Riverine Tsetse Species Research Articles

The continental atlas of tsetse and African animal trypanosomosis in Nigeria

Tsetse-transmitted trypanosomosis remains a major animal health problem in Nigeria, in a context where changes in land cover, climate and control interventions are modifying its epidemiological patterns. Evidence-based decision making for the progressive control of the disease requires spatially-explicit information on its occurrence and prevalence, as well as on the distribution and abundance of the tsetse vector.In the framework of the continental Atlas of tsetse and African animal trypanosomosis (AAT), a geo-referenced database was assembled for Nigeria, based on the systematic review of 133 scientific publications (period January 1990 - March 2019).The three main species of trypanosomes responsible for the disease (i.e. Trypanosoma vivax, T. congolense and T. brucei) were found to be widespread, thus posing a national-level problem. Their geographic distribution extends beyond the tsetse-infested belt, owing to the combined effect of animal movement and mechanical transmission by non-tsetse vectors. T. simiae, the major trypanosomal pathogen in pigs, T. godfreyi and the human-infective T. brucei gambiense were also reported. AAT was reported in a number of susceptible host species, including cattle, sheep, goats, pigs, camels, horses, donkeys and dogs, while no study on wildlife was identified. Estimates of prevalence are heavily influenced by the sensitivity of the diagnostic techniques, ranging from an average of 3.5% for blood films to 31.0% for molecular techniques.Two riverine tsetse species (i.e. Glossina palpalis palpalis and G. tachinoides) were found to have the broadest geographical range, as they were detected in all six geopolitical zones of Nigeria. By contrast, the distribution of savannah species (i.e. G. morsitans submorsitans and G. longipalpis) appears to be highly fragmented, and limited to protected areas. Very little information is available for forest species, with one single paper reporting on G. fusca congolensis and G. nigrofusca nigrofusca in the Niger Delta region.The future development of a national Atlas of tsetse and AAT, relying on both published and unpublished information, could improve on the present review and provide further epidemiological evidence for decision making.

Can sequential aerosol technique be used against riverine tsetse?

For the implementation of the sequential aerosol technique (SAT), several technical constraints such as the calibration of the spray, as well as the meteorological conditions during spray operations, need to be taken into account [1, 2]. Because some of these constraints differ depending on whether savannah or riverine tsetse flies are the target of control, the technique, which has been proven successful in southern Africa against Glossina morsitans, cannot be transferred indiscriminately to combat riverine tsetse flies [3]. This paper aims at addressing these elements and identifying which studies are required before using SAT against riverine tsetse species.

Comparison of single cell sensitivities to acetone, 1-octen-3-ol and 3-methylphenol in the riverine tsetse species Glossina fuscipes fuscipes and G. palpalis palpalis

Action potentials from individual cells were recorded from antennae (funiculi) of living tsetse flies, Glossina p. palpalis and Glossina f. fuscipes using a “surface-contact” recording technique. Stimuli were vapours of 1-octen-3-ol, acetone and 3-methylphenol. Of the 101 and 128 olfactory cells tested for their sensitivity to odour stimuli in G. p. palpalis and G. f. fuscipes, respectively, the majority (83 and 77%) were activated by more than one chemical. The numbers of these “generalist” cells were 20 and 15% higher in females than in males. Response intensity increased with increasing odour dose. Temporal patterns of excitation were phasic-tonic and showed cells with relatively rapid cessation of spike activity after the end of stimulation and cells which continued firing for several seconds or even minutes after stimulation. Inhibition by odours only occurred in a minority of cells and was dose-dependent. For each of the three substances the excitatory response was significantly higher in G. f. fuscipes than in G. p. palpalis, whereas no significant differences between inhibitory responses were found. In G. f. fuscipes each stimulus evoked equal excitatory responses. In G. p. palpalis, however, acetone induced significantly higher responses than 1-octen-3-ol and 3-methylphenol. Response intensities to each of the three chemicals did not differ between male and female G. p. palpalis, whereas in G. f. fuscipes 1-octen-3-ol evoked significantly higher responses in males. Possible mechanisms of receptor cell odour coding and behavioural effects of the various cell type activities are discussed.

Community-based tsetse fly control minimizes the effect of trypanosomosis on livestock in Metekel zone, Ethiopia.

