequence repeats and single nucleotide polymorphisms afford much higher resolution of Glossina and Trypanosoma population structures than heretofore. Even greater resolution is offered by partial and whole genome sequencing. Reproduction in T. brucei sensu lato is principally clonal although genetic recombination in tsetse salivary glands has been demonstrated in T. b. brucei and T. b. rhodesiense but not in T. b. gambiense. In the past decade most genetic attention was given to the chief human African trypanosomiasis vectors in subgenus Nemorhina e.g., Glossina f. fuscipes, G. p. palpalis, and G. p. gambiense. The chief interest in Nemorhina population genetics seemed to be finding vector populations sufficiently isolated to enable efficient and long-lasting suppression. To this end estimates were made of gene flow, derived from F_ST and its analogues, and Ne, the size of a hypothetical population equivalent to that under study. Genetic drift was greater, gene flow and Ne typically lesser in savannah inhabiting tsetse (subgenus Glossina) than in riverine forms (Nemorhina). Population stabilities were examined by sequential sampling and genotypic analysis of nuclear and mitochondrial genomes in both groups and found to be stable. Gene frequencies estimated in sequential samplings differed by drift and allowed estimates of effective population numbers that were greater for Nemorhina spp than Glossina spp. Prospects are examined of genetic methods of vector control. The tsetse long generation time (c. 50 d) is a major contraindication to any suggested genetic method of tsetse population manipulation. Ecological and modelling research convincingly show that conventional methods of targeted insecticide applications and traps/targets can achieve cost-effective reduction in tsetse densities.

Vision and genesis of survival strategies in tsetse flies: A laboratory study

Organisms respond to environmental stimuli in ways that optimize survival and reproduction. Tsetse fly life-history is characterized by high investment in progeny by the pregnant female and low birth rate. This places constraints on tsetse populations across the sub-Saharan biotopes they colonize where extreme climatic conditions militate against survival. Controlling metabolic rate is crucial in biotopes where daily swings in temperature can exceed 20 °C. Tsetse acquire their nutrient requirements from the blood meal. These diurnal flies are otherwise confined for most of their lives to perching sites in the shade. At these locations they are simultaneously threatened by vertebrate and invertebrate predators. Here we describe behaviours of the East African tsetse Glossina pallidipes Austen (Diptera: Glossinidae) that permit it to reduce risk daily. Newly-emerged flies remain immobile at emergence in the photophase but scotophase-emerging flies walk away within seconds to climb (negative geotaxis) vertical substrates to find a perch off the ground. Flies of all ages show the ability to fly in almost total darkness (1.10^-5 lux) in the scotophase to perch on the upper side of horizontally suspended 1 cm diameter bars, simulating branches of vegetation, but perch under the same bars during the photophase. This underlines the predilection of tsetse for objects with a linear aspect that provide a vantage point and shade. Mature G. pallidipes can discriminate between horizontally suspended bars of different diameter and shape. Flicker fusion frequency values established by optomotor and retinogram recordings reveal a higher visual acuity in mature compared to newly-emerged tsetse.

Impact of an integrated control campaign on tsetse populations in Burkina Faso

BACKGROUND

Tsetse flies are the sole vectors of human and animal trypanosomosis. In Burkina Faso, a project aiming to create zones free of tsetse flies and trypanosomosis was executed from June 2006 to December 2013. After the determination of tsetse distribution in the intervention area from December 2007 to November 2008, the control campaign was launched in November 2009 and ended in December 2013. The goal was to eliminate tsetse flies from 40,000 km2 of area, through an integrated control campaign including insecticide targets, traps and cattle, sequential aerial treatment (SAT) and the mass treatment of livestock using trypanocides. The campaign involved assistance of the beneficiary communities at all the steps of the control strategy with insecticide impregnated targets.

METHODS

This study was carried out to assess the impact of the control project on tsetse apparent density per trap per day (ADT). To evaluate the effectiveness of tsetse control, 201 sites were selected based on the baseline survey results carried out from December 2007 to November 2008. These sites were monitored bi-monthly from January 2010 to November 2012. At the end-of-study in 2013 a generalized entomological survey was carried out in 401 infested sites found during the longitudinal survey done before the control. Barrier and tsetse persistence areas were treated by ground spraying and evaluated. Controls were also done before and after aerial spraying.

RESULTS

In the insecticide-impregnated target area, the control showed that ADT of tsetse flies declined from 10.73 (SD 13.27) to 0.43 (SD 2.51) fly/trap/day from the third month of campaign onwards (P < 0.0001) and remained low thereafter. At the end of the campaign in 2013, an 83% reduction of ADT was observed for Glossina palpalis gambiensis and a 92% reduction for G. tachinoides. Tsetse flies were captured only in 29% of the sites found infested in 2008.

CONCLUSIONS

Tsetse flies could be suppressed efficiently but their elimination from the targeted area may require the use integrated methods including the Sterile Insect Technique, which is programmed through the development of the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC Burkina) insectarium. The challenge will remain the sustainability of the achievement.

Galba truncatula and Fasciola hepatica: Genetic costructures and interactions with intermediate host dispersal

Antagonistic interactions between hosts and parasites are key structuring forces in natural populations. Demographic factors like extinction, migration and the effective population size shape host-parasite metapopulational dynamics. Therefore, to understand the evolution of host-parasite systems it is necessary to study the distribution of the genetic variation of both entities simultaneously. In this paper, we investigate the population genetics co-structure of parasites and hosts within a metapopulation of the liver fluke, Fasciola hepatica, and two of its intermediate hosts, the main intermediate host in Europe, Galba truncatula, and a new intermediate host, Omphiscola glabra, in Central France. Our results reveal an absence of specificity of flukes as regard to the two alternative hosts though O. glabra shows higher prevalence of F. hepatica. Host and parasites displayed contrasting population genetics structure with very small, highly inbred (selfing) and strongly isolated G. truncatula populations and much bigger, panmictic and more dispersive F. hepatica. This could indicate a local adaptation of the parasite and a local maladaptation of the host. We also unveil a parasite-mediated biased population genetics structure suggesting that infected G. truncatula disperse more; have higher dispersal survival than uninfected snails or, more likely, that immigrant snails are infected more often than local snails (local parasites are less adapted to local hosts). Finally, an absence, or at least an ambiguous signature of isolation by distance was observed in both host and parasite population. A very weak migration rate for G. truncatula provides a reasonable explanation for this ambiguous result. Alternatively, smaller sample sizes combined with modest migration rates might explain the difficulties to unveil the signal in F. hepatica.

