<p>Study recruitment and laboratory testing results.</p>

Reactive case detection (RACD), or testing and treatment of close contacts of recent malaria cases, is widely used in settings nearing malaria elimination, but often results in very low yield and uncertain impact. In areas where the primary vector exposure occurs outside the home, adapting RACD to target high-risk populations with specific exposure profiles may increase yield, surveillance value, and impact on treatment rates and transmission. We evaluated the feasibility and yield of a risk-based RACD approach based upon forest-based exposure, in comparison with the standard neighborhood and household-based RACD conducted in Aceh Province, Indonesia. Index malaria cases were enrolled from public health facilities in five subdistricts. For each index case, standard RACD included testing members of households within 500 m, while risk-based RACD involved testing non-household contacts sharing a forest worksite or forest travel. Infection prevalence was measured by light microscopy, loop-mediated isothermal amplification (LAMP) and polymerase chain reaction (PCR). The study enrolled 47 index malaria cases during the study period, the majority (28) of which were diagnosed as P. knowlesi infections. Forty-one of the cases reported potential forest exposure outside the home, and follow-up was completed for 40 cases. Standard household-based RACD tested 847 household members from 34 index cases, yielding only 1 infection by LAMP/PCR (0.1%; 95% Confidence Interval (CI) 0.0 – 0.7%). Risk-based RACD tested 179 eligible contacts from 32 index cases, yielding 8 LAMP positive infections (4.5%; 95% CI 1.9 – 8.6%), including 3 confirmed by PCR (1.7%; 95% CI 0.3 – 4.8%). In Aceh Province, targeting RACD to non-household contacts with shared high-risk exposures substantially increased infection yield compared to household-based RACD. Risk-based, exposure-driven approaches may improve the effectiveness of RACD in settings where malaria risk is greater outside the home.