Malaria incidence in children in South-West Burkina Faso: comparison of active and passive case detection methods

Abstract

Background: The aim of this study was to determine the incidence and seasonal pattern of malaria in children in South-West Burkina Faso, and to compare, in a randomized trial, characteristics of cases detected by active and passive surveillance. This study also enabled the planning of a malaria vaccine trial.

Methods: Households with young children, located within 5 kilometers of a health facility, were randomized to one of two malaria surveillance methods. In the first group, children were monitored actively. Each child was visited twice weekly; tympanic temperature was measured, and if the child had a fever or history of fever, a malaria rapid diagnostic test was performed and a blood smear collected. In the second group, children were monitored passively. The child's parent or caregiver was asked to bring the child to the nearest clinic if he was unwell. Follow up lasted 13 months from September 2009.

Results: Incidence of malaria (Fever with parasitaemia ≥5,000/µL) was 1.18 episodes/child/year in the active cohort and 0.89 in the passive cohort (rate ratio 1.32, 95% CI 1.13-1.54). Malaria cases in the passive cohort were more likely to have high grade fever; but parasite densities were similar in the two groups. Incidence was highly seasonal; when a specific case definition was used, about 60% of cases occurred within the 4 months June-September.

Conclusion: Passive case detection required at least a 30%-40% increase in the sample size for vaccine trials, compared to active detection, to achieve the same power. However we did not find any evidence that parasite densities were higher with passive than with active detection. The incidence of malaria is highly seasonal and meets the WHO criteria for Seasonal Malaria Chemoprevention (SMC). At least half of the malaria cases in these children could potentially be prevented if SMC was effectively deployed.

Figure 1. Map of the study area, Banfora district, Burkina Faso Households of study participants are identified as “white circle” when assigned to active case detection and “gray circle” when assigned to passive case detection.

Figure 2. Study profile.

Figure 3. Nelson-Aalen cumulative hazard, mean number of children with a measured fever and a parasite density of 5,000/µL or more, by surveillance method.

Figure 4. Nelson-Aalen cumulative hazard, mean number of children with a measured fever or history of fever in the last 24 hours and a positive RDT, by surveillance method.

Figure 5. Arithmetic mean tympanic temperature (°C) among children with parasite positive malaria cases by calendar month.

Figure 6. Arithmetic mean parasite density among parasite positive malaria cases by calendar month.

Figure 7. Malaria case definition using objective fever (Temperature≥38.0): Sensitivity and specificity of alternatives parasite threshold.

Figure 8. Malaria case definition using objective fever (Temperature≥38.0)/History of fever: Sensitivity and specificity of alternatives parasite threshold.