Investigating population differentiation in a major African agricultural pest: evidence from geometric morphometrics and connectivity suggests high invasion potential

The distribution, spatial pattern and population dynamics of a species can be influenced by differences in the environment across its range. Spatial variation in climatic conditions can cause local populations to undergo disruptive selection and ultimately result in local adaptation. However, local adaptation can be constrained by gene flow and may favour resident individuals over migrants-both are factors critical to the assessment of invasion potential. The Natal fruit fly (Ceratitis rosa) is a major agricultural pest in Africa with a history of island invasions, although its range is largely restricted to south east Africa. Across Africa, C. rosa is genetically structured into two clusters (R1 and R2), with these clusters occurring sympatrically in the north of South Africa. The spatial distribution of these genotypic clusters remains unexamined despite their importance for understanding the pest's invasion potential. Here, C. rosa, sampled from 22 South African locations, were genotyped at 11 polymorphic microsatellite loci and assessed morphologically using geometric morphometric wing shape analyses to investigate patterns of population structure and determine connectedness of pest-occupied sites. Our results show little to no intraspecific (population) differentiation, high population connectivity, high effective population sizes and only one morphological type (R2) within South Africa. The absence of the R1 morphotype at sites where it was previously found may be a consequence of differences in thermal niches of the two morphotypes. Overall, our results suggest high invasion potential of this species, that area-wide pest management should be undertaken on a country-wide scale, and that border control is critical to preventing further invasions.

Mapping landscape friction to locate isolated tsetse populations that are candidates for elimination

Tsetse flies are the cyclical vectors of deadly human and animal trypanosomes in sub-Saharan Africa. Tsetse control is a key component for the integrated management of both plagues, but local eradication successes have been limited to less than 2% of the infested area. This is attributed to either resurgence of residual populations that were omitted from the eradication campaign or reinvasion from neighboring infested areas. Here we focused on Glossina palpalis gambiensis, a riverine tsetse species representing the main vector of trypanosomoses in West Africa. We mapped landscape resistance to tsetse genetic flow, hereafter referred to as friction, to identify natural barriers that isolate tsetse populations. For this purpose, we fitted a statistical model of the genetic distance between 37 tsetse populations sampled in the region, using a set of remotely sensed environmental data as predictors. The least-cost path between these populations was then estimated using the predicted friction map. The method enabled us to avoid the subjectivity inherent in the expert-based weighting of environmental parameters. Finally, we identified potentially isolated clusters of G. p. gambiensis habitat based on a species distribution model and ranked them according to their predicted genetic distance to the main tsetse population. The methodology presented here will inform the choice on the most appropriate intervention strategies to be implemented against tsetse flies in different parts of Africa. It can also be used to control other pests and to support conservation of endangered species.

Mitochondrial DNA sequence divergence and diversity of Glossina fuscipes fuscipes in the Lake Victoria basin of Uganda: implications for control

BACKGROUND

Glossina fuscipes fuscipes is the main vector of African Trypanosomiasis affecting both humans and livestock in Uganda. The human disease (sleeping sickness) manifests itself in two forms: acute and chronic. The Lake Victoria basin in Uganda has the acute form and a history of tsetse re-emergence despite concerted efforts to control tsetse. The government of Uganda has targeted the basin for tsetse eradication. To provide empirical data for this initiative, we screened tsetse flies from the basin for genetic variation at the mitochondrial DNA cytochrome oxidase II (mtDNA COII) gene with the goal of investigating genetic diversity and gene flow among tsetse, tsetse demographic history; and compare these results with results from a previous study based on microsatellite loci data in the same area.

METHODS

We collected 429 Gff tsetse fly samples from 14 localities in the entire Ugandan portion of the Lake Victoria coast, covering 40,000 km(2). We performed genetic analyses on them and added data collected for 56 Gff individuals from 4 additional sampling sites in the basin. The 529 pb partial mitochondrial DNA cytochrome oxidase II (mtDNA COII) sequences totaling 485 were analysed for genetic differentiation, structuring and demographic history. The results were compared with findings from a previous study based on microsatellite loci data from the basin.

RESULTS

The differences within sampling sites explained a significant proportion of the genetic variation. We found three very closely related mtDNA population clusters, which co-occurred in multiple sites. Although Φ ST (0 - 0.592; P < 0.05) and Bayesian analyses suggest some level of weak genetic differentiation, there is no correlation between genetic divergence and geographic distance (r = 0.109, P = 0.185), and demographic tests provide evidence of locality-based demographic history.

CONCLUSION

The mtDNA data analysed here complement inferences made in a previous study based on microsatellite data. Given the differences in mutation rates, mtDNA afforded a look further back in time than microsatellites and revealed that Gff populations were more connected in the past. Microsatellite data revealed more genetic structuring than mtDNA. The differences in connectedness and structuring over time could be related to vector control efforts. Tsetse re-emergence after control interventions may be due to re-invasions from outside the treated areas, which emphasizes the need for an integrated area-wide tsetse eradication strategy for sustainable removal of the tsetse and trypanosomiasis problem from this area.

A Spatio-temporal Model of African Animal Trypanosomosis Risk

BACKGROUND

African animal trypanosomosis (AAT) is a major constraint to sustainable development of cattle farming in sub-Saharan Africa. The habitat of the tsetse fly vector is increasingly fragmented owing to demographic pressure and shifts in climate, which leads to heterogeneous risk of cyclical transmission both in space and time. In Burkina Faso and Ghana, the most important vectors are riverine species, namely Glossina palpalis gambiensis and G. tachinoides, which are more resilient to human-induced changes than the savannah and forest species. Although many authors studied the distribution of AAT risk both in space and time, spatio-temporal models allowing predictions of it are lacking.

METHODOLOGY/PRINCIPAL FINDINGS

We used datasets generated by various projects, including two baseline surveys conducted in Burkina Faso and Ghana within PATTEC (Pan African Tsetse and Trypanosomosis Eradication Campaign) national initiatives. We computed the entomological inoculation rate (EIR) or tsetse challenge using a range of environmental data. The tsetse apparent density and their infection rate were separately estimated and subsequently combined to derive the EIR using a "one layer-one model" approach. The estimated EIR was then projected into suitable habitat. This risk index was finally validated against data on bovine trypanosomosis. It allowed a good prediction of the parasitological status (r2 = 67%), showed a positive correlation but less predictive power with serological status (r2 = 22%) aggregated at the village level but was not related to the illness status (r2 = 2%).

CONCLUSIONS/SIGNIFICANCE

The presented spatio-temporal model provides a fine-scale picture of the dynamics of AAT risk in sub-humid areas of West Africa. The estimated EIR was high in the proximity of rivers during the dry season and more widespread during the rainy season. The present analysis is a first step in a broader framework for an efficient risk management of climate-sensitive vector-borne diseases.

