Probability models to facilitate a declaration of pest-free status, with special reference to tsetse (Diptera: Glossinidae)

A distribution model for Glossina brevipalpis and Glossina austeni in Southern Mozambique, Eswatini and South Africa for enhanced area-wide integrated pest management approaches

Background

Glossina austeni and Glossina brevipalpis (Diptera: Glossinidae) are the sole cyclical vectors of African trypanosomes in South Africa, Eswatini and southern Mozambique. These populations represent the southernmost distribution of tsetse flies on the African continent. Accurate knowledge of infested areas is a prerequisite to develop and implement efficient and cost-effective control strategies, and distribution models may reduce large-scale, extensive entomological surveys that are time consuming and expensive. The objective was to develop a MaxEnt species distribution model and habitat suitability maps for the southern tsetse belt of South Africa, Eswatini and southern Mozambique.

Methodology/Principal findings

The present study used existing entomological survey data of G. austeni and G. brevipalpis to develop a MaxEnt species distribution model and habitat suitability maps. Distribution models and a checkerboard analysis indicated an overlapping presence of the two species and the most suitable habitat for both species were protected areas and the coastal strip in KwaZulu-Natal Province, South Africa and Maputo Province, Mozambique. The predicted presence extents, to a small degree, into communal farming areas adjacent to the protected areas and coastline, especially in the Matutuíne District of Mozambique. The quality of the MaxEnt model was assessed using an independent data set and indicated good performance with high predictive power (AUC > 0.80 for both species).

Conclusions/Significance

The models indicated that cattle density, land surface temperature and protected areas, in relation with vegetation are the main factors contributing to the distribution of the two tsetse species in the area. Changes in the climate, agricultural practices and land-use have had a significant and rapid impact on tsetse abundance in the area. The model predicted low habitat suitability in the Gaza and Inhambane Provinces of Mozambique, i.e., the area north of the Matutuíne District. This might indicate that the southern tsetse population is isolated from the main tsetse belt in the north of Mozambique. The updated distribution models will be useful for planning tsetse and trypanosomosis interventions in the area.

The two tsetse species transmitting nagana in South Africa, Eswatini and southern Mozambique represent the southernmost distribution of this genus on the African continent. Distribution models were developed to support tsetse control. These models indicated that the main factors contributing to tsetse distribution in the area are the presence of host animals, variation in climate and vegetation mostly observed in protected areas, agricultural practises and land-use also had a significant and rapid impact on tsetse abundance in the area. Application of the model to areas north of the southern distribution predict a low presence of suitable habitats in the Gaza and Inhambane Provinces of Mozambique, thereby indicating that this southern population is geographically isolated from the main tsetse belt starting in the north of Mozambique.

Dispersal in heterogeneous environments drives population dynamics and control of tsetse flies

Spatio-temporally heterogeneous environments may lead to unexpected population dynamics. Knowledge is needed on local properties favouring population resilience at large scale. For pathogen vectors, such as tsetse flies transmitting human and animal African trypanosomosis, this is crucial to target management strategies. We developed a mechanistic spatio-temporal model of the age-structured population dynamics of tsetse flies, parametrized with field and laboratory data. It accounts for density- and temperature-dependence. The studied environment is heterogeneous, fragmented and dispersal is suitability-driven. We confirmed that temperature and adult mortality have a strong impact on tsetse populations. When homogeneously increasing adult mortality, control was less effective and induced faster population recovery in the coldest and temperature-stable locations, creating refuges. To optimally select locations to control, we assessed the potential impact of treating them and their contribution to the whole population. This heterogeneous control induced a similar population decrease, with more dispersed individuals. Control efficacy was no longer related to temperature. Dispersal was responsible for refuges at the interface between controlled and uncontrolled zones, where resurgence after control was very high. The early identification of refuges, which could jeopardize control efforts, is crucial. We recommend baseline data collection to characterize the ecosystem before implementing any measures.

Impact of tiny targets on Glossina fuscipes quanzensis, the primary vector of human African trypanosomiasis in the Democratic Republic of the Congo

Over the past 20 years there has been a >95% reduction in the number of Gambian Human African trypanosomiasis (g-HAT) cases reported globally, largely as a result of large-scale active screening and treatment programmes. There are however still foci where the disease persists, particularly in parts of the Democratic Republic of the Congo (DRC). Additional control efforts such as tsetse control using Tiny Targets may therefore be required to achieve g-HAT elimination goals. The purpose of this study was to evaluate the impact of Tiny Targets within DRC. In 2015-2017, pre- and post-intervention tsetse abundance data were collected from 1,234 locations across three neighbouring Health Zones (Yasa Bonga, Mosango, Masi Manimba). Remotely sensed dry season data were combined with pre-intervention tsetse presence/absence data from 332 locations within a species distribution modelling framework to produce a habitat suitability map. The impact of Tiny Targets on the tsetse population was then evaluated by fitting a generalised linear mixed model to the relative fly abundance data collected from 889 post-intervention monitoring sites within Yasa Bonga, with habitat suitability, proximity to the intervention and intervention duration as covariates. Immediately following the introduction of the intervention, we observe a dramatic reduction in fly catches by > 85% (pre-intervention: 0.78 flies/trap/day, 95% CI 0.676-0.900; 3 month post-intervention: 0.11 flies/trap/day, 95% CI 0.070-0.153) which is sustained throughout the study period. Declines in catches were negatively associated with proximity to Tiny Targets, and while habitat suitability is positively associated with abundance its influence is reduced in the presence of the intervention. This study adds to the body of evidence demonstrating the impact of Tiny Targets on tsetse across a range of ecological settings, and further characterises the factors which modify its impact. The habitat suitability maps have the potential to guide the expansion of tsetse control activities in this area.