Tsetse-related trypanosomosis threatens integrated crop-livestock production system in Ethiopia. Metekel zone is among the tsetse-infested areas in Ethiopia. Community-based tsetse fly and trypanosomosis control using targets was conducted from June 2011 to May 2012 in the zone. In coordination with the control project, a questionnaire survey was administered to assess the economic importance of the tsetse fly and trypanosomosis control conducted in the Zone. On average, there were 3 livestock deaths/household before control and this was decreased to 0.07 after control. This death was contributed from all possible livestock diseases. A household loss 4380 (95% CI = 4223.98-4541.79) EB (1 US$ = 17.9EB) only due to the deaths of his/her livestock was reduced to 90.78 (95% CI = 89.15-92.42) EB after control. Morbidity of oxen in the study areas was significantly decreased by 70.7%. As a result, the monetary loss to rent for healthy draught oxen was decreased by 96.4%. When a household with one cattle harvest 10.9 quintals (1090kg), his neighbor's household with ten cattle maximize his/her yield to about 50.5 quintals (5050kg) of crops indicating that trypanosomosis has great economic impact in crop-livestock production system. As the community-based tsetse fly control approach is simple, cost effective, and appropriate for riverine tsetse species, we conclude that farmers can minimize the negative impact of tsetse flies and trypanosomosis in the tsetse-infested areas of African countries.

Tsetse Control and the Elimination of Gambian Sleeping Sickness.

Tsetse Control and the Elimination of Gambian Sleeping Sickness.

Community-based tsetse fly control significantly reduces fly density and trypanosomosis prevalence in Metekel Zone, Northwest, Ethiopia.

African animal trypanosomosis is a great obstacle to livestock production where tsetse flies play a major role. Metekel zone is among the tsetse-infested areas. Community-based tsetse fly and trypanosomosis control using targets was conducted from June 2011 to May 2012 in Metekel zone, Ethiopia, to decrease trypanosomosis and tsetse fly. Cloth screen targets were developed, impregnated with 0.1% deltamethrin, and deployed alongside rivers by the research team together with the community animal health workers. Monthly parasitological and entomological data were collected, processed, and compared with similar data collected before control. Overall average tsetse fly (Glossina tachinoides) density decreased from 1.13 to 0.18fly/trap/day after control. The density was decreased in all sites with no significant difference among the sites. However, higher decrements were observed in the dry and late dry seasons by more than 12 and 6 times, respectively. The reduction in overall apparent prevalence of trypanosomosis caused by Trypanosoma congolense, Trypanosoma brucei, and Trypanosoma vivax from 12.14% before to 3.61% after control coincides with the tsetse fly reduction. In all the study sites, significant reduction was observed before and after control. The highest decrement was observed in the late dry season when the apparent prevalence was reduced from 7.89 to 1.17% before and after control, respectively. As this approach is simple, cost-effective, and appropriate for riverine tsetse species, we recommend to be scaled up to other similar places.

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.

Mapping the benefit-cost ratios of interventions against bovine trypanosomosis in Eastern Africa

This study builds upon earlier work mapping the potential benefits from bovine trypanosomosis control and analysing the costs of different approaches. Updated costs were derived for five intervention techniques: trypanocides, targets, insecticide-treated cattle, aerial spraying and the release of sterile males. Two strategies were considered: continuous control and elimination. For mapping the costs, cattle densities, environmental constraints, and the presence of savannah or riverine tsetse species were taken into account. These were combined with maps of potential benefits to produce maps of benefit-cost ratios.The results illustrate a diverse picture, and they clearly indicate that no single technique or strategy is universally profitable. For control using trypanocide prophylaxis, returns are modest, even without accounting for the risk of drug resistance but, in areas of low cattle densities, this is the only approach that yields a positive return. Where cattle densities are sufficient to support it, the use of insecticide-treated cattle stands out as the most consistently profitable technique, widely achieving benefit-cost ratios above 5. In parts of the high-potential areas such as the mixed farming, high-oxen-use zones of western Ethiopia, the fertile crescent north of Lake Victoria and the dairy production areas in western and central Kenya, all tsetse control strategies achieve benefit-cost ratios from 2 to over 15, and for elimination strategies, ratios from 5 to over 20. By contrast, in some areas, notably where cattle densities are below 20per km2, the costs of interventions against tsetse match or even outweigh the benefits, especially for control scenarios using aerial spraying or the deployment of targets where both savannah and riverine flies are present. If the burden of human African trypanosomosis were factored in, the benefit-cost ratios of some of the low-return areas would be considerably increased.Comparatively, elimination strategies give rise to higher benefit-cost ratios than do those for continuous control. However, the costs calculated for elimination assume problem-free, large scale operations, and they rest on the outputs of entomological models that are difficult to validate in the field. Experience indicates that the conditions underlying successful and sustained elimination campaigns are seldom met.By choosing the most appropriate thresholds for benefit-cost ratios, decision-makers and planners can use the maps to define strategies, assist in prioritising areas for intervention, and help choose among intervention techniques and approaches. The methodology would have wider applicability in analysing other disease constraints with a strong spatial component.

