A comparison of tsetse control
Tsetse flies (Glossina spp) transmit sleeping sickness in humans and nagana in livestock, which attacks the blood and nervous system of its victims. It is caused by the protozoa trypanosomiasis and the tsetse fly acts as a vector as it feeds on blood of animals and humans. Sleeping sickness is always fatal without the appropriate medical treatment and trypanosomiasis is one of the most widespread and important constraints to agricultural development in Africa and also has a major impact on social welfare (African Development Fund 2004:1). Whilst knowledge of the biology and ecology of the tsetse fly is extensive and a wide variety of control and eradication measures utilised, trypanosomosis is still a major problem in Africa. This paper will evaluate the various control and eradication methods employed against the tsetse fly in Africa and attempt to deduce lessons for the future. In order to do this the paper will be divided into a number of parts. First, an overview of the distribution and negative impact of tsetse fly infestation in Africa. Second, a brief description will be given of the biology and ecology of the tsetse fly specifically relevant to control and eradication. Third, an analysis will be made of the various methods of tsetse control such targets, traps, insecticides and the sterile insect techniques looking at both the advantages and disadvantages. Finally, an examination will be conducted of the lessons that can be learned from the history of the fight against the tsetse fly and possible ways of improving the situation.
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Tsetse flies (Glossina spp.) can be ranked among the world's most destructive pests and are a vector for sleeping sickness in humans and African Animal Trypanosomosis (AAT) or Nagana in live stock (Vreyson 2001:397). Tsetse flies infest about 10 million km2 of fertile land spread across 37 countries on the African continent, from Senegal in the North, to South Africa in the south (Oluwafemi et al 2007:382).The term Africa's bane (Nash 1969) was coined to encapsulate the detrimental effects on human health, agriculture, and the economic development of African countries due to the tsetse fly. The prevalence of the disease differs both between and within countries. In 2005, major outbreaks occurred in Angola, the Congo and Sudan. In Chad, Congo, Tanzania, Côte d'Ivoire, Guinea, the Central African Republic, Malawi, and Uganda sleeping sickness is still an important health issue. Countries such as Burkina Faso, Equatorial Guinea, Cameroon, Kenya, Mozambique, Nigeria, Rwanda, Zambia and Zimbabwe are reporting fewer than 50 new cases per year. In in countries such as Botswana, Ghana, Ethiopia, Mali, Gambia, Liberia, Namibia, Niger, Senegal, Sierra Leone Togo transmission seems to have ceased and no new cases have been reported for a number of decades (WHO 2006). Sleeping sickness appeared to be under control in Africa during the 1960s and 1970s. However, recent decades have seen the disease spread to epidemic proportions due to the breakdown of control programmes causing a public health crisis in many affected areas (Smith et al 1998:342).
The economic effects of trypanosomosis in Africa are complex and multifaceted with direct impacts on animal production and human health, and indirect effects on draught power use, animal husbandry, and farming and settlement patterns and land use (Bourne et al 2005:1). Due to the tsetse fly that there are few horses in Africa and there is a separation of crop and animal production, with little or no mixed farming. Without draught animals, farmers have no means of power to pull ploughs and they continue to till the land by hand. Since crop and livestock production are physically separated, soil fertility suffers due to a lack of manure for fertilization. In Asia, an estimated 50 per cent of crop production benefits from the power of draught animals. In Africa, the rate is only 5-10 per cent. As a result, estimates the Food and Agriculture Organization, Africa may lose $4.5 bn in potential crop production each year (Okhoya 2003:17).