Evaluation of the relative roles of the Tabanidae and Glossinidae in the transmission of trypanosomosis in drug resistance hotspots in Mozambique

Background

Tsetse flies (Diptera: Glossinidae) and tabanids (Diptera: Tabanidae) are haematophagous insects of medical and veterinary importance due to their respective role in the biological and mechanical transmission of trypanosomes. Few studies on the distribution and relative abundance of both families have been conducted in Mozambique since the country’s independence. Despite Nicoadala, Mozambique, being a multiple trypanocidal drug resistance hotspot no information regarding the distribution, seasonality or infection rates of fly-vectors are available. This is, however, crucial to understanding the epidemiology of trypanosomosis and to refine vector management.

Methods

For 365 days, 55 traps (20 NGU traps, 20 horizontal traps and 15 Epsilon traps) were deployed in three grazing areas of Nicoadala District: Namitangurine (25 traps); Zalala (15 traps); and Botao (15 traps). Flies were collected weekly and preserved in 70% ethanol. Identification using morphological keys was followed by molecular confirmation using cytochrome c oxidase subunit 1 gene. Trap efficiency, species distribution and seasonal abundance were also assessed. To determine trypanosome infection rates, DNA was extracted from the captured flies, and submitted to 18S PCR-RFLP screening for the detection of Trypanosoma.

Results

In total, 4379 tabanids (of 10 species) and 24 tsetse flies (of 3 species), were caught. NGU traps were more effective in capturing both the Tabanidae and Glossinidae. Higher abundance and species diversity were observed in Namitangurine followed by Zalala and Botao. Tabanid abundance was approximately double during the rainy season compared to the dry season. Trypanosoma congolense and T. theileri were detected in the flies with overall infection rates of 75% for tsetse flies and 13% for tabanids. Atylotus agrestis had the highest infection rate of the tabanid species. The only pathogenic trypanosome detected was T. congolense.

Conclusions

Despite the low numbers of tsetse flies captured, it can be assumed that they are still the cyclical vectors of trypanosomosis in the area. However, the high numbers of tabanids captured, associated to their demonstrated capacity of transmitting trypanosomes mechanically, suggest an important role in the epidemiology of trypanosomosis in the Nicoadala district. These results on the composition of tsetse and tabanid populations as well as the observed infection rates, should be considered when defining strategies to control the disease.

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.

Estimating tsetse fertility: daily averaging versus periodic larviposition

When computing mean daily fertility in adult female tsetse, the common practice of taking the reciprocal of the interlarval period (called averaged fertility) was compared with the method of taking the sum of the products of daily fertility and adult survivorship divided by the sum of daily survivorships (called periodic fertility). The latter method yielded a consistently higher measure of fertility (approximately 10% for tsetse) than the former method. A conversion factor was calculated to convert averaged fertility to periodic fertility. A feasibility criterion was determined for the viability of a tsetse population. Fertility and survivorship data from tsetse populations on Antelope Is. and Redcliff Is., both in Zimbabwe, were used to illustrate the feasibility criterion, as well as the limitations imposed by survivorship and fertility on the viability of tsetse populations. The 10% difference in fertility between the two methods of calculation makes the computation of population feasibility with some parameter combinations sometimes result in a wrong answer. It also underestimates both sterile male release rates required to eradicate a pest population, as well as the speed of resurgence if an eradication attempt fails.

Phased Conditional Approach for Mosquito Management Using Sterile Insect Technique

Mosquito-borne diseases represent a major threat to humankind. Recently, the incidence of malaria has stopped decreasing while that of dengue is increasing exponentially. Alternative mosquito-control methods are urgently needed. The sterile insect technique (SIT) has seen significant developments recently and may play an important role. However, testing and implementing SIT for vector control is challenging, and a phased conditional approach (PCA) is recommended, that is, advancement to the next phase depends on completion of activities in the previous one. We herewith present a PCA to test the SIT against mosquitoes within an area-wide-integrated pest-management programme, taking into account the experience gained with plant and livestock pests and the recent developments of the technique against mosquitoes.

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.

