Genetic diversity and gene flow in morsitans group tsetse flies
E.S. Krafsur. Iowa State University, Ames, Iowa 50011-3222 USA. [ekrafsur@iastateedu]
The question of how to achieve effective levels of tsetse fly control at financially and environmentally acceptable costs is perennial and contentious. Even though tsetse flies are slow to reproduce, populations seem to recover sooner or later after control measures are relaxed. A great capacity and propensity to disperse is said to be characteristic of tsetse flies, and many experts suggest that area-wide control measures and eradication are unobtainable for this reason alone. Others contend that area-wide methods, including the sterile insect technique, can be used successfully to achieve a high degree of control. Can a study of tsetse fly population genetics add anything to the ongoing debate? I believe it can. Here's why.
While the tsetse fly is traditionally shown on maps as being distributed in broad belts, within these belts tsetse fly populations are patchily distributed. Uninfested regions presumably consist of unsuitable or marginally suitable habitat. Even within infested patches, tsetse flies are aggregated into demes among which there may be varying degrees of isolation. Given application of effective control measures, how large an area must be treated to minimize re-invasion? John Hargrove suggests that very large areas are required, greater than 10 000 square kilometres (Hargrove, J.W. 2000. A theoretical study of the invasion of cleared areas by tsetse flies (Diptera: Glossinidae). Bulletin of Entomological Research, 90, 201-209). Suppose, then, that a tsetse patch has been eliminated. What is the risk of invasion from nearby patches? Frontal advance of morsitans group flies has been shown to be of the order of 7 km per year and density-dependent responses might increase that value, but the distances between patches are too great to measure experimentally by mark, release, and recapture methods. Moreover, areas between patches are likely to be unsuitable for tsetse reproduction so that frontal advances fail and long range colonization of cleared areas is necessary.
In principle, we could measure gene frequencies of tsetse flies in two or more patches and derive indices of gene flow within and among them. This is the province of population genetics. Evolutionary insights may also come from such studies, as we shall see. Well developed theory teaches that the exchange of approximately one reproducing fly per generation, on average, is sufficient to prevent fixation of genetic differences between populations. Moreover, this 'critical' migration rate is virtually independent of population size! Whether the 'real' number is 0.5 or 2 is not biologically significant - in principle, numerically little gene flow can overcome local genetic differentiation. In theory, then, we have a powerful tool with which to examine the notion of biologically significant exchange of tsetse flies between patches.
Measurement of gene frequencies nowadays is a fairly routine affair. We can examine genetic diversity at loci that code for enzymes - so called isozymes and allozymes - by using starch, paper, or polyacrylamide electrophoresis, coupled with histochemical staining to demonstrate enzyme activity and allelic variation. The chief drawback is that of preserving enzyme activity in field-collected samples. The preferred method, using liquid nitrogen, is not always available, and airlines often refuse to accept shipments of this innocuous substance.
Another source of gene diversity is microsatellite loci. Microsatellites are short repetitive nucleotide sequences that vary in number, for example, [CA]n, where the number of repeats n varies. DNA can be extracted from rapidly dried or ethanol preserved flies, thereby making sampling and transport to the laboratory easier. But much energy and skill is required to find microsatellite loci and to design and test the primers necessary to amplify them in the polymerase chain reaction (PCR).
Allozyme, isozyme, and microsatellite loci are present in two copies. Thus they can be used to measure genotypic frequencies. Genotypic frequencies, in turn, allow estimates to be made of departures from random mating within populations. Allele frequencies can be used to test hypotheses about the independence of two or more samples i.e. genetic differentiation.
PCR can be used to measure variation in mitochondrial genes. Mitochondria contain single copy loci and are maternally inherited. Variants do not recombine so the mitochondrial genome is inherited as a unit. Thus mitochondrial loci can be used to measure genetic differentiation of populations and maternal lines of descent because of the clonal pattern of inheritance. Moreover, mitochondrial variation is much more sensitive to population (demographic) events than is nuclear variation because of its inheritance pattern and the fact that its genes are represented by single copies, not double, as is the case for nuclear genes.
The foregoing kinds of genetic variation have been applied to some morsitans group tsetse flies. The sampling was carried out by Nigel Griffiths in The Gambia, Kenya, Zambia, Zimbabwe, Mozambique, and Namibia, Reg Allsopp in Botswana, Steve Mihok in Ethiopia, and Marc Vreysen, also in Ethiopia. The laboratory work and analysis was performed in my laboratory at Iowa State University.