Effect of sampling methods, effective population size and migration rate estimation in Glossina palpalis palpalis from Cameroon

Human and animal trypanosomiases are two major constraints to development in Africa. These diseases are mainly transmitted by tsetse flies in particular by Glossina palpalis palpalis in Western and Central Africa. To set up an effective vector control campaign, prior population genetics studies have proved useful. Previous studies on population genetics of G. p. palpalis using microsatellite loci showed high heterozygote deficits, as compared to Hardy-Weinberg expectations, mainly explained by the presence of null alleles and/or the mixing of individuals belonging to several reproductive units (Wahlund effect). In this study we implemented a system of trapping, consisting of a central trap and two to four satellite traps around the central one to evaluate a possible role of the Wahlund effect in tsetse flies from three Cameroon human and animal African trypanosomiases foci (Campo, Bipindi and Fontem). We also estimated effective population sizes and dispersal. No difference was observed between the values of allelic richness, genetic diversity and Wright's FIS, in the samples from central and from satellite traps, suggesting an absence of Wahlund effect. Partitioning of the samples with Bayesian methods showed numerous clusters of 2-3 individuals as expected from a population at demographic equilibrium with two expected offspring per reproducing female. As previously shown, null alleles appeared as the most probable factor inducing these heterozygote deficits in these populations. Effective population sizes varied from 80 to 450 individuals while immigration rates were between 0.05 and 0.43, showing substantial genetic exchanges between different villages within a focus. These results suggest that the "suppression" with establishment of physical barriers may be the best strategy for a vector control campaign in this forest context.

Ecotype evolution in Glossina palpalis subspecies, major vectors of sleeping sickness

Background

The role of environmental factors in driving adaptive trajectories of living organisms is still being debated. This is even more important to understand when dealing with important neglected diseases and their vectors.

Methodology/Principal Findings

In this paper, we analysed genetic divergence, computed from seven microsatellite loci, of 614 tsetse flies (Glossina palpalis gambiensis and Glossina palpalis palpalis, major vectors of animal and human trypanosomes) from 28 sites of West and Central Africa. We found that the two subspecies are so divergent that they deserve the species status. Controlling for geographic and time distances that separate these samples, which have a significant effect, we found that G. p. gambiensis from different landscapes (Niayes of Senegal, savannah and coastal environments) were significantly genetically different and thus represent different ecotypes or subspecies. We also confirm that G. p. palpalis from Ivory Coast, Cameroon and DRC are strongly divergent.

Conclusions/Significance

These results provide an opportunity to examine whether new tsetse fly ecotypes might display different behaviour, dispersal patterns, host preferences and vectorial capacities. This work also urges a revision of taxonomic status of Glossina palpalis subspecies and highlights again how fast ecological divergence can be, especially in host-parasite-vector systems.

The role of environmental factors in driving adaptive trajectories of living organisms is still being debated. This is even more important to understand when dealing with important and /or neglected diseases and their vectors. In this paper, we analysed genetic divergence, computed from several genetic markers, of 614 tsetse flies (Glossina palpalis gambiensis and Glossina palpalis palpalis, major vectors of animal and human trypanosomes) from 28 sites of West and Central Africa. We found that the two subspecies are so divergent that they deserve the species status. We found that G. p. gambiensis from different landscapes (Niayes of Senegal, savannah and coastal environments) were significantly genetically different, and thus represent different adaptive entities or even subspecies. We also confirm that G. p. palpalis from Ivory Coast, Cameroon and DRC are strongly divergent. These results provide an opportunity to examine whether these different types of tsetse fly might display different behaviour, dispersal patterns, host preferences and vectorial capacities. This work also urges a revision of taxonomic status of Glossina palpalis subspecies and highlights again how fast ecological divergence can be, especially in host-parasite-vector systems.

Perspectives on why digital ecologies matter: combining population genetics and ecologically informed agent-based models with GIS for managing dipteran livestock pests

It is becoming clear that handling the inherent complexity found in ecological systems is an essential task for finding ways to control insect pests of tropical livestock such as tsetse flies, and old and new world screwworms. In particular, challenging multivalent management programs, such as Area Wide Integrated Pest Management (AW-IPM), face daunting problems of complexity at multiple spatial scales, ranging from landscape level processes to those of smaller scales such as the parasite loads of individual animals. Daunting temporal challenges also await resolution, such as matching management time frames to those found on ecological and even evolutionary temporal scales. How does one deal with representing processes with models that involve multiple spatial and temporal scales? Agent-based models (ABM), combined with geographic information systems (GIS), may allow for understanding, predicting and managing pest control efforts in livestock pests. This paper argues that by incorporating digital ecologies in our management efforts clearer and more informed decisions can be made. I also point out the power of these models in making better predictions in order to anticipate the range of outcomes possible or likely.

Genetic correlations within and between isolated tsetse populations: what can we learn?

Isolated tsetse populations constitute a target for tsetse control programmes in endemic countries, since their isolation, if demonstrated, allows control without reinvasion risk from neighbouring populations. Population genetic parameters, such as the fixation index, have proven useful to assess isolation status, and should also give important information on the divergence time since isolation. We gathered results obtained from different datasets regarding several examples of putatively totally isolated tsetse populations of different tsetse species: Glossina palpalis gambiensis in Guinea, in the Niayes of Senegal, and in the sacred wood of Bama in Burkina Faso; G. tachinoides from Bitou and Pama in South-East Burkina Faso. The different levels of isolation were compared to differentiation between the two subspecies G. p. gambiensis and G. p. palpalis which both occur allopatrically along the Comoe River in Ivory Coast. We also use some historical evidence to calibrate differentiation speed and give estimates of time since separation for the different cases studied. Discrepancies mostly come from underestimate of effective population sizes, and we propose improving sampling design and genetic markers quality to circumvent such caveats.