A pilot study to delimit tsetse target populations in Zimbabwe

Background

Tsetse (Glossina sensu stricto) are cyclical vectors of human and animal trypanosomoses, that are presently targeted by the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) coordinated by the African Union. In order to achieve effective control of tsetse, there is need to produce elaborate plans to guide intervention programmes. A model intended to aid in the planning of intervention programmes and assist a fuller understanding of tsetse distribution was applied, in a pilot study in the Masoka area, Mid-Zambezi valley in Zimbabwe, and targeting two savannah species, Glossina morsitans morsitans and Glossina pallidipes.

Methodology/Principal findings

The field study was conducted between March and December 2015 in 105 sites following a standardized grid sampling frame. Presence data were used to study habitat suitability of both species based on climatic and environmental data derived from MODIS and SPOT 5 satellite images. Factors influencing distribution were studied using an Ecological Niche Factor Analysis (ENFA) whilst habitat suitability was predicted using a Maximum Entropy (MaxEnt) model at a spatial resolution of 250 m. Area Under the Curve (AUC), an indicator of model performance, was 0.89 for G. m. morsitans and 0.96 for G. pallidipes. We then used the predicted suitable areas to calculate the probability that flies were really absent from the grid cells where they were not captured during the study based on a probability model using a risk threshold of 0.05. Apart from grid cells where G. m. morsitans and G. pallidipes were captured, there was a high probability of presence in an additional 128 km² and 144 km² respectively.

Conclusions/Significance

The modelling process promised to be useful in optimizing the outputs of presence/absence surveys, allowing the definition of tsetse infested areas with improved accuracy. The methodology proposed here can be extended to all the tsetse infested parts of Zimbabwe and may also be useful for other PATTEC national initiatives in other African countries.

Tse-tse flies are vectors of human and animal trypanosomoses, that are presently targeted by the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) coordinated by the African Union. In Zimbabwe, the government has devoted a full section of the veterinary services to tsetse and trypanosomosis control but the delimitation of tsetse infested areas, which is a pre-requisite to achieve effective control still requires improvement. Here we present a methodology that could help delimit target areas throughout the country, in a pilot study area located in the Masoka area, Mid-Zambezi valley in Zimbabwe, and targeting two savannah species, Glossina morsitans morsitans and Glossina pallidipes. The study, which was carried out in preparation for a vector control campaign, allowed to discriminate areas where tsetse presence was certain, likely or unlikely Habitat degradation due to agricultural activities seemed to play a pivotal role in determining the infestation by tsetse since settled areas had low probabilities for both species which was expected in this group. Application of this model will help reduce the cost of delineating tsetse infested areas in other parts of Zimbabwe and may also be useful for other PATTEC national initiatives in other African countries at a time when funding for tsetse control programmes is reduced.

Identification of a Tsal152-75 salivary synthetic peptide to monitor cattle exposure to tsetse flies

Background

The saliva of tsetse flies contains a cocktail of bioactive molecules inducing specific antibody responses in hosts exposed to bites. We have previously shown that an indirect-ELISA test using whole salivary extracts from Glossina morsitans submorsitans was able to discriminate between (i) cattle from tsetse infested and tsetse free areas and (ii) animals experimentally exposed to low or high numbers of tsetse flies. In the present study, our aim was to identify specific salivary synthetic peptides that could be used to develop simple immunoassays to measure cattle exposure to tsetse flies.

Methods

In a first step, 2D-electrophoresis immunoblotting, using sera from animals exposed to a variety of bloodsucking arthropods, was performed to identify specific salivary proteins recognised in cattle exposed to tsetse bites. Linear epitope prediction software and Blast analysis were then used to design synthetic peptides within the identified salivary proteins. Finally, candidate peptides were tested by indirect-ELISA on serum samples from tsetse infested and tsetse free areas, and from exposure experiments.

Results

The combined immunoblotting and bioinformatics analyses led to the identification of five peptides carrying putative linear epitopes within two salivary proteins: the tsetse salivary gland protein 1 (Tsal1) and the Salivary Secreted Adenosine (SSA). Of these, two were synthesised and tested further based on the absence of sequence homology with other arthropods or pathogen species. IgG responses to the Tsal1_52–75 synthetic peptide were shown to be specific of tsetse exposure in both naturally and experimentally exposed hosts. Nevertheless, anti-Tsal1_52–75 IgG responses were absent in animals exposed to high tsetse biting rates.

Conclusions

These results suggest that Tsal1_52–75 specific antibodies represent a biomarker of low cattle exposure to tsetse fly. These results are discussed in the light of the other available tsetse saliva based-immunoassays and in the perspective of developing a simple serological tool for tsetse eradication campaigns to assess the tsetse free status or to detect tsetse reemergence in previously cleared areas.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1414-8) contains supplementary material, which is available to authorized users.

A Molecular Method to Discriminate between Mass-Reared Sterile and Wild Tsetse Flies during Eradication Programmes That Have a Sterile Insect Technique Component

Background

The Government of Senegal has embarked several years ago on a project that aims to eradicate Glossina palpalis gambiensis from the Niayes area. The removal of the animal trypanosomosis would allow the development more efficient livestock production systems. The project was implemented using an area-wide integrated pest management strategy including a sterile insect technique (SIT) component. The released sterile male flies originated from a colony from Burkina Faso.