Genetic diversity
So, what do the data show? First, let's examine diversities, i.e. magnitudes of genetic variation. Later, I'll deal with gene flow. Diversities at mitochondrial loci estimate the probability that two randomly chosen tsetse flies have different haplotypes. For nuclear genes, diversity can be expressed in terms of the number of variants (alleles) at each locus and as heterozygosities - the proportion of loci with different (non-matching) alleles. Mitochondrial diversities, averaged over populations, were 41 percent in G. pallidipes, 35 percent in G. m. morsitans, and 43 percent in G. m. submorsitans, but only 22 percent in G. m. centralis. There were important contrasts in these diversities among regional populations that I'll return to later.
Allozyme data indicate that morsitans group flies are heterozygous at about 6 percent of their loci (heterozygosities at polymorphic loci, however, were about 25 percent). The 6 percent value compares with heterozygosities of about 18 percent in house flies and face flies (Musca domestica and M. autumnalis, respectively). The same methods show similarly high levels of diversity (heterozygosity) in numerous ladybird beetle and leaf beetle species (Chrysomelidae). Microsatellite diversities (heterozygosities) were much greater than the allozyme diversities, largely because they are probably untranscribed, subject to higher mutation rates, and do not respond to natural selection (i.e. they are 'selectively neutral'). Thus, in G. pallidipes, the number of alleles per locus was very much greater at polymorphic microsatellite loci (mean, 20.8 per locus) than at polymorphic allozyme loci (mean, 3 per locus). Microsatellite heterozygosities (diversities), averaged over populations and polymorphic loci, were 71 percent in G. pallidipes, 73 percent in G. m. morsitans, 81 percent in G. m. submorsitans, and 70 percent in G. m. centralis.
The magnitude of genetic diversity is important from evolutionary, ecological, and historical points of view. For example, theory shows a direct relationship between historical population sizes and diversity. Thus, the comparatively low diversities in tsetse flies are an indication that historical mean tsetse population sizes have been considerably less than those of many Diptera and Coleoptera, and are consistent with tsetse's low reproduction rates.
Low diversities can suggest historical 'bottlenecks' in population size, in which one or more successive generations undergo a great reduction in numbers. Bottlenecks in tsetse populations have been conjectured, as a consequence of rinderpest epizootics in the nineteenth and early twentieth centuries; indeed, such was demonstrated in Zimbabwe and claimed in Uganda (reviews can be found in Ford J., 1971, The Role of the Trypanosomiases in African Ecology. Clarendon Press, Oxford, and Leak S.G.A., 1998, Tsetse Biology and Ecology: Their Role in the Epidemiology and Control of Trypanosomiasis. ILRI Nairobi/CABI). Glossina morsitans centralis in Botswana and Mamili National Park in Namibia showed a remarkable paucity of mitochondrial variation (only a 3 percent chance that two randomly chosen flies would have different mitochondrial haplotypes), populations having recovered from extensive control schemes in the Okavango region. Mitochondrial diversities in G. m. submorsitans were much less in The Gambia (26 percent) than in Ethiopia (84 percent); in Zimbabwe, G. pallidipes diversities were only 15 percent, but in Kenya and Ethiopia they averaged 54 percent. Microsatellite variation showed no hint of bottlenecks in G. morsitans s.l. but was reduced in Zimbabwean G. pallidipes. Allozyme diversities, on the other hand, seemed to be totally unaffected by putative bottlenecks. For example, Zimbabwean G. pallidipes showed slightly more heterozygosity than Kenyan populations even though mitochondrial and microsatellite variation was very much less. Thus, it seems that tsetse provide an example of 'balancing selection' acting on allozyme heterozygotes, thereby promoting diversity at allozyme loci.
Theory teaches that recovery from bottlenecks requires tens of thousands of generations, far more than the roughly 800 generations since the rinderpest epizootic. Therefore, the tsetse populations that we study today should still exhibit clear evidence of the earlier bottlenecks. The nature of the evidence includes disequilibrium between forces of mutation, migration, and genetic drift. So far, however, we are unable to reject null hypotheses that populations are in mutation-drift equilibrium. Larger sample sizes and the development of more sensitive statistical tests may, in future, allow more definitive investigations.
Gene flow
Three independent lines of genetic evidence show abundant variation in morsitans group tsetse flies and this variation can be used to estimate gene flow within and among populations. How can this be done?