The Sequential Aerosol Technique: A Major Component in an Integrated Strategy of Intervention against Riverine Tsetse in Ghana

BackgroundAn 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 FindingsThe 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/SignificanceThe 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.

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.

Contrasting Population Structures of Two Vectors of African Trypanosomoses in Burkina Faso: Consequences for Control

BackgroundAfrican 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 FindingsIn 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/SignificanceTherefore, 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.

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.

The prevalence of African animal trypanosomoses and tsetse presence in Western Senegal

In 2005, the Government of Senegal initiated a tsetse eradication campaign in the Niayes and La Petite Côte aiming at the removal of African Animal Trypanosomosis (AAT), which is one of the main constraints to the development of more effective cattle production systems. The target area has particular meteorological and ecological characteristics that provide great potential for animal production, but it is unfortunately still infested by the riverine tsetse species Glossina palpalis gambiensis Vanderplank (Diptera: Glossinidae). The tsetse project in Senegal has adopted an area-wide integrated pest management (AW-IPM) approach that targets the entire tsetse population within a delimited area. During the first phase of the programme, a feasibility study was conducted that included the collection of entomological, veterinary, population genetics, environmental and socioeconomic baseline data. This paper presents the parasitological and serological prevalence data of AAT in cattle residing inside and outside the tsetse-infested areas of the target zone prior to the control effort. At the herd level, a mean parasitological prevalence of 2.4% was observed, whereas a serological prevalence of 28.7%, 4.4%, and 0.3% was obtained for Trypanosoma vivax, T. congolense and T. brucei brucei, respectively. The observed infection risk was 3 times higher for T. congolense and T. vivax in the tsetse-infested than in the assumed tsetse-free areas. Moreover, AAT prevalence decreased significantly with distance from the nearest tsetse captured which indicated that cyclical transmission of the parasites by tsetse was predominant over mechanical transmission by numerous other biting flies present. The importance of these results for the development of a control strategy for the planned AW-IPM campaign is discussed.

Stratified Entomological Sampling in Preparation for an Area-Wide Integrated Pest Management Program: The Example of &lt;I&gt;Glossina palpalis gambiensis&lt;/I&gt; (Diptera: Glossinidae) in the Niayes of Senegal

The riverine tsetse species Glossina palpalis gambiensis Vanderplank 1949 (Diptera: Glossinidae) inhabits riparian forests along river systems in West Africa. The government of Senegal has embarked on a project to eliminate this tsetse species, and African animal trypanosomoses, from the Niayes area using an area-wide integrated pest management approach. A stratified entomological sampling strategy was therefore developed using spatial analytical tools and mathematical modeling. A preliminary phytosociological census identified eight types of suitable habitat, which could be discriminated from LandSat 7 ETM+ satellite images and denominated wet areas. At the end of March 2009, 683 unbaited Vavoua traps had been deployed, and the observed infested area in the Niayes was 525 km2. In the remaining area, a mathematical model was used to assess the risk that flies were present despite a sequence of zero catches. The analysis showed that this risk was above 0.05 in 19% of this area that will be considered as infested during the control operations. The remote sensing analysis that identified the wet areas allowed a restriction of the area to be surveyed to 4% of the total surface area (7,150 km2), whereas the mathematical model provided an efficient method to improve the accuracy and the robustness of the sampling protocol. The final size of the control area will be decided based on the entomological collection data. This entomological sampling procedure might be used for other vector or pest control scenarios.