Methodology/Principal Findings

Monitoring the efficacy of the sterile male releases requires the discrimination between wild and sterile male G. p. gambiensis that are sampled in monitoring traps. Before being released, sterile male flies were marked with a fluorescent dye powder. The marking was however not infallible with some sterile flies only slightly marked or some wild flies contaminated with a few dye particles in the monitoring traps. Trapped flies can also be damaged due to predation by ants, making it difficult to discriminate between wild and sterile males using a fluorescence camera and / or a fluorescence microscope. We developed a molecular technique based on the determination of cytochrome oxidase haplotypes of G. p. gambiensis to discriminate between wild and sterile males. DNA was isolated from the head of flies and a portion of the 5’ end of the mitochondrial gene cytochrome oxidase I was amplified to be finally sequenced. Our results indicated that all the sterile males from the Burkina Faso colony displayed the same haplotype and systematically differed from wild male flies trapped in Senegal and Burkina Faso. This allowed 100% discrimination between sterile and wild male G. p. gambiensis.

Conclusions/Significance

This tool might be useful for other tsetse control campaigns with a SIT component in the framework of the Pan-African Tsetse and Trypanosomosis Eradication Campaign (PATTEC) and, more generally, for other vector or insect pest control programs.

The Government of Senegal has embarked since several years on a project that aims to create a tsetse-free area in the Niayes. The project was implemented using an area-wide integrated pest management (AW-IPM) strategy where the sterile flies used for the sterile insect technique (SIT) component were derived from a colony originating from Burkina Faso. Monitoring the efficacy of the sterile male releases requires the discrimination between wild and sterile males that are sampled in monitoring traps. Before being released, sterile males were marked with a fluorescent dye powder. The marking was however not infallible with some sterile flies only slightly marked or some wild flies contaminated with a few dye particles in the monitoring traps, making it difficult to discriminate between wild and sterile males using a UV camera. We developed a molecular technique based on the cytochrome oxidase gene that efficiently discriminates between wild and sterile males. This tool might be useful for other tsetse control campaigns with a SIT component or for other vector or insect pest control programs.

Tsetse Control and Gambian Sleeping Sickness; Implications for Control Strategy

Background

Gambian sleeping sickness (human African trypanosomiasis, HAT) outbreaks are brought under control by case detection and treatment although it is recognised that this typically only reaches about 75% of the population. Vector control is capable of completely interrupting HAT transmission but is not used because it is considered too expensive and difficult to organise in resource-poor settings. We conducted a full scale field trial of a refined vector control technology to determine its utility in control of Gambian HAT.

Methods and Findings

The major vector of Gambian HAT is the tsetse fly Glossina fuscipes which lives in the humid zone immediately adjacent to water bodies. From a series of preliminary trials we determined the number of tiny targets required to reduce G. fuscipes populations by more than 90%. Using these data for model calibration we predicted we needed a target density of 20 per linear km of river in riverine savannah to achieve >90% tsetse control. We then carried out a full scale, 500 km² field trial covering two HAT foci in Northern Uganda to determine the efficacy of tiny targets (overall target density 5.7/km²). In 12 months, tsetse populations declined by more than 90%. As a guide we used a published HAT transmission model and calculated that a 72% reduction in tsetse population is required to stop transmission in those settings.

Interpretation

The Ugandan census suggests population density in the HAT foci is approximately 500 per km². The estimated cost for a single round of active case detection (excluding treatment), covering 80% of the population, is US$433,333 (WHO figures). One year of vector control organised within the country, which can completely stop HAT transmission, would cost US$42,700. The case for adding this method of vector control to case detection and treatment is strong. We outline how such a component could be organised.

Sleeping sickness is controlled by case detection and treatment but this often only reaches less than 75% of the population. Vector control is capable of completely interrupting HAT transmission but is not used because of expense. We conducted a full scale field trial of a refined vector control technology. From preliminary trials we determined the number of insecticidal tiny targets required to control tsetse populations by more than 90%. We then carried out a full scale, 500 km² field trial covering two HAT foci in Northern Uganda (overall target density 5.7/km²). In 12 months tsetse populations declined by more than 90%. A mathematical model suggested that a 72% reduction in tsetse population is required to stop transmission in those settings. The Ugandan census suggests population density in the HAT foci is approximately 500 per km2. The estimated cost for a single round of active case detection (excluding treatment), covering 80% of the population, is US$433,333 (WHO figures). One year of vector control organised within country, which can completely stop HAT transmission, would cost US$42,700. The case for adding this new method of vector control to case detection and treatment is strong. We outline how such a component could be organised.

Mathematical models of human african trypanosomiasis epidemiology

Human African trypanosomiasis (HAT), commonly called sleeping sickness, is caused by Trypanosoma spp. and transmitted by tsetse flies (Glossina spp.). HAT is usually fatal if untreated and transmission occurs in foci across sub-Saharan Africa. Mathematical modelling of HAT began in the 1980s with extensions of the Ross-Macdonald malaria model and has since consisted, with a few exceptions, of similar deterministic compartmental models. These models have captured the main features of HAT epidemiology and provided insight on the effectiveness of the two main control interventions (treatment of humans and tsetse fly control) in eliminating transmission. However, most existing models have overestimated prevalence of infection and ignored transient dynamics. There is a need for properly validated models, evolving with improved data collection, that can provide quantitative predictions to help guide control and elimination strategies for HAT.

