Alexandra Shaw

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

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

Reply to the letter to the editor by Bouyer et al. (2013)

Who is really misleading decision-makers about costffective approaches to tackling tsetse flies? Bouyer et al. (2013) claim we have given misleading dvice in our paper (Shaw et al., 2013) on the costs of ontrolling or eliminating riverine or savannah species of setse using various techniques, singly or in combination. e suggested that the most cost-effective option is often setse control, rather than elimination. In contrast, Bouyer t al. (2013) focus on the importance of eliminating tsetse rom whole belts of contiguous tsetse infestation, and claim hat for riverine species the only means of achieving this is o rely on the sterile insect technique (SIT) to deliver the nal blow to populations suppressed by other methods. The focus adopted by Bouyer et al. (2013) accords ith their involvement in an SIT-based elimination proramme in Senegal (Vreysen et al., 2013). Given that SIT eals particularly poorly with the problem of tsetse invaion (Vale and Torr, 2005) it is not surprising that they ink the use of SIT to the aim of widespread eliminaion of tsetse up to the natural barriers to invasion – he so-called area-wide policy. Nor is it surprising that hey claim that SIT is essential for final elimination since he use of this complex, costly and protracted technique akes sense only if cheaper and simpler techniques canot perform the task unaided by SIT. To maintain their tance, they ignore evidence detrimental to their cause nd advance only weak arguments, which we challenge elow. First, Bouyer et al. (2013) criticise the model (Vale nd Torr, 2005; Torr and Vale, 2011) that we used to ssess conservatively the time for which various eliminaion techniques need to be applied. The criticisms levelled gainst this model and, implicitly against the model of argrove (2003), distract attention from the pertinent oint that no alternative model has shown anything mateially different. The only model that seemed to attempt his (Barclay and Vreysen, 2011a) considered technical fficacy alone, not costs, and was flawed in its structure nd parameters, as outlined by Hargrove et al. (2011) and