In order to achieve success, any strategies to control or eradicate the tsetse fly must rely on the biology of the tsetse fly and distribution data (Cecchi et al 2008: 1365-1370). The tsetse fly needs two basic resources to persist in an area shade and blood (Bissonette & Storch 2003:286). The distribution of the tsetse is a result of the interaction of climatic and ecological factors such as temperature, humidity, availability of food and type of vegetation, all interact limiting distribution and resulting in belts of tsetse fly. Studies of the distribution and habits of the different species of tsetse fly in relation to certain vegetation has shown that there tends to be a concentration of insects in specific plant communities which are a small part of the bush or woodland as a whole. This is especially true during the hot and dry seasons when climatic conditions are unfavourable to dispersal over a wide area, and the flies prefer a cooler micro climate provided by the vegetation (Hocking et al 1963:813).
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The reproductive and life cycle of the tsetse fly has a number of features which can be exploited when attempting control or eradication programmes. Females mate young, before or around time of taking the first meal of blood, and usually only once in their lives. The males can mate several times. Once a female has mated with male, she can produce larvae for the rest of her life. The reproductive life cycle of tsetse flies is unique among insects (Vreysen 2001:398) and has this implications for both control and eradication programmes. The tsetse reproduces by adenotrophic viviparity, whereby the egg contains yolk which sustains the embryonic development and the larva is nourished in the female by special organs (Hagan 1951:472). This means that the adult stage is the only viable stage for eradication due to the absence of egg or larvae stage and because pupae development occurs in the ground.
A further consequence of the life history of tsetse is their low genetic variability within a population. This is due to a low dispersal rate and reproductive rate, combined with selection for the most energetically-efficient individuals. A female produces only one offspring at a time and up to 12 offspring at intervals of about 9-10 days during the average adult lifespan of 2-3 months. As a result, the rate of tsetse population growth tends to be low. This means that even a small effect on mortality can cause the tsetse population to decline in number which can be utilised in the efforts to control the tsetse fly. For flight, the adult tsetse relies on the metabolism of proline, an amino acid derived from blood, and when this is exhausted they must rest in order to replenish the proline reserve. One result is that tsetse flies are generally unable to fly for long periods, flying instead for short periods with a relatively low capacity for active dispersal (Kuzoe & Schofield 2004:14).
The main methods of control of tsetse and sleeping sickness in Africa have been based on hemoprophylaxis, chemotherapy and the elimination of vectors using insecticides (Grant 2001:1). However, over time there have been a number of different methods employed in an attempt to control the tsetse fly population, including game reduction and vegetation clearance. Game destruction was used as a routine method of game control in Bechuanaland, Uganda, Portuguese East Africa and Rhodesia. Carefully applied game destruction was and can still be an economical and practical control method. However, alternative methods have been developed which are just as cheap and reliable and game reduction has been increasingly replaced by other methods. Similarly the complete clearing of areas of vegetation is no longer widely practised as a method of control. Although there have been circumstances where vegetation was cleared, such as serious outbreaks or to preserve land for cattle (Hocking et al 1963: 812). There has been a growing awareness of the environment as a resource that must not be squandered. Game and wildlife parks have become major tourist attractions earning valuable foreign revenue for some African countries.
In the 1940's the use of DDT and related organochlorides was a major advance in the war against the tsetse fly. The main advantage of using insecticides was their effectiveness. Calculations showed that aerial spraying could have up to a 90% kill rate (Yeo & Simpson 1960:631). The selective spraying of residual insecticides on tsetse resting sites in the dry season from the ground or with helicopters eliminated tsetse from an area of 200000 km2 in Northern Nigeria (Spielberger et al 1977:589-598). Nevertheless, the widespread use of insecticides can have a number of negative impacts on potentially fragile ecosystems. These include: insecticide resistance, a reduction in the number of beneficial insects, the elimination of predators which feed on harmful insects, and environmental pollution (Vreyson 2001: 398-399). There are also a number of non-ecological disadvantages including the need for funding, trained personnel and the fact that spraying is limited seasonally (Cavalloro 1987:59).
However, Grant argues that modern efficient techniques of discriminative ground spraying, non residual aerial techniques , odour baited targets and pour ons have reshaped the environmental acceptability of insecticide based tsetse control (Grant 2001:2). Furthermore, as knowledge of the habitats and behaviour grows the discriminative application of insecticides to eradicate these species can be more precise, economical and effective (Hockling et al 1963:823).