Using species distribution models to optimize vector control in the framework of the tsetse eradication campaign in Senegal

Tsetse flies are vectors of human and animal trypanosomoses in sub-Saharan Africa and are the target of the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC). Glossina palpalis gambiensis (Diptera: Glossinidae) is a riverine species that is still present as an isolated metapopulation in the Niayes area of Senegal. It is targeted by a national eradication campaign combining a population reduction phase based on insecticide-treated targets (ITTs) and cattle and an eradication phase based on the sterile insect technique. In this study, we used species distribution models to optimize control operations. We compared the probability of the presence of G. p. gambiensis and habitat suitability using a regularized logistic regression and Maxent, respectively. Both models performed well, with an area under the curve of 0.89 and 0.92, respectively. Only the Maxent model predicted an expert-based classification of landscapes correctly. Maxent predictions were therefore used throughout the eradication campaign in the Niayes to make control operations more efficient in terms of deployment of ITTs, release density of sterile males, and location of monitoring traps used to assess program progress. We discuss how the models' results informed about the particular ecology of tsetse in the target area. Maxent predictions allowed optimizing efficiency and cost within our project, and might be useful for other tsetse control campaigns in the framework of the PATTEC and, more generally, other vector or insect pest control programs.

From integrated pest management to integrated pest eradication: technologies and future needs

BACKGROUND

With growing globalization and trade, insect incursions are increasing worldwide. A proportion of incursions involve pests of major economic crops (e.g.Mediterranean fruit fly), conservation value (e.g. tramp ants) or health significance(e.g.mosquitoes), and maybe the targets of eradication programmes. Historically, such responses have included the use of broad spectrum insecticides. However, with increasing public awareness of the negative aspects of pesticides, new environmentally friendly and effective techniques are needed. Here, we review and evaluate a range of selective to broad-spectrum tactical options for suppression which either have, or show potential for, integration within arthropod eradication programmes.

RESULTS

Most of the available technologies have their roots in pest management, but higher efficacy is required. Further refinement may be needed for use in eradication. Integration of several tactics is usually needed, as compatible tools can be used simultaneously to target different parts of the pest life cycle. However, not all technologies are fully compatible; for example, the simultaneous use of mass trapping and the sterile insect technique (SIT) may be suboptimal, although sequential application may still be effective.

CONCLUSIONS

Broad-spectrum insecticides are generally incompatible with some biologically based technologies such as the SIT, but may be used to reduce the population so that density-dependent tactics can be used. Several novel technologies with fewer nontarget impacts have been proposed in recent years, and need to be properly evaluated for their applicability to insecteradication. Overall, there are still major gaps in surveillance and selective eradication technologies for most insects.

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.

First insights into the cattle serological response to tsetse salivary antigens: a promising direct biomarker of exposure to tsetse bites

In the context of the Pan African Tsetse and Trypanosomiasis Eradication Campaign, the value of tsetse saliva antibodies as a biomarker of cattle exposure to tsetse flies was evaluated, as this could provide an alternative and complementary tool to conventional entomological methods. Serum immune reactivity to Glossina (G.) palpalis (p.) gambiensis, G. tachinoides and G. morsitans (m.) submorsitans whole saliva extracts (WSE) were monitored in cattle from both tsetse free and tsetse infested areas, and in cows experimentally exposed to tsetse flies and other hematophagous arthropods. In the tsetse infested area, cattle IgG responses to Glossina WSE were significantly higher during the dry season (p<0.0001) when herds are most exposed to tsetse flies and in infected animals (p=0.01) as expected in the case of a biomarker of exposure. Experimental studies further confirmed this as a quick rise of specific IgGs was observed in animals exposed to tsetse flies (within weeks), followed by a rapid clearance after exposure was stopped. In contrast to the two other tsetse species, G. m. submorsitans WSE enabled to detect exposure to all tsetse species and were associated with low level of cross-reactivity to other blood sucking arthropods. Finally, IgG responses to G. m. submorsitans salivary antigens enabled to distinguish different groups of cows according to exposure levels, thus indicating that tsetse saliva antibodies are not only indicators of tsetse exposure but also are correlated to the intensity of tsetse contacts (p=0.0031). Implementation of this new sero-epidemiological marker of cattle exposure to tsetse flies in the framework of tsetse elimination campaigns is discussed.

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.

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.

