The tsetse and trypanosomiasis problem is characterized by many interdependencies involving agro-economical, social and environmental issues. Any intervention (or non-intervention) will have a wide range of immediate and longer-term implications.
In most African countries, demographic developments demand that agricultural systems become more productive. Increased productivity is difficult to implement because of the tsetse and the trypanosomiasis problem, which appears to have worsened over the past years: the number of sleeping sickness cases is exceeding even the level recorded during the epidemics of the 1920s. In several African countries, tsetse flies reinfest formerly reclaimed areas, invade previously uninfested agricultural areas, cause escalating problems in areas suffering from civil unrest and further decrease the area of land available for high-productivity agricultural systems. Controlled or substantially reduced vector populations can still be very efficient transmitters of trypanosomes. Some intervention measures have undesirable side-effects. Trypanosomes have developed resistance and cross-resistance to various trypanocides. The extensive use of insecticides on cattle for tsetse control appears to have the potential to interfere with the zootic stability/immunity of cattle to several tick-borne diseases, so long-term tsetse control may exacerbate other secondary problems. The environmentally acceptable methods for tsetse and trypanosomiasis management that are currently available all have specific limitations. In infested areas, only a combination of several methods in an integrated, phased and area-wide approach (Knipling, 1972; Chandler and Faust, 1998; Klassen, 2000) can effectively advance the establishment of viable agricultural systems that suit the needs of the rapidly growing human population.
The trypanosomiasis problem is not restricted to individual countries but is transnational and must be tackled on a regional, or at least a subregional, level. Although several donors currently favour integrated disease management through interventions in selected areas, an area-wide integrated pest management approach should be incorporated into such broader development concepts, and the option of creating tsetse fly-free zones should be pursued wherever this is feasible and sustainable. It is essential that a variety of options be retained, including the elimination of the tsetse and trypanosomiasis problem from large areas.
The potential for integrating several available intervention methods and for new supportive technologies has not been sufficiently explored. This is particularly the case for the sterile insect technique (SIT) which, unlike other conventional methods of tsetse control, has a unique efficiency pattern: efficiency increases as target pest population density decreases. A sequence of conventional methods, with SIT as a final component, would have maximum efficiency throughout an intervention campaign.
The recent eradication1of the tsetse fly in Zanzibar (Vreysen et al., 2000) by means of aerial releases of large numbers of sterile males has received considerable attention. The major difference between the Zanzibar operations and previous tsetse SIT projects is aerial release capability, which allows for the systematic and area-wide application of this environmentally friendly intervention method, especially in inaccessible areas.
FAO and the International Atomic Energy Agency (IAEA) have launched an initiative to upgrade SIT to make it an economically attractive alternative for integration into area-wide subregional tsetse and trypanosomiasis intervention campaigns. This is a focused component of an overall initiative in livestock disease and wildlife management and agricultural development. Methods are under development for the release of at least 500 000 sterile males per week in the near future, which would enable operation in areas as extensive as 5 000 to 10 000 km2 at a time. The initiative consists of three components: 1) research and development on tsetse rearing automation and more efficient aerial release systems, on tsetse attractants and on tsetse genetics; 2) an effort to increase awareness and determination among Africa's top decision-makers and more concerted, impact-oriented technical assistance support from the UN family and other major stakeholders, including declared milestones and verifiable implementation indicators; and 3) the identification of priority intervention areas along with the preparation, and eventual implementation, of feasibility demonstrations of the SIT package as a component of area-wide, integrated tsetse and trypanosomiasis management efforts. Substantial progress is being recorded on all three points, and one achievement relevant to point 2) deserves particular attention and represents a challenge for all the partners involved in tsetse/trypanosomiasis research and intervention: the African Heads of State and Government decision on tsetse eradication, AHG/Dec.156(XXXVI) (PAAT, 2000), passed at their 36th summit in Lomé, Togo. The summit "commends those African countries that have initiated the application of sterile insect technology (SIT) for their pioneering effort" and "urges Member States to act collectively to rise to the challenge of eliminating the problem through concerted efforts in mobilizing the necessary human, financial and material resources required to render Africa tsetse-free within the shortest time possible". As part of an integrated area-wide approach, SIT appears to be the most environmentally friendly component for implementing this decision. This publication addresses some of the challenges that remain and the major points of criticism raised in connection with SIT.
1Although the term "eradication" means the extinction of a species from the earth, in this publication it is applied to localized complete removal of a population of the species, i.e. creation of a fly-free zone. Colonies of the "eradicated" insect population will be maintained, ensuring that various genetic information and molecular and other tools, which may be desired in the future, are preserved. The mass colonization of a pest species for SIT releases also implies that the "eradication" situation could be reversed in a very short time by intentionally releasing fertile adults. The World Health Organization (WHO, 2001) defines "eradication of disease" as a status, whereby no further cases of a disease occur anywhere, and continued control measures are unnecessary. "Elimination of disease" is defined as the reduction of case transmission to a predetermined very low level (e.g. one case per 10 000 or 1 million of the population). "Control of disease" is defined as ongoing operations or programmes aimed at reducing the incidence and/or prevalence, or eliminating, such conditions.