Genetic comparison of Glossina tachinoides populations in three river basins of the Upper West Region of Ghana and implications for tsetse control

Tsetse flies are the cyclical vectors of African animal trypanosomosis (AAT) and human African trypanosomosis (HAT). In March 2010, the Government of Ghana initiated a large scale integrated tsetse eradication campaign in the Upper West Region (UWR) (≈18,000 km(2)) under the umbrella of the Pan-African Tsetse and Trypanosomosis Eradication Campaign (PATTEC). We investigated the structuring of Glossina tachinoides populations within and between the three main river basins of the target area in the UWR. Out of a total sample of 884 flies, a sub-sample of 266 was genotyped at nine microsatellite loci. The significance of the different hierarchical levels was tested using Yang's parameters estimated with Weir and Cockerham's method. A significant effect of traps within groups (pooling traps no more than 3 km distant from each other), of groups within river basins and of river basins within the whole target area was observed. Isolation by distance between traps was highly significant. A local density of 0.48-0.61 flies/m(2) was estimated and a dispersal distance that approximated 11 m per generation [CI 9, 17]. No significant sex-biased dispersal was detected. Dispersal distances of G. tachinoides in the UWR were relatively low, possibly as a result of the fragmentation of the habitat and the seasonality of the Kulpawn and Sissili rivers. Moreover, very high fly population densities were observed in the sample sites, which potentially reduces dispersal at constant habitat saturation, because the probability that migrants can established is reduced (density dependent dispersal). However, the observed spatial dispersal was deemed sufficient for a G. tachinoides-cleared area to be reinvaded from neighboring populations in adjacent river basins. These data corroborate results from other population genetics studies in West Africa, which indicate that G. tachinoides populations from different river basins cannot be considered isolated.

Assessing introgression of Sahelian zebu genes into native Bos taurus breeds in Burkina Faso

A total of 350 samples were analyzed to estimate zebu gene proportions into two different taurine cattle breeds of Burkina Faso (Lobi and N'Dama) using 38 microsatellites and various statistical methodologies. West African and East African zebu samples were sequentially used as reference parental populations. Furthermore, N'Dama cattle from Congo, the composite South African Bonsmara cattle breed and a pool of European cattle were used successively as second parental populations. Independently of the methodology applied: (a) the use of West African zebu samples gave higher admixture coefficients than the East African zebu; (b) the higher zebu proportions were estimated when the European cattle was used as parental population 2; and (c) the use of the N'Dama population from Congo as parental population 2 gave the more consistent zebu proportion estimates for both the Lobi and the N'Dama breeds. In any case, the zebu admixture proportions estimated were not negligible and were always higher in the N'Dama cattle than in the Lobi cattle of Burkina Faso. This suggested that the introgression of Sahelian zebu genes into the taurine cattle of Southern West Africa can follow a complex pattern that can depend on local agro-ecological features. The current research pointed out that the estimation of admixture coefficients is highly dependent on both the assumptions underlying the methodologies applied and the selection of parental populations. Our analyses suggest that either too high or nil genetic identity between the parental and the expectedly derived populations must be avoided.

Genetically distinct Glossina fuscipes fuscipes populations in the Lake Kyoga region of Uganda and its relevance for human African trypanosomiasis

Tsetse flies (Glossina spp.) are the sole vectors of Trypanosoma brucei—the agent of human (HAT) and animal (AAT) trypanosomiasis. Glossina fuscipes fuscipes (Gff) is the main vector species in Uganda—the only country where the two forms of HAT disease (rhodesiense and gambiense) occur, with gambiense limited to the northwest. Gff populations cluster in three genetically distinct groups in northern, southern, and western Uganda, respectively, with a contact zone present in central Uganda. Understanding the dynamics of this contact zone is epidemiologically important as the merger of the two diseases is a major health concern. We used mitochondrial and microsatellite DNA data from Gff samples in the contact zone to understand its spatial extent and temporal stability. We show that this zone is relatively narrow, extending through central Uganda along major rivers with south to north introgression but displaying no sex-biased dispersal. Lack of obvious vicariant barriers suggests that either environmental conditions or reciprocal competitive exclusion could explain the patterns of genetic differentiation observed. Lack of admixture between northern and southern populations may prevent the sympatry of the two forms of HAT disease, although continued control efforts are needed to prevent the recolonization of tsetse-free regions by neighboring populations.

Dynamics of tsetse natural infection rates in the Mouhoun river, Burkina Faso, in relation with environmental factors

In Burkina Faso, the cyclical vectors of African animal trypanosomoses (AAT) are riverine tsetse species, namely Glossina palpalis gambiensis Vanderplank (G.p.g.) and Glossina tachinoides Westwood (G.t.) (Diptera: Glossinidae). Experimental work demonstrated that environmental stress can increase the sensitivity of tsetse to trypanosome infection. Seasonal variations of the tsetse infection rates were monitored monthly over 17 months (May 2006–September 2007) in two sites (Douroula and Kadomba). In total, 1423 flies were dissected and the infection of the proboscis, middle intestine and salivary glands was noted. All the positive organs were analyzed using monospecific polymerase chain reaction (PCR) primers. To investigate the role of different environmental factors, fly infection rates were analyzed using generalized linear mixed binomial models using the species, sex, and monthly averages of the maximum, minimum and mean daily temperatures, rainfalls, Land Surface Temperature day (LSTd) and night (LSTn) as fixed effects and the trap position as a random effect. The overall infection rate was 10% from which the predominant species was T. congolense (7.6% of the flies), followed by T. vivax (2.2% of the flies). The best model (lowest AICc) for the global infection rates was the one with the maximum daily temperature only as fixed effect (p < 0.001). For T. congolense, the best model was the one with the tsetse species, sex, maximum daily temperature and rainfalls as fixed effect, where the maximum daily temperature was the main effect (p < 0.001). The number of T. vivax infections was too low to allow the models to converge. The maturation rate of T. congolense was very high (94%), and G. t. harbored a higher maturation rate (p = 0.03). The results are discussed in view of former laboratory studies showing that temperature stress can increase the susceptibility of tsetse to trypanosomes, as well as the possibility to improve AAT risk mapping using satellite images.

Glossina fuscipes populations provide insights for human African trypanosomiasis transmission in Uganda

Uganda has both forms of human African trypanosomiasis (HAT): the chronic gambiense disease in the northwest and the acute rhodesiense disease in the south. The recent spread of rhodesiense into central Uganda has raised concerns given the different control strategies the two diseases require. We present knowledge on the population genetics of the major vector species Glossina fuscipes fuscipes in Uganda with a focus on population structure, measures of gene flow between populations, and the occurrence of polyandry. The microbiome composition and diversity is discussed, focusing on their potential role on trypanosome infection outcomes. We discuss the implications of these findings for large-scale tsetse control programs, including suppression or eradication, being undertaken in Uganda, and potential future genetic applications.

Treating cattle to protect people? Impact of footbath insecticide treatment on tsetse density in Chad

BACKGROUND

In Chad, several species of tsetse flies (Genus: Glossina) transmit African animal trypanosomoses (AAT), which represents a major obstacle to cattle rearing, and sleeping sickness, which impacts public health. After the failure of past interventions to eradicate tsetse, the government of Chad is now looking for other approaches that integrate cost-effective intervention techniques, which can be applied by the stake holders to control tsetse-transmitted trypanosomoses in a sustainable manner. The present study thus attempted to assess the efficacy of restricted application of insecticides to cattle leg extremities using footbaths for controlling Glossina m. submorsitans, G. tachinoides and G. f. fuscipes in southern Chad.