Combining tactics to exploit Allee effects for eradication of alien insect populations

Invasive species increasingly threaten ecosystems, food production, and human welfare worldwide. Hundreds of eradication programs have targeted a wide range of nonnative insect species to mitigate the economic and ecological impacts of biological invasions. Many such programs used multiple tactics to achieve this goal, but interactions between tactics have received little formal consideration, specifically as they interact with Allee dynamics. If a population can be driven below an Allee threshold, extinction becomes more probable because of factors such as the failure to find mates, satiate natural enemies, or successfully exploit food resources, as well as demographic and environmental stochasticity. A key implication of an Allee threshold is that the population can be eradicated without the need and expense of killing the last individuals. Some combinations of control tactics could interact with Allee dynamics to increase the probability of successful eradication. Combinations of tactics can be considered to have synergistic (greater efficiency in achieving extinction from the combination), additive (no improvement over single tactics alone), or antagonistic (reduced efficiency from the combination) effects on Allee dynamics. We highlight examples of combinations of tactics likely to act synergistically, additively, or antagonistically on pest populations. By exploiting the interacting effects of multiple tactics on Allee dynamics, the success and cost-effectiveness of eradication programs can be enhanced.

Using molecular data for epidemiological inference: assessing the prevalence of Trypanosoma brucei rhodesiense in tsetse in Serengeti, Tanzania

Background

Measuring the prevalence of transmissible Trypanosoma brucei rhodesiense in tsetse populations is essential for understanding transmission dynamics, assessing human disease risk and monitoring spatio-temporal trends and the impact of control interventions. Although an important epidemiological variable, identifying flies which carry transmissible infections is difficult, with challenges including low prevalence, presence of other trypanosome species in the same fly, and concurrent detection of immature non-transmissible infections. Diagnostic tests to measure the prevalence of T. b. rhodesiense in tsetse are applied and interpreted inconsistently, and discrepancies between studies suggest this value is not consistently estimated even to within an order of magnitude.

Methodology/Principal Findings

Three approaches were used to estimate the prevalence of transmissible Trypanosoma brucei s.l. and T. b. rhodesiense in Glossina swynnertoni and G. pallidipes in Serengeti National Park, Tanzania: (i) dissection/microscopy; (ii) PCR on infected tsetse midguts; and (iii) inference from a mathematical model. Using dissection/microscopy the prevalence of transmissible T. brucei s.l. was 0% (95% CI 0–0.085) for G. swynnertoni and 0% (0–0.18) G. pallidipes; using PCR the prevalence of transmissible T. b. rhodesiense was 0.010% (0–0.054) and 0.0089% (0–0.059) respectively, and by model inference 0.0064% and 0.00085% respectively.

Conclusions/Significance

The zero prevalence result by dissection/microscopy (likely really greater than zero given the results of other approaches) is not unusual by this technique, often ascribed to poor sensitivity. The application of additional techniques confirmed the very low prevalence of T. brucei suggesting the zero prevalence result was attributable to insufficient sample size (despite examination of 6000 tsetse). Given the prohibitively high sample sizes required to obtain meaningful results by dissection/microscopy, PCR-based approaches offer the current best option for assessing trypanosome prevalence in tsetse but inconsistencies in relating PCR results to transmissibility highlight the need for a consensus approach to generate meaningful and comparable data.

Human African trypanosomiasis is a fatal disease that is carried by a tsetse vector. Assessing the proportion of tsetse which carries human-infective trypanosomes is important in assessing human disease risk and understanding disease transmission dynamics. However, identifying flies which carry transmissible infections is difficult, due to potential presence of other trypanosome species in the same fly, and concurrent detection of immature infections which are not transmissible. We used three methods to estimate the proportion of flies carrying human-infective trypanosomes: dissection and microscopic examination of flies to visualise trypanosomes directly in the fly; PCR of fly midguts in which trypanosomes were observed by microscopy; and theoretical analysis using a mathematical model of disease transmission. All three methods found the prevalence to be extremely low. Given the low prevalence, dissection/microscopy requires prohibitively large sample sizes and therefore PCR-based approaches are likely to be of most value. However, interpretation of PCR data is not straightforward; whilst PCR identifies flies carrying pathogen genetic material it does not directly identify flies with transmissible infections. This study highlights the need for a consensus approach on the analysis and interpretation of PCR data to generate reliable and comparable measures of the proportion of flies which carry transmissible human-infective trypanosomes.

Is the even distribution of insecticide-treated cattle essential for tsetse control? Modelling the impact of baits in heterogeneous environments

Background

Eliminating Rhodesian sleeping sickness, the zoonotic form of Human African Trypanosomiasis, can be achieved only through interventions against the vectors, species of tsetse (Glossina). The use of insecticide-treated cattle is the most cost-effective method of controlling tsetse but its impact might be compromised by the patchy distribution of livestock. A deterministic simulation model was used to analyse the effects of spatial heterogeneities in habitat and baits (insecticide-treated cattle and targets) on the distribution and abundance of tsetse.