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Despite this the use if insecticides have been superseded by traps and targets in recent decades. The female flies are attracted and land on the trap or target and receive a lethal dose of insecticide or die to of heat or starvation. A small mortality rate of 2-3 % each day is sufficient to cause a decline in the population (Willemese 1991:351-357). The traps use unsophisticated technology and the materials are relatively cheap. The use of traps and targets has been promoted as the most sustainable method of tsetse control, especially in the context of community participation (Holmes 1997:121-135), and experiments have led to increasing sophistication in terms of colour, use of odours and location.
Nevertheless the use of traps is not without difficulty. There are number of technical requirements so that the traps operate efficiently. For example, the importance of location, maintenance, and replacement are all factors that may influence the effectiveness of traps and targets. All these factors mean that reports of the sustainability of traps and targets have been mixed and a more profitable approach seems to be the integration of several methods with an area wide approach (Black & Seed 2001: 40).
The sterile insect technique (SIT) is one area-wide insect pest management method by which the insect pest is controlled or eradicated by hampering its reproductive capacity. This is an environmentally friendly method in comparison to insecticides which have the potential to harm the environment. Although there is an initial investment in technology, over the time there is a very positive cost benefit ratio using the SIT, and it can easily be combined with other methods. In addition it is species specific and is effective even as the population of the targeted species declines.
Other methods of control are usually applied first to achieve a reduction in the tsetse fly population before the SIT is utilised. Sterilisation is achieved by radiation, although it is also possible using chemicals. The technique relies on producing in large numbers of the target insect in mass-rearing facilities, sterilisation of the male and the release in large numbers so as to outnumber the wild population (Knipling 1959: 902-904). The sterile insect technique successfully eradicated tsetse flies from the island of Zanzibar. However the success of this method largely depends on the isolation of the area, as reinvasion from neighbouring populations is a major problem.
Although very promising as a method of eradicating the tsetse fly there are also some hazards which can occur during the production, shipping and release of the sterile males. The most serious risk is due to use of the irradiator to sterilise the laboratory reared males. There is the possibility of operators' exposure to high levels of radiation or environmental contamination. In addition, workers may also develop allergic reactions from frequent exposure to blood. Other risks include the risk of contamination of the environment through disposal of contaminated waste and the risk of male flies escaping prior to sterilisation. There is also a risk that males, fed with contaminated blood may carry trypanosomiasis or even other diseases. Since sterile males can transmit trypanosomiasis, their releases can even temporarily increase the incidence of the disease. Nevertheless with population numbers below 5%, the probability that disease incidence will increase is very low (African Development Fund 2004:7).
In addition to the technical and mechanical requirements it is important to realise that all these measures of control and eradication do not operate in a vacuum but rather within a socio-economic context which can hamper effective implementation. Difficulties such as maintaining the interest of the farmer over time, different perception of farmers regarding the scale of the problem and their willingness to contribute, the problem of participation in a sparsely populated area, theft and accessibility (Holmes 1997:121-135). Research driven solutions to the tsetse problem are inadequate if the communities affected are not involved.
The growing concerns related to the massive use of insecticides resulted in increased demands for alternative methods of tsetse control and eradication. This led to the development of integrated pest management (IPM). According to the Council on Environmental Quality, IPM is defined as 'the selection, integration and implementation of pest control methods based on predicted economic, ecological and sociological consequences' (Ridgway et al 1993:3-15). This means in effect employing several control methods in an environmentally and ecologically friendly way while at the same time attempting to minimize the disadvantages of each.
