Chagas Disease Control Programme in Brazil: A Study of the Effectiveness of 13 Years of Intervention(*)

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Introduction

Public health services and programmes can be evaluated by different theoretical and methodological approaches which take into account the triad of structure-process-results (1). Since structure and process can rapidly be assessed, the evaluation conclusions can provide important information to programme administrators. On the other hand, only the evaluation of results can identify changes in the mortality and morbidity of populations as a result of the programme's intervention (2, 3). Evaluation of the impact of the programme requires a considerable investment of time in order to observe, analyse and measure the changes in the health indicators being studied (2). Epidemiological studies are often used for evaluating the impact (results) of health programmes, as well as estimating their efficacy and effectiveness, after controlling for extrinsic or confounding factors and identifying other changes (4-6).

The actions of the Chagas Disease Control Programme in Brazil are based on house spraying with insecticide, the objective being to control the Triatoma bug population (intermediate host of Trypanosoma cruzi, the etiological agent of Chagas disease). Until now, the assessment of the programme's impact has been mainly based on entomological indicators, i.e. measuring the reduction of the household triatomine population. The results have shown success in controlling Triatoma infestans, the main vector of T. cruzi in Brazil. However, the replacement of this species by secondary ones in household infestation, which could lead to a possible recrudescence in T. cruzi infection rates, has focused attention on this type of habitat in the transmission of Chagas disease (7, 8) and emphasized the need to evaluate the programme's activities in terms of a reduction in transmission rates. Hence, in addition to the entomological results, serological evaluations of selected cohorts have shown a reduction of T. cruzi infection among those who were born after the start of the programme (9-12). Seroprevalence of Chagas disease in this population is considered to be a sensitive indicator of triatomine household infestation rates, capable of estimating the risk of parasite transmission in the community (3).

A quasi-experimental epidemiological study was conducted in Minas Gerais State in 1987, with the objective of evaluating the effectiveness of the programme. Areas with interventions lasting 5 and 10 years were compared to a control area with no intervention. A comparison of the infection rates found in the study areas, among those born after the programme's implementation, with those published by Chagas Disease Serological Survey showed a decrease in T. cruzi infection rates (cohort of 2-6-year-olds) in the area with 10 years of intervention. However, in the area with only 5 years of intervention (Montalvania), there was an increase in the T. cruzi infection rates in a similar cohort. The authors suggested that 5 years of programme activities were not sufficient to induce changes in the infection rates (13, 14).

The present study was conducted in Montalvania, Minas Gerais State, in 1995 after 13 years of interventions under the Chagas Disease Control Programme. The objective was to investigate (1) the T. cruzi infection transmission pattern in a cohort born after the intervention, and (2) whether the duration of the programme was a determinant factor in inducing changes in the incidences. The panel study was the design chosen in order to evaluate the efficacy of the control programme, a series of cross-sectional studies being carried out in the same population at various time intervals (15). The results of this study were compared with two previous investigations conducted in this area: the Chagas Disease National Serological Survey (1975-1980) (16) and the quasi-experimental study conducted in 1987 (13).

Materials and methods

Study population. This investigation was carried out in the rural area of Montalvania, situated in the north-east of Minas Gerais State, south-east Brazil. …