If gene flow is unrestricted, gene frequencies among populations will be statistically homogeneous. But what does it mean if they differ significantly? And why should they differ at all, if they do not respond to natural selection and have a common ancestry? The answer is that differences in gene frequencies arise because the laws of chance operate in the transmission of alleles from one generation to the next; thus, the smaller the breeding population the greater is the variance in gene frequencies. This is termed 'genetic drift' and is a major evolutionary force. The major result of drift is that gene frequencies of populations tend to diverge from each other in proportion to their isolation from each other. The isolation may be spatial, temporal, behavioural, premating, postmating, etc. Opposed to drift is immigration, and, as we have seen, numerically little exchange of reproducing migrants is effective in reversing the effects of drift.
We can do better than a simple test for differences by measuring the magnitudes of departures from random mating. The most commonly used index of departures from random mating is F, the so-called inbreeding coefficient. Now F may be viewed as a correlation of genes. In theory, F can take values from -1 (matings only between unlike) to 1 (like mates only with like: completely inbred). It makes sense that flies in a particular location are more likely to mate with each other than with flies in another location, so there will be a measure of drift that leads to genetic differentiation. The classical estimate of drift (or genetic differentiation) among populations is termed FST. The null hypothesis is FST = 0. Generally speaking, FST estimates of ³ 0.05 are considered to indicate a biologically significant degree of differentiation. Genetic differentiation provides a continuous scale of reproductive isolation, in principle varying from zero to unity.
Gene flow among conspecific tsetse fly populations
Estimates of FST based on allozyme, mitochondrial, and microsatellite variation were consistent. In G. m. submorsitans, FST » 0.17 - 0.35 (depending on the method of analysis) among seven populations in The Gambia and Ethiopia, but FST was only 0.016 among samples within countries. We find that FST » 0.18 among six G. m. morsitans populations, five of which originated in Zambia and Zimbabwe. More extensive sampling of G. m. morsitans is necessary to get an estimate of gene flow among fly belts. Among seven G. m. centralis populations extending from Tanzania to Botswana, it was found that FST » 0.19. A much greater estimate was obtained at mitochondrial loci, for which FST » 0.87 (recall that mitochondrial loci are much more sensitive to demographic upheavals than are nuclear loci).
Glossina pallidipes showed, among 11 populations, a very high degree of differentiation at allozyme loci (FST » 0.19-0.24, depending on method of analysis), microsatellite (FST » 0.29), and mitochondrial loci (FST » 0.51). Study revealed that among northern populations, mitochondrial FST » 0.52, whereas FST » 0.28 among southern populations. Microsatellite loci showed the same trends but allozyme loci did not. These data indicate that genetic drift at allozyme loci is greatly dampened and provides further evidence of balancing selection acting on allozyme loci (Krafsur, E.S., 2002, Population structure of the tsetse fly Glossina pallidipes estimated by allozyme, microsatellite, and mitochondrial gene diversities. Insect Molecular Biology, 11, 37-45).
Estimates of FST can be converted to hypothetical estimates of the mean number of reproducing organisms exchanged per generation, Nm. The mathematical relationship is deceptively simple and entails many assumptions, but Nm can provide useful perspective by indicating the amount of gene flow in simple terms. The means, taken over all populations, were exchanges of 1.1 reproductives per generation among G. m. morsitans, 0.04-1.1 among G. m. centralis (based on microsatellite vs. mitochondrial loci, respectively), 0.5-1.2 among G. m. submorsitans, and 0.6-0.8 among G. pallidipes. Most of the foregoing estimates do not differ greatly from 1, that is, the 'critical' amount of gene flow below which genetic drift can proceed to fixation of different genotypes in different populations. Most insect species show Nm values that vary from five or ten to hundreds and thousands.
The high degree of genetic differentiation among morsitans group tsetse in general and G. pallidipes in particular is surprising when ecological and experimental data are considered. Glossina pallidipes is highly mobile - indeed, it is said to be the most vagile tsetse. I shall return to the contrast between ecological and genetical evidences later.
Gene flow within populations
Genotypic frequencies, in terms of expected and observed heterozygosities, can be used to test hypothesis that matings are random within populations. For example, a deficiency of heterozygotes within a population is evidence that two or more demes of different gene frequencies were sampled; this can happen when samples from different locations are pooled. The chief estimator is FIS (inbreeding F for individuals I in S (sub)populations). Sampling morsitans group tsetse indicated that matings were random within populations. Such data indicate no large scale immigration.