Methodology/Principal Findings

The simulated area comprised an operational block extending 32 km from an area of good habitat from which tsetse might invade. Within the operational block, habitat comprised good areas mixed with poor ones where survival probabilities and population densities were lower. In good habitat, the natural daily mortalities of adults averaged 6.14% for males and 3.07% for females; the population grew 8.4× in a year following a 90% reduction in densities of adults and pupae, but expired when the population density of males was reduced to <0.1/km²; daily movement of adults averaged 249 m for males and 367 m for females. Baits were placed throughout the operational area, or patchily to simulate uneven distributions of cattle and targets. Gaps of 2–3 km between baits were inconsequential provided the average imposed mortality per km² across the entire operational area was maintained. Leaving gaps 5–7 km wide inside an area where baits killed 10% per day delayed effective control by 4–11 years. Corrective measures that put a few baits within the gaps were more effective than deploying extra baits on the edges.

Conclusions/Significance

The uneven distribution of cattle within settled areas is unlikely to compromise the impact of insecticide-treated cattle on tsetse. However, where areas of >3 km wide are cattle-free then insecticide-treated targets should be deployed to compensate for the lack of cattle.

Eliminating Rhodesian sleeping sickness, the zoonotic form of Human African Trypanosomiasis found in East and Southern Africa, can be achieved only through eliminating the vectors, species of tsetse fly (Glossina). The deployment of insecticide-treated cattle is the most cost-effective means of achieving this. However, the even distribution of insecticide-treated cattle is seldom possible due to the patchy distribution of grazing, water and human settlement. We used a simulation model to explore the likely impact of such patchiness on the outcome of control operations against tsetse. The results suggest that even in areas that are highly suitable for tsetse, gaps of up to 3 km in the distribution of insecticide-treated cattle will not have a material impact on the success of an operation provided the overall mean density of cattle across all areas is adequate to achieve control (e.g., ∼4 insecticide-treated cattle/km² killing 10% per day of the tsetse in the area treated). If the gaps are larger than 3 km, then deploying insecticide-treated targets at densities of 4/km² in the cattle-free areas will ensure success.

Progress towards the eradication of Tsetse from the Loos islands, Guinea

BACKGROUND

The tsetse fly Glossina palpalis gambiensis is the main vector of sleeping sickness (Human African Trypanosomiasis - HAT) in West Africa, in particular in littoral Guinea where this disease is currently very active. The Loos islands constitute a small archipelago some 5 km from mainland Guinea, where G. p. gambiensis is well known as a nuisance and potential disease vector by inhabitants of the three main islands, Fotoba, Room, and Kassa. The National Control Program against HAT of Guinea has decided to eradicate tsetse in Loos islands in order to sustainably protect humans and economic activities. After baseline data collection, tsetse control began on the islands in 2006. On each of the three islands a specific combination of control methods was implemented according to the entomological situation found.

RESULTS

Starting densities before control operations were 10, 3 and 1 tsetse/trap/day in Kassa, Room and Fotoba respectively, but by July 2010, tsetse were no longer caught in any of the sentinel traps used for monitoring. The reduction rate was faster where several control methods were implemented as a combination (impregnated traps and targets ITT, selective groundspraying, epicutaneous insecticide treatment of pigs, and impregnated fences around pig pens), whereas it was slower when ITT were used as the only control method.

CONCLUSIONS

This 100% suppression is a promising step in the eradication process, but G. p. gambiensis may still occur at very low, undetectable, densities on the archipelago. Next step will consist in assessing a 0.05 probability of tsetse absence to ascertain a provisional eradication status. Throughout these operations, a key factor has been the involvement of local teams and local communities without whom such results would be impossible to obtain. Work will continue thanks to the partners involved until total eradication of the tsetse on Loos islands can be declared.

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.

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.

Cocoa Pod Borer (Conopomorpha cramerella Snellen) in Papua New Guinea: biosecurity models for New Ireland and the autonomous region of Bougainville

Cocoa Pod Borer (Conopomorpha cramerella Snellen) (CPB) is an important pest of cocoa. Following its emergence as a pest in East New Britain, Papua New Guinea, in 2006, it was considered relevant to assess its potential spread to other cocoa growing regions. Its likelihood of introduction to the islands of Bougainville and New Ireland from East New Britain Province, Papua New Guinea, was modeled using Monte Carlo simulation. This dispersal model was based around different scenarios, identifying trends rather than explicitly attempting to encapsulate true values. The model suggested that CPB is far more likely to establish on New Ireland than on Bougainville. More important, incertitude resulting from incomplete knowledge of the amount and frequency of cocoa transported between islands had a significant effect on model outputs. Quarantine and agriculture officials will be able to refine these parameter values, and then use the relevant scenarios from those presented here as a guide to develop quarantine procedures. In addition, a contingency model was employed to estimate the optimal sampling effort to use following an incursion of CPB into Bougainville or New Ireland and the seemingly successful implementation of an initial eradication program. The model suggests that at a 1% infestation level, sampling should continue for 2.5-2.7 years (90% CI) after claiming eradication, and this estimate changed little for higher infestation levels. Through modeling variations in sampling intensity, the model also suggested that determining the full spread of CPB is more important than increased sampling within one region.