Stratified Entomological Sampling in Preparation for an Area-Wide Integrated Pest Management Program: The Example of Glossina palpalis gambiensis (Diptera: Glossinidae) in the Niayes of Senegal

The riverine tsetse species Glossina palpalis gambiensis Vanderplank 1949 (Diptera: Glossinidae) inhabits riparian forests along river systems in West Africa. The government of Senegal has embarked on a project to eliminate this tsetse species, and African animal trypanosomoses, from the Niayes area using an area-wide integrated pest management approach. A stratified entomological sampling strategy was therefore developed using spatial analytical tools and mathematical modeling. A preliminary phytosociological census identified eight types of suitable habitat, which could be discriminated from LandSat 7 ETM + satellite images and denominated wet areas. At the end of March 2009, 683 unbaited Vavoua traps had been deployed, and the observed infested area in the Niayes was 525 km2. In the remaining area, a mathematical model was used to assess the risk that flies were present despite a sequence of zero catches. The analysis showed that this risk was above 0.05 in 19% of this area that will be considered as infested during the control operations. The remote sensing analysis that identified the wet areas allowed a restriction of the area to be surveyed to 4% of the total surface area (7,150 km2), whereas the mathematical model provided an efficient method to improve the accuracy and the robustness of the sampling protocol. The final size of the control area will be decided based on the entomological collection data. This entomological sampling procedure might be used for other vector or pest control scenarios.

Population structure of Glossina palpalis gambiensis (Diptera: Glossinidae) between river basins in Burkina Faso: Consequences for area-wide integrated pest management

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 are their major vectors. A wide variety of control tactics is available to manage these vectors, but their elimination will only be sustainable if control is exercised following area-wide integrated pest management (AW-IPM) principles, i.e. the control effort is targeting an entire tsetse population within a circumscribed area. In the present study, genetic variation at microsatellite DNA loci was used to examine the population structure of G. p. gambiensis inhabiting two adjacent river basins, i.e. the Comoé and the Mouhoun River basins in Burkina Faso. A remote sensing analysis revealed that the woodland savannah habitats between the river basins have remained unchanged during the last two decades. In addition, genetic variation was studied in two populations that were separated by a man-made lake originating from a dam built in 1991 on the Comoé. Low genetic differentiation was observed between the samples from the Mouhoun and the Comoé River basins and no differentiation was found between the samples separated by the dam. The data presented indicate that the overall genetic differentiation of G. p. gambiensis populations inhabiting two adjacent river basins in Burkina Faso is low (F(ST)=0.016). The results of this study suggest that either G. p. gambiensis populations from the Mouhoun are not isolated from those of the Comoé, or that the isolation is too recent to be detected. If elimination of the G. p. gambiensis population from the Mouhoun River basin is the selected control strategy, re-invasion from adjacent river basins may need to be prevented by establishing a buffer zone between the Mouhoun and the other river basin(s).

Fragmentation Analysis for Prediction of Suitable Habitat for Vectors: Example of Riverine Tsetse Flies in Burkina Faso

Tsetse flies are the cyclic vectors of sleeping sickness and African animal trypanosomosis. The possibility to classify the natural habitat of riverine tsetse species is explored in the Mouhoun River basin, Burkina Faso: the objectives were to discriminate the riverine forests community types and their fragmentation levels by using Landsat 7 enhanced thematic mapper images, to map tsetse densities. Glossina palpalis gambiensis Vanderplank 1949 (Diptera: Glossinidae) and G. tachinoides Westwood, 1850 are the vectors of trypanosomoses in this area. After a supervised classification, the community types were discriminated using the water area in 400-m-wide polygons around the river. A fragmentation analysis of the swamp forest unit, cross-tabulated with the community types, lead to identification of the final landscapes where tsetse apparent densities (ADT) were implemented using a training data set of 608 trap locations. The predicted ADT were then compared with an independent validation data set of 78 trap locations. The correlation between the model predictions and the validation data set was high, validating this approach (P < 0.001). The riverine forest community type and fragmentation level are critical factors for riverine tsetse species, which should be taken into consideration to map their suitable habitat.

Current status of tsetse fly and animal trypanosomosis on the Jos Plateau, Nigeria

Blood samples were collected from 1035 systematically selected zebu (White Fulani) cattle resident in two Local Government Areas (LGAs) of the Jos Plateau, and analysed for infections with trypanosomes. Their grazing grounds were also sampled for tsetse flies using unbaited blue Challier (biconical) traps. Mean prevalence of trypanosome infection was 6.4% (95% confidence interval (CI) ± 0.02) and 9.1% (CI ± 0.02) in Barkin-Ladi and Bassa LGAs, respectively. Higher infection rate was recorded during the rains (9.3%; CI ± 0.02) than in the dry season months (1.5%; CI ± 0.02). In both localities prevalence in cows (8.9%; CI ± 0.04) consistently doubled that in bulls (4.0%; CI ± 0.01). Also, infection rates increased with age until the 7th year. Trypanosoma vivax was both the predominant species and the only one diagnosed among yearlings. Only riverine tsetse species were encountered (apparently at low density) as infected vectors.