Operational research funded by RAI has investigated mass screening of forest-goers, and found that while the forest-goers had a high prevalence of malaria, as measured by PCR test, the use of rapid diagnostic tests (RDTs) was not sensitive enough to pick up the infections, which were mostly asymptomatic. RDTs may therefore not be a good tool in the mass screening of asymptomatic people.
One of the key remaining challenges in malaria elimination in the Greater Mekong lies in optimal control of persistent malaria reservoirs in forests, where malaria continues to infect forest-going individuals, who then act as sources of transmission to others. Forest-going populations are diverse. They include people who enter the forest to make a living (including agriculture, logging, and raising domestic animals), migrants, border guards, and forest rangers. Many forest goers have limited access to health services due to the remote location of forests and the high mobility of these populations.
Institut Pasteur du Cambodge (IPC), a research organisation and public health institute based in Phnom Penh, worked with Malaria Consortium and Partners for Development to conduct research to find out if forest malaria could be eliminated by training forest goers to carry out malaria control activities directly with their own communities.
IPC trained Forest Malaria Workers (FMWs) to implement malaria testing, treatment, and health education in two large forests in the Cambodian provinces of Stung Treng and Kratie. From August 2019 to December 2020, the FMWs used conventional RDTs to conduct mass screening and treatment of forest goers. To assess the accuracy of the RDTs, the researchers also collected blood samples for PCR testing, a type of test that is more accurate, but is also more expensive, takes longer to deliver a result and needs to be performed in a laboratory. The FMWs treated all positive cases they found with the antimalarial drug artesunate-mefloquine and followed up the cases for 42 days. Furthermore, the FMWs used smartphones to collect real-time data, which enabled a better understanding of how malaria cases were concentrated within the forests.
The study enrolled 6,380 forest goers and found a high prevalence of asymptomatic malaria infections among this population, as measured by PCR. However, the sensitivity (ability of a test to detect an infection) of rapid diagnostic tests was extremely low among the forest goers, who were almost all asymptomatic. The sensitivity of the rapid diagnostic tests was 9.0% for Plasmodium falciparum and 3.8% for Plasmodium vivax.
Fewer than 1 in 25 asymptomatic Plasmodium vivax infections detected with RDTs
Fewer than 1 in 10 asymptomatic Plasmodium falciparum infections detected with RDTs
These important findings showed that rapid diagnostic tests are not suitable for use as a screening tool. However, PCR-based mass screening is currently not feasible due to significant logistic and economic challenges. Unfortunately, because testing populations to determine who needs treatment has been shown to not be very effective, strategies based on targeted mass administration of preventative treatment (without prior testing) to high-risk populations would likely be more successful. PCR tests would be useful in regular cross-sectional prevalence surveys to monitor the efficacy of mass treatment.
The study also showed that although hotspots do exist for forest transmission of malaria, half of the infections in the study occurred outside of these hotspot areas. This means that interventions may be most effective if they include all at-risk forest-going individuals. In addition, the research found that the rate of mosquito bites during the daytime was high and that the forest goers did not consistently use their bed nets and insecticide (image to the right). This complicates the roll-out of vector control interventions to forest goers, which should be complemented by other types of malaria elimination tools.
Eliminating Plasmodium falciparum malaria: results from tailoring active case detection approaches to remote populations in forested border areas in north-eastern Cambodia. Ann-Sophie Stratil, Lieven Vernaeve, Sergio Lopes, Yves Bourny, Kylie Mannion, Prudence Hamade, Arantxa Roca-Feltrer, James K. Tibenderana, Siv Sovannaroth & Mark Debackere