Asymptomatic malaria reservoirs are the last challenge in the elimination in Cambodia.

Background: Malaria, a mosquito-borne disease, is a serious public health issue globally and a leading cause of morbidity and mortality in many developing countries worldwide. Cambodia is in the last stages of malaria elimination and aims to eliminate all species of human malaria by 2025. Despite tremendous progress, eliminating malaria in Cambodia has proven to be challenging due to pockets of residual transmission in high-risk populations sustained by untreated asymptomatic malaria reservoirs. Understanding the extent of asymptomatic malaria reservoirs in 'last-mile' communities such as those in Mondulkiri and Kampong Speu, is vital for an effective malaria elimination strategy.

Methods: Malaria cross-sectional surveys were conducted in high-risk populations (forest dwellers, forest goers and forest rangers) at three different time points (T0, T1, T2) from October 2022 to February 2023, overlapping the rainy, malaria transmission season and into the dry season. Blood samples (n = 6350) collected on filter paper from participants from all target groups were screened for Plasmodium species using qPCR.

Results: All qPCR-diagnosed cases were asymptomatic, indicating an untreated parasite reservoir. In Mondulkiri, the prevalence of Plasmodium falciparum was 0.63% at T0, increasing to 0.81% at T1, and decreasing to 0.18% at T2. Plasmodium vivax decreased from 4.80% at T0 to 1.97% at T1 and 1.65% at T2. In Kampong Speu, overall prevalence was 7.06% at T0, declining to 5.19% at T1 and 4.59% at T2. Plasmodium falciparum prevalence was 0.30% at T0, decreasing to 0.09% at T1 and rising slightly to 0.10% at T2. The forest goers showed a prevalence increase to 1.95% at T1 and decrease to 1.46% by T2, while forest dwellers decreased to 3.25% at T1 and further to 3.13% at T2. Passively reported malaria case showed that 1.09% of cases in Mondulkiri and 0.21% of cases in Kampong Speu were rapid diagnostic test (RDT) positive.

Conclusion: Evidence generated during this study point to the continued presence of an untreated asymptomatic reservoir in high-risk populations. Targeted epidemiological and/or vector-based intervention strategies tailored to specific risk groups may enable a reduction of this sustaining reservoir of parasites, thereby leading to eliminating malaria in Cambodia.