What may we conclude?
First, with two important exceptions, the foregoing data on gene diversities do not indicate genetically detectable bottlenecks in population sizes. The exceptions concern G. m. centralis in Botswana and G. pallidipes in Zimbabwe where the historical record suggests that such bottlenecks occurred in G. morsitans s.l. and G. pallidipes, due to host animal reduction caused by the rinderpest outbreak. The same rinderpest epizootics swept through East and West Africa, but the genetic data suggest that tsetse populations were not so greatly affected as in southern Africa.
Second, tsetse populations are highly structured by genetic drift, with surprisingly little gene flow among them, a seemingly curious result considering the well known propensity for tsetse flies to disperse. Moreover, comparable work on other Diptera and many economically and medically important insects generally show much higher rates of gene flow than those recorded in tsetse. The literature suggests frontal advances of tsetse populations averaging 5 to 7 km yearly.
How can the foregoing contradiction be resolved? Are the genetic data faulty or, more likely, is their interpretation simply wrong? Genetic analysis is built on Hardy-Weinberg assumptions which are rarely satisfied when dealing with natural populations and are certainly inapplicable to tsetse. If tsetse populations are recovering from earlier severe bottlenecks and disruptive population fragmentations, they would not be at mutation-drift equilibrium, and conclusions based on an assumption of equilibrium could be in error. Statistical tests for equilibrium, however, provide no evidence that the assumption is false.
Natural selection offers another rationalization for the apparent contradiction between ecological and genetical conclusions. In principle, tsetse far removed from their home territories may be at a significant reproductive disadvantage. Adaptation to local environments may be necessary and most immigrants may die without issue.
We also should consider scale. Ecological data pertain to distances in tens of kilometres. The genetic sampling summarized here involved distances ranging from tens to thousands of kilometres. Means taken over samples that vary so greatly in distance are apt to be misleading because the relationship between Nm and FST is nonlinear. But pairwise population estimates show correlations between genetic and geographical distances and they also generally confirm the low rates of gene flow. This is encouraging because low rates of gene flow support the concept of area-wide control, and predict low rates of re-colonization of habitat lost to tsetse. In principle, the sterile insect technique (SIT) involving sterile fly releases by aircraft may well prove to be more effective in reducing natural populations than are traps, targets, and targeted sprays because larger areas can be treated uniformly and efficaciously. And released, sterile males may turn out to be much more effective in finding small, hard-to-reach tsetse foci than are entomologists, rural sociologists, economists, and stakeholders everywhere.
Let's bear in mind that genetic and ecological research measure different things that are not strictly comparable; indeed, the contrast between ecological and genetic dispersals may be less in practice than in theory. Further research now underway should bring into better focus relationships between geographical distances, genetic distances, and, via physiological adaptation, the possibility of natural selection in explaining the breeding structure of tsetse flies.
As for area-wide control, in my opinion experimental sterile fly releases over large areas should first be made in order to learn something of interactions between released and wild flies in terms of sterile mating rates and target population responses to 'birth control'. Confounding treatments designed to maximize sterile to wild ratios should be avoided in such experiments. Later, treatment by SIT of entire patches, as defined by satellite imagery and ground reconnaissance, might provide levels of control that would endure for many, many years.
Acknowledgement. This paper of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project 6592, was supported by USPHS Grant AI-5245601, Hatch Act, and State of Iowa funds.
Registration for the 27th Meeting of the International Scientific Council for Trypanosomiasis Research and Control (ISCTRC) has opened. The meeting is organized under the auspices of the African Union, and will be held in Pretoria, South Africa, September 29th to October 3rd 2003.
The draft agenda has the following main sections: Review of Research and Control (which will include country reports); Protozoology, Immunology and Diagnosis; Entomology; Human Trypanosomiasis; Animal Trypanosomiasis; Glossina control.
The scientific articles for oral presentation should not exceed 3 000 words and should contain an abstract not exceeding 300 words. The abstract should contain: Title, Objective of the Study, Outline of Methodology, Results in Brief and Conclusions.
There will be a Poster Session with brief oral presentation. The abstract and manuscript must conform to the format indicated for oral presentation. Posters will be 1 × 1.25m; the title should be concise, and followed by the author(s) name and affiliations. Character heights recommended: Title at least 2cm; Subtitles at least 1cm; Text at least 0.25cm. The posters should be legible in comfort from a distance of 1m.
