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Cost-Utility Analysis: the Cost of Gaining a Disability-adjusted Life-year
Nematodes have their main effects on morbidity rather than mortality, so the impact of treatment
cannot be determined in terms of commonly used indicators such as the number of lives saved. As discussed above, the DALY metric, on the other hand, may provide a satisfactory effectiveness measure in this context, not only in calculating the cost-effectiveness of treatment alternatives for a single parasitic infection, but also with regard to comparability with interventions against other diseases. Note that technically, for the current purposes, carrying out the cost-effectiveness analysis of health interventions using utilities, such as DALYs, in the numerator would be called a cost-utility analysis (Drummond, 1987).
The first published attempt at assessing the cost per DALY gained for anthelminthic chemotherapy was undertaken by Warren et al. (1993), using a simple static framework. The total cost per child for a 10 year program (albendazole, praziquantel and delivery costs) was assumed to be US$8-18. The estimated gains in DALYs arose from a reduction in morbidity and mortality in children during the intervention, postintervention health benefits for the target group, and indirect health benefits for the families of the target group during the intervention. The reduction in morbidity was assessed as functions of the proportion of children with high or mild-moderate infections, and the disability weights for the morbidity associated with high or mild-moderate infections. The mortality prevented was determined from the estimated number of deaths and the life expectancy at death. It was estimated that the program would cost US$6-12 (moderateeffect estimates) or US$15-33 (low-effect estimates) per DALY gained.
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
The model by Warren et al. (1993) was essentially static. A more recent cost-utility analysis in terms of cost per DALYs gained for school-based delivery of anthelminthics has been undertaken using a population dynamic model in relation to A. lumbricoides infection (Chan et al., 1994a). The model is an extension of the Chan et al. (1994b) age-structured model for intestinal nematodes alluded to before, with three rather than two age-groups: pre-school children, schoolage children and adults. Morbidity is classified into four types, defined by group affected, duration of disability and the number of worms associated with the condition. A low-threshold worm burden is associated with reversible growth faltering in children and/or reduced physical fitness in children and adults (Type A), and permanent growth retardation in 3% of children (Type B). A high-threshold burden is associated with clinically overt acute illness of short duration (Type C) and acute complications in 70% of those with Type C (Type D). Mortality was assumed to occur in 5% of Type D cases. The low and high-threshold worm burdens were age-dependent, and each morbidity type was associated with assumed disability weights. Chan (1994b) predicted that 70% of the total DALY loss in a community with a high prevalence of infection could be averted by treating 60% of school-aged children every year for 10 years, at a cost per DALY averted of US$8.
Both these estimates would suggest that helminth control is an exceptionally good ‘buy’ in public health terms (World Bank, 1993). They also show that gaining an understanding of the impact of many of the critical components of cost-effective treatment, such as frequency and duration of treatment, coverage and age-targeting, can only come from incorporating parasite population dynamics and epidemiology into economic evaluation frameworks.
CLASSICAL PARASITE EPIDEMIOLOGY
No chapter on epidemiology would be complete without some explanation of the application of medical statistical methods to understanding the patterns of disease and their causation and control. Here we give examples of two
approaches—the randomized trial and metaanalysis—that have been fundamental to understanding infectious disease, and indeed much else, in epidemiology. The reader is referred to specific texts (see Hedges and Olkin, 1985) to explore this in more detail.
A Randomized Trial
Intestinal nematodes infection constrains the physical and intellectual development of school children in low-income countries (Bundy and de Silva, 1998). Few studies have examined the impact of infection on younger children, partly because the burden of worms and, it has been assumed, disease is light at this early age, but perhaps mainly because of the practical difficulty in reaching this pre-school population. In Uttar Pradesh in northern India there are high levels of worm infection and malnutrition in the preschool group (Gaitonde and Renapurkar, 1979) and a study of supplementary nutrition has shown a benefit of deworming these children (Gupta et al., 1977). Furthermore, the State Integrated Child Development Scheme (ICDS) now provides a health care infrastructure that reaches all children under 5 years of age.