The global burden of malaria decreased substantially between 2000 to 2018, including in The Gambia, thanks to the scale up of malaria control interventions such as Long-Lasting Insecticide Treated Nets (LLINs), Indoor Residual Spraying (IRS) and effective antimalarial combination treatment. As malaria transmission declines, the proportion of asymptomatic malaria infections that may be patent (detected by microscopy) or sub-patent (molecular methods) will increase. The literature review highlights the dynamics of asymptomatic P. falciparum infections, the factors associated with their occurrence, the contribution of asymptomatic infections to ongoing malaria transmission, the diagnostic tests that detect low-density infections and the strategies for malaria elimination.
We identified areas with on-going malaria transmission in the five rural regions of The Gambia at the peak of the malaria transmission season. Coverage of control interventions was high, LLINs use >80%. The prevalence of malaria infections was 16% and varied between and within regions. Malaria prevalence was highest in eastern Gambia (prevalence > 30%) and low (prevalence <10%) in the central and western region. More than 60% of infections were asymptomatic. There is ongoing residual malaria transmission in The Gambia despite a high coverage of vector control interventions. To determine the spatial-temporal dynamics of malaria infections, between June 2013 and April 2014, a prospective cohort study was conducted in two villages per region. All residents aged ≥6 months were enrolled; individual data and blood samples were collected monthly during the transmission season (June-December) and in April (dry season). Mosquito data was collected for three nights monthly (June to December). The results found LLINs use was high, 71.5%. The incidence of malaria infection and clinical disease varied significantly across the country, with higher incidence in the eastern (1.7/PYAR) than in the central (0.2 /PYAR) and western (0.1/PYAR) regions. Sub-patent infections occurred more frequently in older than in younger children (HR=1.4, p<0.01). The risk of clinical malaria was significantly higher in households with at least one infected individual at the beginning of the transmission season (HR=1.8, p<0.01). Vector parity was significantly higher in the eastern region (>90%), than in the western region, indicating higher vector survival. The eastern region of The Gambia has the highest burden of malaria that is maintained by the high survival of the local vector population.
We determined the impact of annual mass drug administration (MDA) with dihydroartemisinin-piperaquine on malaria. The MDA was done at the beginning of the transmission season, in 2014 and 2015. Individual blood samples and data were collected from July to December. The MDA had a coverage of 68% (2014) and 66% (2015) with a high compliance (>83%). Incidence of malaria infections and clinical disease were significantly lower after that before the intervention (2013). Malaria prevalence decreased in the first three months post-MDA but remained significantly higher in the eastern region compared to other regions. Individuals with malaria infections before the MDA had higher risk of infection post-MDA. Multiple MDA rounds done in larger geographical areas are required to achieve a more sustained reduction on malaria transmission.
We evaluated the field performance of the new Alere® highly sensitive rapid diagnostic test (HS-RDT) in areas of varying malaria transmission. The results were used in a mathematical dynamic malaria transmission model to predict the impact of mass testing using HS-RDT and treatment (MTAT) campaigns on malaria prevalence. Compared to qPCR, HS-RDT sensitivity was low (38%) and varied by transmission intensity. The MTAT model predicted three rounds of MTAT with HS-RDT at 85% coverage would decrease malaria prevalence from 15% to below 5%.
The discussion highlights the impact of these results on policy and identifies additional research gaps. Malaria endemic countries are increasingly being divided into two groups, i.e. those that could achieve malaria elimination within the next decade, and high burden countries. As sub-patent infections represent an important proportion of malaria infections in low transmission areas, the contribution of low-density infections to maintaining transmission should be established. Community treatment strategies like MDA and MTAT will have a major role in further reducing the human reservoir of infection and in turn malaria transmission. Asymptomatic individuals transmitting to vectors will need to be targeted. Eliminating malaria will be achieved through regional country collaborations, new strategic approaches, novel diagnostics and treatment technologies.