However such an approach requires finance and political will. To address this problem, the Heads of State and Government of the African Union collectively launched the Pan African Tsetse and Trypanosomiasis Eradication Campaign (PATTEC) in 2001 with the aim of eradicating tsetse flies and trypanosomiasis (African development Fund 2004:1). Funding of US$80 million was approved by the African Development Bank for the first stage of a Pan-African Tsetse and Trypanosomosis Eradication Campaign (PATTEC), as part of the African Union's New Partnership for Africa's Development (NEPAD) initiative. The first, six-year stage of the project intended to eliminate the tsetse fly and trypanosomosis from 13 million hectares in two regions of West and East Africa, including areas of Ghana, Burkina Faso, Mali, Ethiopia, Kenya and Uganda (Bourne et al 2005:1).
There are a number of potential benefits if tsetse and trypanosomosis is eradicated from Africa including improved human and livestock heath, diversification of agriculture, and improved food production and security (Oluwafemi 2009). Many currently infested areas could be very productive and therefore support a higher density of population. The increasing population could in terms help prevent re-infestation by reducing the amount of shade, one of the requirements for the tsetse fly (U.S. Congress Office of Technology Assessment 1988: 270).
Interestingly, there were some possible benefits due to tsetse fly infestation. It restricted some of the naive development plans especially during the colonial period of livestock overstocking and intensification of agriculture (Rogers & Randolf 2005:57). Despite the improvements that could be attained in human health and food production by the elimination of the tsetse fly there are also a number of disadvantages such as environmental degradation from land use, intensification of livestock production and environmental contamination from the widespread use of insecticides (Grant 2001:1).
The lessons from previous efforts to eliminate the tsetse fly can be summarised as: first, the tsetse fly has been largely neglected by the various African governments in the last few decades as they struggled with the problems of the burden of debt, civil war, and state formation. Along with this instability there has been a decentralisation of tsetse control resulting in a shift from large scale eradication approaches to localised tsetse control efforts by the local farmer communities. This combination of factors in many African countries has contributed to the decline of most tsetse control efforts in the last decades (Vreyson 2006:2). The de-centralization means that many of the initiatives to reduce the tsetse fly are inadequate, because the belts of tsetse infestation cross borders and thus require both regional and international co-operation. Second, many projects were foreign-driven, and as soon as foreign support was withdrawn, the projects collapsed, with no continuity. The political will was not translated into viable and sustainable national policies that could withstand the withdrawal of foreign aid and support. Third, control and eradication methods were often applied singularly and isolation. A far more effective and rational approach would have been to use a number of control and eradication methods taking into account the ecological and social context so as to maximise advantages and minimize disadvantages of each. Fourth, the different international agencies involved the campaign to eliminate the tsetse had different priorities. The World Health Organization focused on the tsetse fly as a health issue and whilst the Food and Agricultural Organization focused on the effect on livestock this led to a divergence of approaches when combating the tsetse fly and trypanosomosis. Fifth, was the problem of re- infestation, whereby, if a small area was cleared but was still within a region or area infested with tsetse it quickly became uneconomical and unfeasible to prevent re-infestation. Rather a more sensible approach is an area wide approach making using of natural barriers such as temperature and humidity to prevent the re-infestation of an area. Lastly is the importance of scientific knowledge and advances which will be crucial in combating the tsetse fly in the future and includes such as SIT, genetics and environmentally friendly methods of control and eradication.
In conclusion, the struggle against the tsetse fly has been long with some successes and many failures. The success of the eradication programme in Zanzibar showed that there have been real advances made both in scientific knowledge and methods of control and eradication. However, the focus up to now seems to have been very much research driven with little progress into integrating the different methods into a comprehensive eradication programmes
at national, regional and international levels. The challenge for the future lies in producing integrated pest control programmes that recognise political and social realities on the ground. The challenge facing African governments is how to implement control and eradication measures when faced with political and economic problem and instability that make resourcing of any programmes difficult. Aid by foreign donors is only part of the solution since withdrawal often results in a lack of continuity. Therefore the solution must be based in African with a mixture of local and community participation alongside regional co-operation.
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