Summaries of scientific articles in duplicate English and French should be sent preferably by e-mail so as to reach the Secretariat not later than 30 April 2003.
The working languages of the meeting will be English and French, and there will be simultaneous translation.
Arrival particulars for participants in Pretoria should be communicated to the ISCTRC Secretary, AU/IBAR, P.O. Box 30786, Nairobi, Kenya. Fax Nos. 254-2-220546/226565. E-mail: [email protected] or [email protected], with copy to Dr Rob Bagnall [email protected] or [email protected].
The following is a summary of a position paper having the above title, by Dr Mulumba Kamuanga, presented at the Meeting of the Panel of Advisory Group Co-ordinators, Ouagadougou, Burkina Faso, 26-28 September 2001.
Tsetse-transmitted trypanosomiasis still stands as an important constraint to agricultural development in the subhumid (including the wetter areas of the semi-arid zones) and humid zones of Africa. Generally the benefits of tsetse and trypanosomiasis (T&T) control will derive from the reduced risk of contracting the disease, both human and animal. There will also be a diminution in the expenses incurred in prevention and disease treatment. These factors will thus improve human health and the productivity of existing livestock.
The paper is organized around three main topics: (1) an overview of socio-economic factors to account for in the research and control of tsetse and trypanosomiasis; (2) the role of socio-cultural factors affecting community participation in the control activities to ensure sustainability of the benefits derived from T&T control, and (3) the importance of past lessons and experiences in strategic planning incorporating socio-cultural aspects in design, monitoring and evaluation of T&T control programmes. There is a substantial bibliography including 97 items drawn from published and unpublished sources.
A list (with brief comments) of available control techniques is provided. It includes ground and aerial spraying; sterile insect technique (SIT); traps and targets/screens often enhanced with attractant odours; insecticide-treated livestock; husbandry of trypanotolerant livestock; and drug therapy. Items three and four are sometimes bracketed under the term bait technology.
The development of bait technologies has triggered two important shifts in the research and control of trypanosomiasis beyond the issue of costs and returns. The first is the involvement of beneficiaries as partners. The second is the move away from large scale, government-supported schemes to small-scale community-based participation where tsetse control interventions can be regarded as local public goods. Broadly speaking, with the variety of technical options now available, there is consensus that good opportunities and possibilities exist for effective control of the disease.
The technicalities of handling the required concepts of economic analysis are discussed. Most of the failures of development projects have been attributed to the fact that the communities concerned were left out of all the process related to design, formulation and implementation of policy. Can this weakness be rectified? The present paper advocates community participation and related notions in T&T control programmes, in the context of a more effective approach to sustainable rural development, whether an area-wide or a farmer/community-based approach is envisaged.
What is meant by a community, community participation, and related terms, is defined. Community involvement may range from token participation on the one hand to full participation and empowerment on the other. Theorists also distinguish between "top-down" and "bottom-up" programmes in community participation. The weakness of the top-down approach, initiated and directed by central government or affiliated agencies, is that there is a tendency for a uniform strategy that does not reflect local social, cultural or political conditions. On the other hand, bottom-up strategies are difficult to implement because very often members of the community (farmers, labourers, local opinion leaders) must accept enhanced responsibilities in decision-making actions to fulfil their dreams and aspirations.
Certain case studies of community participation in T&T control are examined in more detail. These include: Community-based T&T control in Burkina Faso, in Côte d'Ivoire; in Busia District, Kenya; and in Lambwe Valley, Kenya.
The major findings are summarized concerning socio-economic and cultural factors that determine when and how it might be appropriate to involve communities and individual livestock farmers in T&T control operations. Information to date is most readily available when targets and traps are the principal techniques being proposed for T&T control. However, experience is slowly emerging with other non-bait technologies, including integrated systems of control involving several approaches to ensure sustainability.
All the research and experiences in sub-Saharan areas suggest that in locations where there is sleeping sickness at present, or where serious outbreaks of human African trypanosomiasis (HAT) have occurred within living memory, there would logically be a major incentive for community action. Outside these locations, it is not easy to identify similar incentives that might mobilise the whole population. Ownership of cattle, and in some instances that of other livestock, have been indicated as a significant factor determining individual willingness to contribute resources to T&T control. Where communities depend on cattle for their livelihood, as is the case in most pastoral societies, it is the costs/benefits calculations of alternative strategies that will influence people in their decisions to participate individually or as a community in T&T control operations. Further related issues are discussed, including whether the community has experienced previous externally initiated research-development action; knowledge by the farming community of the symptoms of animal trypanosomiasis; the amount of time that the community can donate to the project; village and social structure; age of community participants, their level of education, and the distance from the point of action (e.g. the traps to be serviced).
