Nested Case-Control Study in a Serological Survey to Evaluate the Effectiveness of a Chagas Disease Control Programme in Brazil

Article excerpt

Objective To identify risk factors associated with Trypanosoma cruzi infections in areas under surveillance in the State of Minas Gerais, Brazil.

Methods A model using a nested case-control design incorporated within a serological survey of schoolchildren which was employed to evaluate the effectiveness of the Chagas disease control programme.

Findings In a sample of 40 374 schoolchildren (aged 7-14 years) surveyed, 16 children tested positive for T. cruzi antibody (by indirect immunofluorescence and indirect haemagglutination). In the case-control study, each case was randomly matched to three seronegative controls (classroom and age [+ or -] year). Compared to controls, T. cruzi-seropositive children were more likely to have a seropositive mother (odds ratio (OR) = 6.8; 95% confidence interval (CI) = 0.71-63.9) or a seropositive family member (OR = 8.6; 95% CI = 1.0-75.5).

Conclusion Use of the nested case-control model in a sero-epidemiological survey to evaluate risk factors for T. cruzi transmission was adequate for assessing the effectiveness of a Chagas disease control programme.

Keywords Chagas disease/prevention and control; Child; Program evaluation/methods; Risk factors; Logistic models; Seroepidemiologic studies; Case-control studies; Brazil (source: MeSH).

Mots cles Trypanosomiase sud-americaine/prevention et controle; Enfant; Evaluation programme/methodes; Facteur risque; Modele logistique; Etude sero-epidemiologique; Etude cas-temoins; Bresil (source: INSERM).

Palabras clave Enfermedad de Chagas/prevencion y control; Nino; Evaluacion de programas/metodos; Factores de riesgo; Modelos logisticos; Estudios seroepidemiologicos; Estudios de casos y controles; Brasil (fuente: BIREME).

Bulletin of the World Health Organization, 2001, 79: 409-414.

Voir page 413 le resume en francais. En la pagina 413 figura un resumen en espanol.

Introduction

Epidemiological studies are often used to evaluate the impact of health programmes, by estimating their efficacy and effectiveness after controlling for confounding factors. External comparison (i.e. comparison between areas with and without an intervention) can identify changes in health indicators of populations covered by the programme and can associate programme actions with the observed changes (1-5). The quasi-experimental model has been proposed as the method of choice to evaluate public health programmes when comparison areas are available, especially for programmes affecting large populations (3, 6). When a control area is not available, internal comparison methods may be used. One such approach is the use of several cross-sectional studies in the same area at different times. Unlike conventional cross-sectional studies, this method -- known as a panel study -- can attribute changes in morbidity or mortality to the intervention programme because time trends can be analysed (7). Another design that uses internal comparison to assess impact is the case-control study, which compares infected and uninfected individuals and their exposure to the programme. An advantage of the case-control method is that it can be initiated relatively early in relation to programme actions, thereby providing results more rapidly. Furthermore, the method is quick, inexpensive, easy to carry out, and effective in identifying risk factors associated with infection (8-11).

The Chagas disease control programme in Brazil employs insecticide spraying of residences to control the triatomine population--the intermediate host of Trypanosoma cruzi, the etiological agent of Chagas disease. Programme assessment has been based mainly on entomological indicators, such as measuring the reduction in the household triatomine population. The programme has been successful in controlling Triatoma infestans, the most important vector of T. cruzi in Brazil (12-17). In addition to the entomological results, epidemiological studies using a quasi-experimental model and panel studies conducted in the intervention area have shown reductions in T. …