Elimination of the Djodji form of the blackfly Simulium sanctipauli sensu stricto as a result of larviciding by the WHO Onchocerciasis Control Programme in West Africa

Cytotaxonomic identifications of larvae of members of the Simulium damnosum Theobald (Diptera: Simuliidae) complex collected in forest zones of southeast Ghana and southwest Togo between 1977 and 1996 showed that the Djodji form of Simulium sanctipauli Vajime & Dunbar, a vector of onchocerciasis, was eliminated in 1988 by larvicide operations conducted by the World Health Organization (WHO) Onchocerciasis Control Programme (OCP) in West Africa. No members of the form were identified amongst 997 larvae collected up to 8 years after systematic control operations began in February 1988. The results are discussed in relation to estimates of the numbers of samples required to certify elimination and the possibility that other members of the S. damnosum complex were also eliminated by the OCP.

The use of aerial spraying to eliminate tsetse from the Okavango Delta of Botswana

In Botswana, 16,000 km(2) of the Okavango Delta were aerial sprayed five times with deltamethrin, applied at 0.26-0.3g/ha, to control Glossina morsitans centralis Machado (Diptera: Glossinidae) over a period of approximately 8 weeks. The northern half of the Delta (7180 km(2)) was sprayed in June-September 2001 and the southern half (8720 km(2)) in May-August 2002. A barrier (mean width approximately 10 km) of 12,000 deltamethrin-treated targets was deployed at the interface of these two blocks to prevent tsetse from invading from the southern to the northern block. Prior to spraying, the mean catches of tsetse from man fly-rounds were 44.6 round/day in the northern block and 101 in the southern. Between September 2002 and November 2005, surveys ( approximately 820 daily fly-rounds and approximately 2050 trap-days) in the northern and southern blocks failed to detect tsetse. Simulations of tsetse populations suggest that while spraying operations can reduce tsetse populations to levels that are difficult to detect by standard survey techniques, such populations will recover to densities >100 tsetse/km(2) after 1000 days, at which density there is a very high probability (>0.999) that the survey methods will catch at least one fly. Since none was caught, it is argued that tsetse have been eliminated from the Delta. The particular success of this operation in comparison to the 18 aerial spraying operations conducted in the Delta prior to 2001 is attributed to the application of an adequate dose of insecticide, the use of a GPS-based navigation system to ensure even application of insecticide, and the large size and spatial arrangement of the spray blocks coupled with the use of a barrier of targets which prevented tsetse from re-invading the northern sprayed block before the southern one was treated.

Optimal eradication: when to stop looking for an invasive plant

The notion of being sure that you have completely eradicated an invasive species is fanciful because of imperfect detection and persistent seed banks. Eradication is commonly declared either on an ad hoc basis, on notions of seed bank longevity, or on setting arbitrary thresholds of 1% or 5% confidence that the species is not present. Rather than declaring eradication at some arbitrary level of confidence, we take an economic approach in which we stop looking when the expected costs outweigh the expected benefits. We develop theory that determines the number of years of absent surveys required to minimize the net expected cost. Given detection of a species is imperfect, the optimal stopping time is a trade-off between the cost of continued surveying and the cost of escape and damage if eradication is declared too soon. A simple rule of thumb compares well to the exact optimal solution using stochastic dynamic programming. Application of the approach to the eradication programme of Helenium amarum reveals that the actual stopping time was a precautionary one given the ranges for each parameter.

Extinction probabilities and times to extinction for populations of tsetse flies Glossina spp. (Diptera: Glossinidae) subjected to various control measures

A stochastic branching process was used to derive equations for the mean and variance of the probability of, and time to, extinction in tsetse populations. If the remnant population is a single inseminated female, the extinction probability increases linearly with adult mortality and is always certain if this mortality >3.5% per day even for zero pupal mortality. If the latter mortality is 4% per day, certain extinction is only avoided if adult mortality <1.5% per day. For remnant female populations >1, the extinction probability increases in a non-linear manner with adult mortality. Extinction is still certain for adult mortality >3.5% per day but, when the remnant population is >16, extinction is highly unlikely for adult mortality <2.5% per day if all females are inseminated. Extinction probability increases with increasing probability of sterile mating in much the same way as it does with increasing adult mortality. Extinction is assured if the probability of insemination can be reduced to 0.1. The required reduction decreases with increasing adult mortality. For adult mortality = 6-8% per day, the time to extinction increases only by one generation per order of magnitude increase in the starting population. Time to extinction is less sensitive to changes in the pupal than in the adult mortality. Reductions in the probability of insemination only become important when adult mortality is small; if the adult mortality is 8% per day, reducing the insemination probability from 1 to 0.1 only reduces the expected time to extinction by two generations. Conversely, increases in adult mortality produce important reductions in the required time even when the probability of insemination is 0.1. The practical, economic implication for the sterile insect technique is that the low-tech methods used to suppress tsetse populations should not be halted when the release of sterile males is initiated. The sterile insect technique should only be contemplated when it has been demonstrated that the low-tech methods have failed to effect eradication. The theory is shown to be in good accord with the observed results of tsetse control campaigns involving the use of odour-baited targets in Zimbabwe and the sterile insect technique on Unguja Island, Zanzibar.