METHODOLOGY/PRINCIPAL FINDINGS

Two sites were included, one close to the historical human African trypanosomiasis (HAT) focus of Moundou and the other to the active foci of Bodo and Moissala. At both sites, a treated and an untreated herd were compared. In the treatment sites, cattle were treated on a regular basis using a formulation of deltamethrin 0.005% (67 to 98 cattle were treated in one of the sites and 88 to 102 in the other one). For each herd, tsetse densities were monthly monitored using 7 biconical traps set along the river and beside the cattle pen from February to December 2009. The impact of footbath treatment on tsetse populations was strong (p < 10(-3)) with a reduction of 80% in total tsetse catches by the end of the 6-month footbath treatment.

CONCLUSIONS/SIGNIFICANCE

The impact of footbath treatment as a vector control tool within an integrated strategy to manage AAT and HAT is discussed in the framework of the "One Health" concept. Like other techniques based on the treatment of cattle, this technology should be used under controlled conditions, in order to avoid the development of insecticide and acaricide resistance in tsetse and tick populations, respectively.

The sequential aerosol technique: a major component in an integrated strategy of intervention against Riverine Tsetse in Ghana

Background

An integrated strategy of intervention against tsetse flies was implemented in the Upper West Region of Ghana (9.62°–11.00° N, 1.40°–2.76° W), covering an area of ≈18,000 km² within the framework of the Pan-African Tsetse and Trypanosomosis Eradication Campaign. Two species were targeted: Glossina tachinoides and Glossina palpalis gambiensis.

Methodology/Principal Findings

The objectives were to test the potentiality of the sequential aerosol technique (SAT) to eliminate riverine tsetse species in a challenging subsection (dense tree canopy and high tsetse densities) of the total sprayed area (6,745 km²) and the subsequent efficacy of an integrated strategy including ground spraying (≈100 km²), insecticide treated targets (20,000) and insecticide treated cattle (45,000) in sustaining the results of tsetse suppression in the whole intervention area. The aerial application of low-dosage deltamethrin aerosols (0.33–0.35 g a.i/ha) was conducted along the three main rivers using five custom designed fixed-wings Turbo thrush aircraft. The impact of SAT on tsetse densities was monitored using 30 biconical traps deployed from two weeks before until two weeks after the operations. Results of the SAT monitoring indicated an overall reduction rate of 98% (from a pre-intervention mean apparent density per trap per day (ADT) of 16.7 to 0.3 at the end of the fourth and last cycle). One year after the SAT operations, a second survey using 200 biconical traps set in 20 sites during 3 weeks was conducted throughout the intervention area to measure the impact of the integrated control strategy. Both target species were still detected, albeit at very low densities (ADT of 0.27 inside sprayed blocks and 0.10 outside sprayed blocks).

Conclusions/Significance

The SAT operations failed to achieve elimination in the monitored section, but the subsequent integrated strategy maintained high levels of suppression throughout the intervention area, which will contribute to improving animal health, increasing animal production and fostering food security.

We document the impact of an integrated strategy of intervention against riverine tsetse flies in the Upper West Region of Ghana within the framework of the Pan-African Tsetse and Trypanosomosis Eradication Campaign, in an area of ≈18,000 km². The strategy included a sequential aerosol technique (SAT) component, i.e. four applications of low-dosage deltamethrin aerosols, conducted along the three main rivers. The impact of SAT on tsetse densities was monitored in a challenging subsection (dense tree canopy and high tsetse densities) from two weeks before until two weeks after the operations. The SAT operations succeeded in reducing tsetse populations by 98% within one month but fell short of achieving elimination. Insecticide ground spraying, deltamethrin-treated targets and cattle were used as complementary tools to maintain tsetse suppression in the intervention area. An entomological survey conducted one year after SAT operations showed that both target species were still present, albeit at drastically reduced densities as compared to the baseline levels. This integrated strategy of intervention will contribute to improving animal health, increasing animal production and fostering food security in the target area.

Climate, cattle rearing systems and African Animal Trypanosomosis risk in Burkina Faso

Background

In sub-Saharan countries infested by tsetse flies, African Animal Trypanosomosis (AAT) is considered as the main pathological constraint to cattle breeding. Africa has known a strong climatic change and its population was multiplied by four during the last half-century. The aim of this study was to characterize the impact of production practices and climate on tsetse occurrence and abundance, and the associated prevalence of AAT in Burkina Faso.

Methodology/Principal Findings

Four sites were selected along a South-north transect of increasing aridity. The study combines parasitological and entomological surveys. For the parasitological aspect, blood samples were collected from 1,041 cattle selected through a stratified sampling procedure including location and livestock management system (long transhumance, short transhumance, sedentary). Parasitological and serological prevalence specific to livestock management systems show a gradual increase from the Sahelian to the Sudano-Guinean area (P<0.05). Livestock management system had also a significant impact on parasitological prevalence (P<0.05). Tsetse diversity, apparent densities and their infection rates overall decreased with aridity, from four species, an apparent density of 53.1 flies/trap/day and an infection rate of 13.7% to an absence at the northern edge of the transect, where the density and diversity of other biting flies were on the contrary highest (p<0.001).

Conclusions/Significance

The climatic pressure clearly had a negative impact on tsetse abundance and AAT risk. However, the persistency of tsetse habitats along the Mouhoun river loop maintains a high risk of cyclical transmission of T. vivax. Moreover, an “epidemic mechanical livestock trypanosomosis” cycle is likely to occur in the northern site, where trypanosomes are brought in by cattle transhuming from the tsetse infested area and are locally transmitted by mechanical vectors. In Burkina Faso, the impact of tsetse thus extends to a buffer area around their distribution belt, corresponding to the herd transhumance radius.

The population structure of Glossina fuscipes fuscipes in the Lake Victoria basin in Uganda: implications for vector control

Background

Glossina fuscipes fuscipes is the primary vector of trypanosomiasis in humans and livestock in Uganda. The Lake Victoria basin has been targeted for tsetse eradication using a rolling carpet initiative, from west to east, with four operational blocks (3 in Uganda and 1 in Kenya), under a Pan-African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC). We screened tsetse flies from the three Ugandan PATTEC blocks for genetic diversity at 15 microsatellite loci from continental and offshore populations to provide empirical data to support this initiative.

Methods

We collected tsetse samples from 11 sites across the Lake Victoria basin in Uganda. We performed genetic analyses on 409 of the collected tsetse flies and added data collected for 278 individuals in a previous study. The flies were screened across 15 microsatellite loci and the resulting data were used to assess the temporal stability of populations, to analyze patterns of genetic exchange and structuring, to estimate dispersal rates and evaluate the sex bias in dispersal, as well as to estimate demographic parameters (N_E and N_C).

