Trapping sylvatic Triatominae (Reduviidae) in hollow trees.

. infection below which mass peatment will become unfavourable in terrns of cost-effectiveness. However, it also implies that above this value variations in the prevalence of infection have little effect on the cost per infected person treated. The precise prevalence value at which this non-linearity is evident is dependent on the cost per person treated: the higher this cost, the lower the prevalence valye. This relationship has important practical implications for the cost-effectiveness of mass treatment implemented in areas with a prevalence of infection above a given level. Based on WHO'S recommendation of mass treatment at a prevalence of 50%, the estimated cost per infected child treated for school-based delivery is US460.46 for albendazole and US$1.58 for praziquantel. The costeffectiveness ratio is higher for praziquantel owing to the higher drug and delivery costs (which incorporate a Screening component to identify the schools for mass treatment and require the dose for each child to be calculated). In the intervention areas of the PCD (Tanzania) the prevalence of infection with Ascaris, TricJzuris orhookwormwas 73% (PCD, 1998b). Based onthe cost of school-based albendazole treatment of US$0.23, this translates into a cost per infected child treated of US$0*32. Successive interventions will be expected to reduce the prevalence of intestinal worms in these communities, and as a result one may need to readdress the cut-off prevalence level that justifies mass treatment. Indeed, the analysis here suggests that since mass treatment with albendazole delivered through schools can be achieved at such a low cost, mass treatment in areas with more than 25% infected children would still result in a cost per infected child treated less than US$l*OO. This would be equivalent to using mass treatment in areas with greater than 50% infected children if a mobile team were used (see Figure). In conclusion, delivery of anthelmintics through the school system as opposed to using a mobile team can reduce delivery costs by an order of mawitude. In addition, due to these low costs, mass treatment for intestinal nematodes may be 'good value' even when only 25% of the community are infected.