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DALY, disability-adjusted life year. Figures in bold within brackets denote the proportion of DALYs lost contributed by communicable diseases to the total DALYs lost (1.36 billion) in 199o. Data from World Bank (1993).

DALY, disability-adjusted life year. Figures in bold within brackets denote the proportion of DALYs lost contributed by communicable diseases to the total DALYs lost (1.36 billion) in 199o. Data from World Bank (1993).

al., 1984; Hammer, 1993; Prescott, 1987). Although used gainfully to investigate the optimal chemotherapy strategy for the control of schistosomiasis (Prescott, 1993) and the combined control of intestinal nematodes and schistosomes (Warren et al., 1993), this approach is felt to be limited by the inability to account for dynamic responses in infection (e.g. changes in immunity development, parasite or vector resistance development and human disease in malaria following either drug treatment or vector control, and changes in parasite population and human morbidity as a result of treatment and reinfection in helminth infections) following interventions. As such, the effectiveness measures addressed using static models are usually simple immediate outcomes that reflect coverage (e.g. proportion of infected individuals treated) or both coverage and cure rate (e.g. proportion of infected individuals cured). More recent approaches have therefore sought to develop dynamic models for undertaking cost-effectiveness investigations (Guyatt et al., 1993, 1995). These models have a major advantage in that, since they can monitor changes in the parasite population over time in response to intervention, they not only permit the effectiveness of treatment to be assessed in terms of cases that were prevented because of intervention, but also lend themselves to reliable comparability between strategies varying in the level of coverage, drug efficacy and frequency of mass treatment in areas differing in rate of infection transmission.

This section will address how the use of dynamic models for undertaking cost-effectiveness analysis can provide an understanding of the process, costs and effectiveness of different parasite control strategies. The focus will be on intestinal nematode infections, since most of this new work has been carried out for these infections. Mention will also be made regarding the use of DALYs as an effectiveness measure in carrying out such analyses.

Cost-Effectiveness Analysis using a Dynamic Model of Intestinal Nematode Control

Most cost-effectiveness analyses of intestinal nematode control programs have focused on strategies for delivering chemotherapy, as this is generally considered to be the most cost-effective approach to controlling infection (Bundy and de Silva, 1998). In the application of mass chemotherapy, an important indecision lies in the frequency of treatment, which depends in part on the intensity, and hence dynamics, of parasite transmission in that endemic locality (Anderson and May, 1985). Guyatt et al. (1993) incorporated cost analysis into the dynamic framework for helminth transmission of Medley et al. (1993), to assess the cost-effectiveness of alternative mass chemotherapy strategies, which varied in the frequency of treatment of Ascaris lumbricoides infection in high- and low-transmission areas. The effectiveness of a 5 year program, with treatment at intervals of 4 months, 6 months, 1 year and 2 years, was assessed over 10 years. Since the control program component is assumed to last for 5 years, it is obvious that the less frequent the treatments, the fewer treatments that are given.

One of the most important qualitative results from this analysis was that measuring effectiveness and cost-effectiveness in terms of the reduction in prevalence of infection gives conclusions that conflict with assessment in terms of reduction in intensity. This is an important observation, given that most control programs are evaluated in terms only of the reduction in prevalence of infection. Figure 2.5 illustrates the relationship between cost and effectiveness for the four treatment options in the high- and low-transmission areas, in terms of reduction in infection and in heavy infection. Reduction in heavy infection was maximized in the hightransmission area and when treating frequently (every 4 months). Maximal reduction in infection prevalence, in contrast, was observed in the low-transmission area. Since, in this model, the costs are independent of endemicity, the cost-effectiveness ratios for heavy infection reduction are consistently lower in the high-transmission area than in the low-transmission area (see Table 2.4), suggesting it is more cost-effective to intervene in the high-transmission area. The cost-effectiveness ratios also indicate that the most cost-effective intervention in terms of heavy infection reduction is to treat every 2 years.

Although treating every 2 years minimizes the cost per heavy infection case prevented per person, treating every year provides an extra

Cost per capita (US$)

Fig 2.5 The relationship between effectiveness (average infection and heavy infection cases prevented per person) and costs per capita at increasing frequency of treatment directed at Ascaris lumbricoides in a high- and low-transmission area. Modified from Guyatt et al. (1993), with permission

Table 2.4 Cost-effectiveness ratios and incremental cost-effectiveness ratios for delivering anthelminthic treatment directed at Ascaris lumbricoides at different frequencies in a high- and low-transmission area

High-transmission area Low-transmission area

Heavy infection Infection Heavy infection Infection

Cost per case prevented

Heavy infection Infection Heavy infection Infection

Cost per case prevented

2 yearly treatment

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