Results

We found that tsetse populations in this region were stable over 4-16 generations and belong to 4 genetic clusters. Two genetic clusters (1 and 2) corresponded approximately to PATTEC blocks 1 and 2, while the other two (3 and 4) fell within PATTEC block 3. Island populations grouped into the same genetic clusters as neighboring mainland sites, suggesting presence of gene flow between these sites. There was no evidence of the stretch of water separating islands from the mainland forming a significant barrier to dispersal. Dispersal rates ranged from 2.5 km per generation in cluster 1 to 14 km per generation in clusters 3 and 4. We found evidence of male-biased dispersal. Few breeders are successfully dispersing over large distances. Effective population size estimates were low (33–310 individuals), while census size estimates ranged from 1200 (cluster 1) to 4100 (clusters 3 and 4). We present here a novel technique that adapts an existing census size estimation method to sampling without replacement, the scheme used in sampling tsetse flies.

Conclusion

Our study suggests that different control strategies should be implemented for the three PATTEC blocks and that, given the high potential for re-invasion from island sites, mainland and offshore sites in each block should be targeted at the same time.

Mathematical modeling, spatial complexity, and critical decisions in tsetse control

The tsetse fly complex (Glossina spp.) is widely recognized as a key contributor to the African continent's continuing struggle to emerge from deep economic, social, and political problems. Vector control, the backbone of intensive efforts to remove the human and livestock trypanosomosis problem, has been typified by spectacular successes and failures. There is widespread agreement that integrated vector control, combined with direct disease treatment and prevention, has to play a major role in alleviating the tsetse burden in Africa. Mathematical and computer-based simulation models have been extensively used to try to understand how best to manage these control efforts. Such models in ecology have been helpful in giving broad generalizations about population dynamics and control. Unfortunately, in many ways they have inadequately addressed key aspects of the fly's biology and ecology, particularly the spatio-temporal variability of its habitats. These too must factor in any control efforts. Mathematical models have inherent limitations that must be considered in their use for control programs. In this review, we consider some of the controversies being debated within the field of ecology and evolution about the use of mathematical models and critically review several models that have been influential in structuring tsetse control efforts. We also make recommendations on the appropriate role that mathematical and simulation models should play when used for these purposes. Management programs are often vulnerable to naively using these models inappropriately. The questions raised in this review will apply broadly to many conservation and area-wide pest control programs with an ecological component relying on mathematical and computer simulation models to inform their decisions.

Tsetse flies: their biology and control using area-wide integrated pest management approaches

Tsetse flies are the cyclical vectors of trypanosomes, the causative agents of 'sleeping sickness' or human African trypanosomosis (HAT) in humans and 'nagana' or African animal trypanosomosis (AAT) in livestock in Sub-saharan Africa. Many consider HAT as one of the major neglected tropical diseases and AAT as the single greatest health constraint to increased livestock production. This review provides some background information on the taxonomy of tsetse flies, their unique way of reproduction (adenotrophic viviparity) making the adult stage the only one easily accessible for control, and how their ecological affinities, their distribution and population dynamics influence and dictate control efforts. The paper likewise reviews four control tactics (sequential aerosol technique, stationary attractive devices, live bait technique and the sterile insect technique) that are currently accepted as friendly to the environment, and describes their limitations and advantages and how they can best be put to practise in an IPM context. The paper discusses the different strategies for tsetse control i.e. localised versus area-wide and focusses thereafter on the principles of area-wide integrated pest management (AW-IPM) and the phased-conditional approach with the tsetse project in Senegal as a recent example. We argue that sustainable tsetse-free zones can be created on Africa mainland provided certain managerial and technical prerequisites are in place.

Irradiated male tsetse from a 40-year-old colony are still competitive in a Riparian forest in Burkina Faso

BACKGROUND

Tsetse flies are the cyclical vectors of African trypanosomosis that constitute a major constraint to development in Africa. Their control is an important component of the integrated management of these diseases, and among the techniques available, the sterile insect technique (SIT) is the sole that is efficient at low densities. The government of Burkina Faso has embarked on a tsetse eradication programme in the framework of the PATTEC, where SIT is an important component. The project plans to use flies from a Glossina palpalis gambiensis colony that has been maintained for about 40 years at the Centre International de Recherche-Développement sur l'Elevage en zone Subhumide (CIRDES). It was thus necessary to test the competitiveness of the sterile males originating from this colony.

METHODOLOGY/PRINCIPAL FINDINGS

During the period January-February 2010, 16,000 sterile male G. p. gambiensis were released along a tributary of the Mouhoun river. The study revealed that with a mean sterile to wild male ratio of 1.16 (s.d. 0.38), the abortion rate of the wild female flies was significantly higher than before (p = 0.026) and after (p = 0.019) the release period. The estimated competitiveness of the sterile males (Fried index) was 0.07 (s.d. 0.02), indicating that a sterile to wild male ratio of 14.4 would be necessary to obtain nearly complete induced sterility in the female population. The aggregation patterns of sterile and wild male flies were similar. The survival rate of the released sterile male flies was similar to that observed in 1983-1985 for the same colony.

CONCLUSIONS/SIGNIFICANCE

We conclude that gamma sterilised male G. p. gambiensis derived from the CIRDES colony have a competitiveness that is comparable to their competitiveness obtained 35 years ago and can still be used for an area-wide integrated pest management campaign with a sterile insect component in Burkina Faso.

Ascertaining gene flow patterns in livestock populations of developing countries: a case study in Burkina Faso goat

Background

Introgression of Sahel livestock genes southwards in West Africa may be favoured by human activity and the increase of the duration of the dry seasons since the 1970’s. The aim of this study is to assess the gene flow patterns in Burkina Faso goat and to ascertain the most likely factors influencing geographic patterns of genetic variation in the Burkina Faso goat population.