Most of the failures of development projects in general, and tsetse and trypanosomiasis control programmes in particular, have been attributed to the fact that potential and actual beneficiaries were left out of the process related to design, formulation and implementation of policy. There is now an urgent need for the new approach to become demand- rather than supply-driven. There are several lessons to be learnt from the growing disillusionment with both large-scale, government-managed schemes and the questionable sustainability of most small-scale, community-based programmes that will help to determine when and how it might be appropriate to involve communities and individual livestock owners in T&T control.
The following is a draft summary of a position paper having the above title, by Dr Alexandra P.M. Shaw, presented at the Meeting of the Panel of Advisory Group Co-ordinators, Ouagadougou, Burkina Faso, 26-28 September 2001.
This paper seeks to address the issue of how to integrate economic criteria into the strategic planning process for tsetse and trypanosomosis control in West Africa. It was originally prepared for the FAO/IAEA workshop held in Ouagadougou in May 2001, and focuses on the issues raised at that workshop. It takes as its starting point Brent Swallow's PAAT position paper, which reviewed the growing literature on the economic impact of the disease, and complements this with recent references and a rapid overview of the benefit-cost studies undertaken.
Since this has been much debated, and has profound implications for the type of strategy adopted, the methodological issues involved in the economic appraisal of potential projects to control the disease are first discussed in some detail. This discussion is particularly timely in the light of the current Pan-African initiatives, which reveal a need for the wider scientific community and planners to understand the implications for policy and decision-making of the economic techniques used. The literature on the economic appraisal of livestock projects universally advocates putting some value on the use of money over time, reflecting its opportunity cost in terms of resources diverted from other projects and the need to fix some minimum acceptable rate of return on public investments. The use of 'discount' rates is accordingly recommended here, while applying low discount rates in the examples used, so as to reduce the effect of deflating benefits occurring in the distant future as compared to present costs.
The terms of reference for this work were to produce economic guidelines for planners in the tsetse/trypanosomosis field, accordingly it is argued that in the current institutional context, each individual project or zone should be the subject of a separate benefit-cost analysis, so that it is assessed on its own merits, not on its possible technical contribution to a potential continent-wide programme. This again is part of sound economic practice. The setting out of benefits and costs according to the rules of partial analysis is explained for the case of tsetse and trypanosomosis control. This discussion, in particular, emphasises the importance of incorporating farmers' current strategies to control the disease into the analysis. Studies have shown that in many areas their use of trypanocides is effective; this means that a proportion of disease losses are already being successfully avoided. The benefits from introducing tsetse control in this situation would not be the elimination of all possible losses due to trypanosomosis, but would consist of savings in the use of trypanocides plus a further reduction in the losses due to the disease. A dynamic herd model incorporating animal traction is used to simulate the benefits and costs of tsetse eradication, trypanocide use, and the switch from one to the other. This implies that farmers' current strategy of targeting productive animals brings high returns. Tsetse control becomes more profitable in higher challenge areas if sufficiently large cattle populations exist to make up the 'benefit units' per square kilometre and where there is evidence of drug resistance.
Secondly, from this discussion on methodology, it is argued that there is a need for planners to adopt a standardised and transparent approach for assessing tsetse and trypanosomosis control schemes. This would aim firstly to be cost-effective and secondly to produce results for different projects that could be compared and used for ranking and priority setting. In this context there is an urgent need for updated and fully comparable costings on the various forms of tsetse control, as they would apply in West Africa and including overheads, so that they can be applied by planners.