Results

A total of 520 goat were sampled in 23 different locations of Burkina Faso and genotyped for a set of 19 microsatellites. Data deposited in the Dryad repository: http://dx.doi.org/10.5061/dryad.41h46j37. Although overall differentiation is poor (F_ST = 0.067 ± 0.003), the goat population of Burkina Faso is far from being homogeneous. Barrier analysis pointed out the existence of: a) genetic discontinuities in the Central and Southeast Burkina Faso; and b) genetic differences within the goat sampled in the Sahel or the Sudan areas of Burkina Faso. Principal component analysis and admixture proportion scores were computed for each population sampled and used to construct interpolation maps. Furthermore, Population Graph analysis revealed that the Sahel and the Sudan environmental areas of Burkina Faso were connected through a significant number of extended edges, which would be consistent with the hypothesis of long-distance dispersal. Genetic variation of Burkina Faso goat followed a geographic-related pattern. This pattern of variation is likely to be related to the presence of vectors of African animal trypanosomosis. Partial Mantel test identified the present Northern limit of trypanosome vectors as the most significant landscape boundary influencing the genetic variability of Burkina Faso goat (p = 0.008). The contribution of Sahel goat genes to the goat populations in the Northern and Eastern parts of the Sudan-Sahel area of Burkina Faso was substantial. The presence of perennial streams explains the existence of trypanosome vectors. The South half of the Nakambé river (Southern Ouagadougou) and the Mouhoun river loop determined, respectively, the Eastern and Northern limits for the expansion of Sahelian goat genes. Furthermore, results from partial Mantel test suggest that the introgression of Sahelian goat genes into Djallonké goat using human-influenced genetic corridors has a limited influence when compared to the biological boundary defined by the northern limits for the distribution of the tsetse fly. However, the genetic differences found between the goat sampled in Bobo Dioulasso and the other populations located in the Sudan area of Burkina Faso may be explained by the broad goat trade favoured by the main road of the country.

Conclusions

The current analysis clearly suggests that genetic variation in Burkina Faso goat: a) follows a North to South clinal; and b) is affected by the distribution of the tsetse fly that imposes a limit to the Sahelian goat expansion due to their trypanosusceptibility. Here we show how extensive surveys on livestock populations can be useful to indirectly assess the consequences of climate change and human action in developing countries.

Understanding local population genetics of tsetse: the case of an isolated population of Glossina palpalis gambiensis in Burkina Faso

Tsetse flies are the vectors of human and animal trypanosomiases. For tsetse eradication programs, it is crucial to be able to identify and target isolated populations, because they can be targeted for eradication without risk of reinvasion. However, most data that are available on non-isolated populations fail to find how these populations are locally structured, because Wahlund effect (admixture of individuals from genetically different units) always interfere with interpretations. In this paper, we investigated the genetic population structure of a possibly isolated population of Glossina palpalis gambiensis in a sacred wood in South Burkina Faso, using microsatellite DNA markers. We found that genotypic proportions in this population were in agreement with random mating model and that these tsetse were genetically highly differentiated from other populations of the same Mouhoun river basin only a few kilometers away, confirming its genetic isolation. The population also displayed substantial temporal differentiation in a two years period that lead to an estimate of effective population size of ∼100 individuals. The fact that no Wahlund effect was identified allowed us to accurately measure the basic genetic parameters of this isolated population. Identifying such isolated and small populations is crucial for eradication programs and should be implemented more often.

Contrasting population structures of two vectors of African trypanosomoses in Burkina Faso: consequences for control

Background

African animal trypanosomosis is a major obstacle to the development of more efficient and sustainable livestock production systems in West Africa. Riverine tsetse species such as Glossina palpalis gambiensis Vanderplank and Glossina tachinoides Westwood are the major vectors. A wide variety of control tactics is available to manage these vectors, but their removal will in most cases only be sustainable if the control effort is targeting an entire tsetse population within a circumscribed area.

Methodology/Principal Findings

In the present study, genetic variation at microsatellite DNA loci was used to examine the population structure of G. p. gambiensis and G. tachinoides inhabiting four adjacent river basins in Burkina Faso, i.e. the Mouhoun, the Comoé, the Niger and the Sissili River Basins. Isolation by distance was significant for both species across river basins, and dispersal of G. tachinoides was ∼3 times higher than that of G. p. gambiensis. Thus, the data presented indicate that no strong barriers to gene flow exists between riverine tsetse populations in adjacent river basins, especially so for G. tachinoides.

Conclusions/Significance

Therefore, potential re-invasion of flies from adjacent river basins will have to be prevented by establishing buffer zones between the Mouhoun and the other river basin(s), in the framework of the PATTEC (Pan African Tsetse and Trypanosomosis Eradication Campaign) eradication project that is presently targeting the northern part of the Mouhoun River Basin. We argue that these genetic analyses should always be part of the baseline data collection before any tsetse control project is initiated.

Tsetse flies are insects that transmit trypanosomes to humans (sleeping sickness) and animals (nagana). Controlling these vectors is a very efficient way to control these diseases. In Burkina Faso, a tsetse eradication campaign is presently targeting the northern part of the Mouhoun River Basin. To attain this objective, the approach has to be area-wide, i.e. the control effort targets an entire pest population within a circumscribed area. To assess the level of this isolation, we studied the genetic structure of Glossina palpalis gambiensis and Glossina tachinoides populations in the target area and in the adjacent river basins of the Comoé, the Niger and the Sissili River Basins. Our results suggest an absence of strong genetic isolation of the target populations. We therefore recommend establishing permanent buffer zones between the Mouhoun and the other river basin(s) to prevent reinvasion. This kind of study may be extended to other areas on other tsetse species.

Feeding behaviour of potential vectors of West Nile virus in Senegal

Background

West Nile virus (WNV) is a widespread pathogen maintained in an enzootic cycle between mosquitoes and birds with occasional spill-over into dead-end hosts such as horses and humans. Migratory birds are believed to play an important role in its dissemination from and to the Palaearctic area, as well as its local dispersion between wintering sites. The Djoudj Park, located in Senegal, is a major wintering site for birds migrating from Europe during the study period (Sept. 2008- Jan. 2009). In this work, we studied the seasonal feeding behaviour dynamics of the potential WNV mosquito vectors at the border of the Djoudj Park, using a reference trapping method (CDC light CO₂-baited traps) and two host-specific methods (horse- and pigeon-baited traps). Blood meals of engorged females were analysed to determine their origin.

Results

Results indicated that Culex tritaeniorhynchus and Cx. neavei may play a key role in the WNV transmission dynamics, the latter being the best candidate bridging-vector species between mammals and birds. Moreover, the attractiveness of pigeon- and horse-baited traps for Cx. neavei and Cx. tritaeniorhynchus varied with time. Finally, Cx. tritaeniorhynchus was only active when the night temperature was above 20°C, whereas Cx. neavei was active throughout the observation period.

Conclusions

Cx. neavei and Cx. tritaeniorhynchus are the main candidate vectors for the transmission of WNV in the area. The changes in host attractiveness might be related to variable densities of the migratory birds during the trapping period. We discuss the importance of these results on the risk of WNV transmission in horses and humans.