Thirdly, the paper tries to complement the GIS work on the spatial distribution of the factors influencing the economics, and the predictions of the likely rates of changes, by looking at the dynamics of benefits and costs over time, especially in relation to the densities of human and cattle populations. A conceptual model shows tsetse control costs falling with rising human populations. Benefits, however, initially rise, then peak when mixed farming is well established but tsetse challenge persists, and lastly, fall when human populations rise to a level where the fly's habitat becomes eroded and/or fewer cattle are kept. This points to the existence of two turning points in the economics of tsetse control: firstly, below a certain cattle or human population density there are insufficient benefit units to make it profitable; and secondly, above a certain level, fly challenge is reduced, losses due to the disease decline, and cattle numbers may also be lower as the amount of grazing land is reduced. This model is used to characterise situations where controlling the disease may or may not be profitable. These situations and the profitability limits or turning points identified coincide to a large extent with those emerging from the GIS priority-setting exercises. The two approaches thus very much complement each other, suggesting that the economic appraisals should focus on those zones that emerge as priority areas from the GIS filtering process.
Sleeping sickness affects 100 000 people in Angola, the director of the country's Institute for the Eradication of Trypanosomiasis, Teofilo Josenando, has revealed.
Josenando told PANA in Luanda that the country's long civil war was to blame for the alarming spread of the disease, whose prevalence shot up from 0.06 percent [of the population] ten years ago to 10 percent now.
Tsetse flies (Glossina) exist in 14 of Angola's 18 provinces; sleeping sickness is a health problem in Bengo, Kuanza-Norte, Kwanza-Sul, Malanje, Uije, Zaire, and Luanda provinces. Some 27 000 cases of sleeping sickness, including 4 000 in Luanda, the capital, have been reported since fighting resumed in 1992, Josenando said, adding that his institute needs at least US $5 million per year to combat the disease. The funds are required to finance the operations of 22 mobile and 43 fixed teams and to buy the required products.
According to Josenando, trypanosomiasis will be eradicated only if there is simultaneous treatment of the disease in Angola, the Democratic Republic of Congo, and Sudan, where it is widespread. Although sleeping sickness has been present in Angola since the 9th century, it was only in 1949 that mobile treatment teams started visiting the affected regions. Since that era, the situation improved considerably, and the number of cases was reduced from 5 000 to only 3 in 1974. However, the outbreak of civil war after the country's independence in November 1975 severely hampered the Institute's activities. Its infrastructure was destroyed or plundered, while most experts fled abroad. At the end of the civil war in April 2002, the institute decided to extend its operations to all the affected provinces in the country, Josenando said.
The Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, and the FAO/IAEA Agriculture and Biotechnology Laboratory, Seiberdorf, IAEA, Vienna, have listed the ongoing Technical Co-operation Projects relating to tsetse and trypanosomiasis control, in the Insect Pest Control Newsletter No.59, July 2002. The reader is referred to this Newsletter for fuller details, as the list below does not necessarily cover all the activities of the respective projects.
ETH/5/012: Integrating SIT for Tsetse Eradication. This supports the construction of a modern mass-rearing facility for Glossina pallidipes.
KEN/5/022: Integrated Area-Wide Tsetse and Trypanosomosis Management in Lambwe Valley. High mortality at a local G. pallidipes colony is being investigated, to trace the source of the problem.
MLI/5/017: Integrated Control of Animal Trypanosomosis through creation of a Tsetse Fly Free Zone. Intensive and extensive surveys for Glossina palpalis gambiensis are under way in the La Faya System (of the River Niger basin) in Mali; the results will be used to develop a fly suppression strategy.
RAF/5/051: SIT for Tsetse and Trypanosomosis Management in Africa. Technical assistance and equipment are provided for a new tsetse rearing facility at CIRDES, in order to produce flies for use in the Mali project (see above).
SAF/5/005: Situation Analysis of the Feasibility and Desirability of Tsetse Fly Eradication. Samples of Glossina brevipalpis have been shipped to South Africa to start a colony there (ARC-OVI, Pretoria). Field-collected G. brevipalpis and G. austeni will also be transferred to this insectary.
URT/5/019: Support to National Tsetse and Trypanosomosis Management: Tsetse (G. brevipalpis) and trypanosomosis are being surveyed on Mafia Island. Extensions to the existing tsetse rearing facility at Tanga are under way, for this to become regional centre for the rearing of different species of fly.
UGA/5/023: Integrated Sterile Insect Technique Based Intervention against Tsetse in Buvuma Island. Insectaries at the Livestock Research Institute, Tororo, are to be upgraded, and a Glossina fuscipes fuscipes colony will be established. A strategic programme document has been prepared, outlining a strategy for the creation of a tsetse free zone around the Lake Victoria Shore.
The interested reader may also wish to refer to http://www.iaea.org/programmes/nafa/d4/index.html, and http://www.fao.org/WAICENT/Agricul.htm.