Spatio-temporal distribution of tsetse and other biting flies in the Mouhoun River basin, Burkina Faso

In the Mouhoun River basin, Burkina Faso, the main vectors of African animal trypanosomoses are Glossina palpalis gambiensis Vanderplank and Glossina tachinoides Westwood (Diptera: Glossinidae), both of which are riverine tsetse species. The aim of our study was to understand the impact of landscape anthropogenic changes on the seasonal dynamics of vectors and associated trypanosomosis risk. Three sites were selected on the basis of the level of disturbance of tsetse habitats and predominant tsetse species: disturbed (Boromo, for G. tachinoides) and half-disturbed (Douroula for G. tachinoides and Kadomba for G. p. gambiensis). At each of these sites, seasonal variations in the apparent densities of tsetse and mechanical vectors and tsetse infection rates were monitored over 17 months. Tsetse densities differed significantly between sites and seasons. Of 5613 captured tsetse, 1897 were dissected; 34 of these were found to be infected with trypanosomes. The most frequent infection was Trypanosoma vivax (1.4%), followed by Trypanosoma congolense (0.3%) and Trypanosoma brucei (0.05%). The mean physiological age of 703 tsetse females was investigated to better characterize the transmission risk. Despite the environmental changes, it appeared that tsetse lived long enough to transmit trypanosomes, especially in half-disturbed landscapes. A total of 3021 other biting flies from 15 species (mainly Tabanidae and Stomoxyinae) were also caught: their densities also differed significantly among sites and seasons. Their relative importance regarding trypanosome transmission is discussed; the trypanosomosis risk in cattle was similar at all sites despite very low tsetse densities (but high mechanical vector densities) in one of them.

Temporal stability of Glossina fuscipes fuscipes populations in Uganda

BACKGROUND

Glossina fuscipes, a riverine species of tsetse, is the major vector of human African trypanosomiasis (HAT) in sub-Saharan Africa. Understanding the population dynamics, and specifically the temporal stability, of G. fuscipes will be important for informing vector control activities. We evaluated genetic changes over time in seven populations of the subspecies G. f. fuscipes distributed across southeastern Uganda, including a zone of contact between two historically isolated lineages. A total of 667 tsetse flies were genotyped at 16 microsatellite loci and at one mitochondrial locus.

RESULTS

Results of an AMOVA indicated that time of sampling did not explain a significant proportion of the variance in allele frequencies observed across all samples. Estimates of differentiation between samples from a single population ranged from approximately 0 to 0.019, using Jost's DEST. Effective population size estimates using momentum-based and likelihood methods were generally large. We observed significant change in mitochondrial haplotype frequencies in just one population, located along the zone of contact. The change in haplotypes was not accompanied by changes in microsatellite frequencies, raising the possibility of asymmetric mating compatibility in this zone.

CONCLUSION

Our results suggest that populations of G. f. fuscipes were stable over the 8-12 generations studied. Future studies should aim to reconcile these data with observed seasonal fluctuations in the apparent density of tsetse.

A geographical approach to identify sleeping sickness risk factors in a mangrove ecosystem

OBJECTIVES

To provide a better understanding of sleeping sickness transmission and spread in mangrove areas to optimize its control.

METHODS

In the Forecariah mangrove area, Guinea, 19 sleeping sickness cases and 19 matched controls were followed up in their living areas (at home, in fields and at water points). All occupational sites and pathways were mapped and then placed in their environmental context.

RESULTS

The sleeping sickness cases displayed a significantly broader and more diverse spatial occupation than the controls. They covered double the daily walking distances of controls and had on average two more occupational sites, most of which were located in mangrove forests. Activities with a higher transmission risk (rice culture, attendance of pirogue jetties) were identified as well as high-risk areas and pathways.

CONCLUSIONS

An entomological control strategy targeting transmission risk areas is proposed. Its implementation in a control programme would reduce by 86% the efforts needed for a classical vector control programme throughout the area. Medical surveys set up at specific locations, such as pirogue jetties and high-risk paths, should also enable better targeting of the population at highest risk.

Population genetics as a tool to select tsetse control strategies: suppression or eradication of Glossina palpalis gambiensis in the Niayes of Senegal

BACKGROUND

The Government of Senegal has initiated the "Projet de lutte contre les glossines dans les Niayes" to remove the trypanosomosis problem from this area in a sustainable way. Due to past failures to sustainably eradicate Glossina palpalis gambiensis from the Niayes area, controversies remain as to the best strategy implement, i.e. "eradication" versus "suppression." To inform this debate, we used population genetics to measure genetic differentiation between G. palpalis gambiensis from the Niayes and those from the southern tsetse belt (Missira).

METHODOLOGY/PRINCIPAL FINDINGS

Three different markers (microsatellite DNA, mitochondrial CO1 DNA, and geometric morphometrics of the wings) were used on 153 individuals and revealed that the G. p. gambiensis populations of the Niayes were genetically isolated from the nearest proximate known population of Missira. The genetic differentiation measured between these two areas (theta = 0.12 using microsatellites) was equivalent to a between-taxa differentiation. We also demonstrated that within the Niayes, the population from Dakar - Hann was isolated from the others and had probably experienced a bottleneck.

CONCLUSION/SIGNIFICANCE

The information presented in this paper leads to the recommendation that an eradication strategy for the Niayes populations is advisable. This kind of study may be repeated in other habitats and for other tsetse species to (i) help decision on appropriate tsetse control strategies and (ii) find other possible discontinuities in tsetse distribution.

Updating the northern tsetse limit in Burkina Faso (1949-2009): impact of global change

The northern distribution limit of tsetse flies was updated in Burkina Faso and compared to previous limits to revise the existing map of these vectors of African trypanosomiases dating from several decades ago. From 1949 to 2009, a 25- to 150-km shift has appeared toward the south. Tsetse are now discontinuously distributed in Burkina Faso with a western and an eastern tsetse belt. This range shift can be explained by a combination of decreased rainfall and increased human density. Within a context of international control, this study provides a better understanding of the factors influencing the distribution of tsetse flies.

A changing environment and the epidemiology of tsetse-transmitted livestock trypanosomiasis

The distribution, prevalence and impact of vector-borne diseases are often affected by anthropogenic environmental changes that alter the interactions between the host, the parasite and the vector. In the case of tsetse-transmitted livestock trypanosomiasis these changes are a result of the encroachment of people and their livestock into tsetse-infected wild areas. This has created a sequence of new epidemiological settings that is changing the relative importance of the domestic or sylvatic trypanosome transmission cycles and is causing concomitant changes in the impact of the disease on livestock. These changes in the dynamics of the epidemiology have an important impact on the factors that need to be considered when developing area-specific strategies for the future management of tsetse-transmitted livestock trypanosomiasis.